ECSE 309 MEGA QUIZLET
Wave propagation may be studied in the context of Any function of a voltage or current in time Digital signals, because they're not sinusoidal Sinusoidal waves only, because standing waves are a phenomenon of sinusoids Wavelengths no shorter than millimeters, because light travels in quanta, not waves
Any function of a voltage or current in time
In equation 7.2 and footnote 12, what is meant by a conservative vs. a non-conservative electric field? What is the distinction between an induced electric field and an electrostatic one?
Any path taken inside a Conservative field would not require work. An induced electric field like one for a loop of wire induced by a magnetic field does require work to move in a certain direction
Electrons in a wire made of a highly-conductive metal such as copper and with the wire not connected to anything or in any electromagnetic fields and with the wire at room temperature (Sears Lab gets chilly, I know, but it's not absolute zero and these wires aren't superconductive) Are stationary in the metal's crystal lattice. Are in a zero-energy state. are in Brownian motion. Correct! Are in random motion and that motion causes noise voltage.
Are in random motion and that motion causes noise voltage.
For a wave in free space (or another simple lossless medium), the phase constant ββ and the wavelength λλ Correct! Are inversely proportional to each other, radians per meter vs. meters per cycle. Are directly proportional to each other, radians per meter vs. cycles per meter. Are directly proportional to each other, meters per radian vs. meters per cycle. are not simply related, because the phase constant includes a loss term and wavelength does not.
Are inversely proportional to each other, radians per meter vs. meters per cycle.
Magnetic field lines... Can arise from convective current only, because that's free charge carriers. Can arise from drift current only, since that's what's in a wire. Can arise from displacement current only, that was Maxwell's addition. Can arise from any of the three current types
Can arise from any of the three current types
A magnetic field associated with a nearby current: Can be time-invariant (i.e., there can be static magnetic fields) Exists only as the current is changing in time Follows "right grip" rule with electron flow in the thumb direction is proportional to the square of the associated current.
Can be time-invariant (i.e., there can be static magnetic fields)
Changing magnetic fields can arise from DC-powered heater resistors. Changing electric fields. Constant displacement current. Constant current in a wire.
Changing electric fields.
Explain Faraday's law itself, as described in the last paragraph of p. 557, and what the term electromotive force means in this context.
Changing magnetic field moves charges (current). This is what we call the EMF
Maxwell's statement of Faraday's law says that Changing magnetic fields can produce electric fields. Changing magnetic fields can produce divergence of an electric field. Changing magnetic fields have divergence. Magnetic monopoles do not exist.
Changing magnetic fields can produce electric fields.
In the second scenario, the PVC pipe is given a charge and then brought near the neutral soda can. The attraction between the two is because of The attraction between net charge and neutrality Faraday induction on the metal can Charge induction on the metal can an illusion--there's actually alternating attraction and repulsion between the pipe and the can.
Charge induction on the metal can
The third scenario illustrates the difference between Charge decay through humidification Charge transfer by contact and by grounding Charge transfer by induction and by contact Charge transfer by microparticles of metal in the matchstick
Charge transfer by induction and by contact
A DC transmission line (they're common in the southwestern United States) consists of two long, parallel wires 50 cm (0.5 meters) apart. The wires carry identical current I in opposite directions. Find I if the force per unit length experienced by each wire is 0.5 N/m. 1118 amperes 1,250,000 amperes 1581 amperes 11,180 amperes
1118 amperes
Normally we do this demo in the classroom and use a ham radio transmitter as the frequency source. We measured a distance between two voltage maxima of 1.027 meters. What was the transmitter's frequency in megahertz?
146 MHz
Calculate the resistance, in ohms, of an equivalent diameter and length of mercury, such as might be contained in a stretchable rubber tubing to form a strain gauge. Mercury strain gauges were the original electronic version of tocometry, or measurement of uterine contractions during labor. Here, the upper band is holding a fetal stethoscope, the lower is the strain gauge:
3
Now stretch the strain gauge to 1.25 meters. Find its new resistance, in ohms.
3.692
Suppose you want to control a stepper motor to play a song, like this: https://www.youtube.com/watch?v=w68qZ8JvBds (Links to an external site.) ...and that song includes the A above middle C. Per ISO 16, what frequency (in hertz) do you have your microcontroller send to the STEP pin of your DRV8825 motor controller during the time that note is playing?
440 Hz
You are given a two-terminal device that is a quarter wavelength of Z0 = 50 ohm transmission line at your transmitter's frequency. You don't know what the termination of the line is--i.e., you don't know what's connected to the load end of the line. You apply a small signal voltage to your two terminals at the generator end and measure the resulting current. You find that the ratio of the voltage to current is 50 ohms, same as the Z0 for the line. You conclude that the termination is 50 ohms, pure real. 50 ohms purely reactive. open-circuit. short-circuit. All of these could be true--the question doesn't have enough information.
50 ohms, pure real.
From Example 6.3: A wire producing a B field strength comparable to that of a medical MRI machine, 1 tesla, would have current of 50,000 amperes 224 amperes 2.5 x 10^9 amperes It depends on the metal used in the wire
50,000 amperes
American Wire Gage (AWG) No. 22 wire, such as you use on the protoboards in lab, has a diameter of 0.64516 millimeters. Calculate the resistance of in milliohms of a one meter length of 22 G. copper wire.
51.3
From Example 6.6, with a loop of wire 1 cm radius (2 cm diameter) carrying 10 amperes of current, find magnitude B-field at the center, and in the line of the loop axis 0.5 cm out of the plane of the loop. 630 microtesla; 0.29 microtesla About a refrigerator memo magnet in the plane; about earth's magnetic field at that distance away from the plane. 630 gauss in the plane, 0.29 gauss away It's not computable without knowing the wire thickness.
630 microtesla; 0.29 microtesla
Select the SI dimensioned value for the resistivity of mercury from the Wikipedia article "Resistivity (Links to an external site.)." Consider the units carefully: They indicate that resistance is inversely proportional to cross-sectional area and directly proportional to length. Write them out more fully (without cancellations) if this isn't clear.
9.8 * 10^7 Ohms-Meter
In the second part of the demo (2 minute mark), the disaffected grad student fixes the lamp at the line's generator side and slides a shorting block along the line. You can see the light bulb turn on and off as she slides the shorting block. When she has it shorted 1/4 wavelength away from the generator, the bulb is maximally illuminated. Thus shorted, the effective length of the transmission line is equivalent to:
?? (not sure) Short at 0 Distance Infinitely Long Line (open) A termination at 3/4 wavelength
Reflected waves for sinusoidal steady state signals on a transmission line... All of these answers are correct Result in standing waves along the length of the line. Add to forward-traveling waves by superposition. Can cause zero voltage between the line's conductors at specific points. Can cause zero current to flow in the line's conductors at certain points.
All of these answers are correct
You build a circuit that connects a digital output from an Arduino to a digital input on a motor controller board via a 2 meter long coaxial cable transmission line. The board isn't responding as it should. You connect a two-channel oscilloscope to the Arduino output and to the controller board input, and both signals have bizarre looking waveforms, nothing like the digital signals you were expecting. The expert lab supervisor suggests that... The controller board's input impedance is not the same as the cable characteristic impedance and is causing ringing. The Arduino's output impedance is not the same as the cable characteristic impedance and is causing ringing. You can correct the problem by adding appropriate terminating resistors. Correct! All of these answers are correct
All of these answers are correct
In the moving bar setup of Figure 7.11, Example 7.5 in the text (p. 570), Correct! All of these answers are correct. If the load resistor is removed, no work is required to move the bar. For a given load resistance, the external force on the bar in the positive x-direction is dependent on velocity v For a given velocity v, force on the bar varies with the load resistor's value
All of these answers are correct.
Changing electric fields can occur from Radio transmitter antennas. All of these answers are true. Charges in motion. Changing magnetic fields
All of these answers are true.
The first of Maxwell's equations, Coulomb's law, indicates that That static electric field lines originate from and terminate on charges. That electric monopoles exist. All of these answers are true. Electric charges are the source of divergence in the electric field. That a closed surface in space (such as a sphere) containing net charge has electric flux lines passing through the surface's area in proportion to the magnitude of that charge.
All of these answers are true.
A circularly polarized wave is said to be right-hand polarized All these are correct. if the electric field vector is rotating clockwise when view from the signal source. by an electrical engineer when a physicist says it is left-hand polarized. if the vertical electric field component lags the horizontal component by 90 degrees.
All these are correct.
ηη, the impedance of free space (also sometimes written Z0, but not the same thing as the characteristic impedance of a transmission line) may be described as Correct! All these are correct. roughly 377 ohms a function of the ratio of the permittivity of free space and the permeability of free space the ratio of magnitude E to magnitude H everywhere along the path of propagation of the wave.
All these are correct.
In equation 7.3, explain the significance of the induced electric field existing from the changing magnetic field alone, but producing current if conducting material is present.
Alone this is proof of relationship b/tw electric and magnetic fields. Presence of conductor allows use to extract energy
Coulomb got credit for the law of inverse-square electrostatics, but similar work was done earlier by an uncredited American newspaper publisher. French writer of grammar texts. chemist who devised phlogiston theory. Japanese physician.
American newspaper publisher.
A phasor, or phase vector, encodes which parameters of a sinusoid? Amplitude, and phase relative to a reference signal. Frequency, and phase relative to a reference signal. Amplitude and frequency Amplitude, phase relative to a reference signal, and frequency.
Amplitude, and phase relative to a reference signal.
Match the use to the motor type. wheel on a mobile robot elevator on a model airplane position control on a 3D printer
DC Motor Servo Motor Stepper Motor
Moving that electron from the outer surface of the metal shell to the inner surface Doesn't take any work, because there's no electric field in the shell. Doesn't take any work despite the electrical potential difference across the shell from the metal's charge separation. Takes work, but of the opposite sign than it would if the shell weren't there. Takes greater overall work than it would if the shell weren't there, because of the shell's polarization.
Doesn't take any work, because there's no electric field in the shell.
The electromotive force differs from the voltage produced by static charges, even though it has units of volts, in that Charges moving in an EMF device's field return to a given point with no net change in their energy. Charges moving in an electrostatic field require outside energy. EMF comes from chemicals and solar cells, not from charges. EMF is nonconservative; the voltage of electrostatic fields is conservative.
EMF is nonconservative; the voltage of electrostatic fields is conservative.
Manufactured materials with permanent surface charges ("permanent" meaning "last long enough to be useful in an device that is expected to be replaced within ten years") Exist, are called electrets, and are the electric analog of a permanent magnet Cannot exist, the free electrons would break loose and find oppositely charged ions too quickly Cannot exist, the free electrons would combine with the material they're on too quickly. Exist and are made from ferromagnetic materials such as iron
Exist, are called electrets, and are the electric analog of a permanent magnet
The rotor of an induction motor is magnetic. True Correct! False
F
You can freely turn an induction motor while it is powered on. True Correct! False
F
A remote operator on Earth could drive a car on Mars at speeds similar to those used in Earth-bound vehicles. False, because the radio time delay for telemetry and control is much longer than required control responses True, because radio propagation from Earth to Mars is within the time required for control responses. True, because the time delay from Earth to Mars is fixed. False, because the radio signals over that distance are too weak.
False, because the radio time delay for telemetry and control is much longer than required control responses
Which of these is the fundamental operating principle of induction motors? Hofstadter's Law Gauss's Law Pournelle's Law Correct! Faraday's Law
Faraday's Law
Select the most correct answers. In order for Kirchoff's voltage law to hold, the size of components and connections must be "small" compared to "wavelengths" . The net charges on components and interconnections must be "zero" . There must not be negative magnetic flux in any circuit meshes (loops) causing nonconservative electric fields. ... [TIME CHANGING, magnetic flux]
Fill in the blank
How does Faraday's homopolar generator work? (mentioned without illustration on p. 557; article in the usual font of all wisdom here: https://en.wikipedia.org/wiki/Homopolar_generator (Links to an external site.)).
Generator uses energy provided by spinning disc to electric potential. Spinning disc means moving charges, means charge separation and a voltage
A magnetic dipole is a far-field depiction of magnetic field lines that: Is described by a scalar dipole moment, m Describes the magnetic field of a toroidal inductor Describes the magnetic field of a long, current-carrying wire Has similar shape and algebraic form to an electric dipole
Has similar shape and algebraic form to an electric dipole
With nonzero conductivity σσ in a material, loss is from Correct! Heating due to current induced by the wave's electric field. Reactive power in the material causing reflections. the electric and magnetic fields staying in phase within the material. investors withdrawing funds when they realize how expensive good coaxial cable is.
Heating due to current induced by the wave's electric field.
Maxwell's verification of the wave equation part of the equations was that his muse, Maxwell's demon, whispered it to him. He created radio waves and measured them. he created standing waves and measured them. His equations correctly predicted relationships between the recently-measured speed of light, permittivity of free space, and permeability of free space.
His equations correctly predicted relationships between the recently-measured speed of light, permittivity of free space, and permeability of free space.
An electric current going through a wire can be calculated using the cross-section of the wire A, the average speed ov the charges along the wire v, the volume density of charges n=Number of chargesvolumen=Number of chargesvolume and the magnitude of each charge, e. The current through the wire is given by the rate of charge passing a point along the wire: I=dqdtI=dqdt By considering a set of electrons inside a volume V=AlV=Al , what is the current in terms of A, e, n, and l?
I = −nAe * (dl / dt)
The result of the wave equation computations for the transmission line is the boxed equation 2.1 (for voltage) and 2.2 (for current). They predict that... That only transmission lines that are long compared to signal wavelength can be used practically. That matching of wire resistance and insulator conductance can prevent loss along the length of the line. Correct! If the wire resistance is not zero, the line will have voltage loss along its length. That only transmission lines that are short compared to signal wavelength can be used practically.
If the wire resistance is not zero, the line will have voltage loss along its length.
The discussion of lossless lines indicates that signals can travel along a transmission line In both directions, but each will be distorted by the other. In both directions only if the signals have identical frequency components Correct! In both directions, even if the signals are completely different In both directions only if the signals have different frequency components
In both directions, even if the signals are completely different
Discuss the electric potentials available from the induced electric field, as shown in figure 7.4; this is the basis of electric generators. (Note carefully in that figure the distinction between the length vector l and the current I, as they look very similar in the typesetting.)
Increasing mag field out of page leads to induced current clockwise. Charge builds up on the wire loop, creating an electrostatic field
Same setup as question 1, but you put the ohm-meter on one wire at one end of the cable and the other wire at the other end. The meter reads... 900 ohms, that's the resistance of the two conductors. 27,000 ohms, that's the resistance of the two conductors in series across the dielectric. Correct! Infinite resistance: it's reading two wires that aren't connected. 450 ohms, that's the resistance across the two conductors.
Infinite resistance: it's reading two wires that aren't connected.
KCL... Is a statement of conservation of charge. Does not apply to an open-ended transmission line, where current is flowing to an open circuit. Applies even if stray capacitive currents are ignored. Doesn't apply to a short circuit such as a shorted transmission line, because the voltage boundary condition is zero.
Is a statement of conservation of charge.
The electric field E Is a vector field. Has units of coulombs per square meter Points from negative charges (electrons) to positive ones (protons) Is the same as the potential field.
Is a vector field.
In classical physics, outside of special relativity, Coulomb's law (the first of Maxwell's equations) Is always true. Is true only for static (unchanging) charges. Is true only for conservative fields. Is true only for liberal arts fields.
Is always true.
Coulomb's law Is in the form of Newton's law of gravitation. Is in the mathematical form of vector fields. Is in Maxwell's equations.
Is in the form of Newton's law of gravitation.
Kirchhoff's voltage law, that the algebraic sum of EMFs around a closed circuit equals the algebraic sum of the voltage drops such as a resistor's IR drop... Is true if all the EMF energy sources in the circuit are considered. Has solutions with nonzero voltages if the electric field in the circuit is conservative. May be applied without considering the presence of changing magnetic fields going through the circuit. Is true for EMF energy sources connected directly to the circuit such as a battery; others may be ignored.
Is true if all the EMF energy sources in the circuit are considered.
Look at equations 4.6 and 4.7 in the text. They look very similar except for an exponent of 2 for distance in the boxed equation, 4.6 and 3 in the next derivation, 4.7. The 2 is for inverse-square law; what's the 3 doing there? Is there an inverse cube law? It accounts for a change in the equation's form from computing with the unit vector from one charge to another to computing with the actual vector in the same direction. It's a special case of Coulomb's law. It shows the deviation from superposition for multiple close charges in space. It is for the near-field computation of the force between two charges; inverse square law is only for far-field.
It accounts for a change in the equation's form from computing with the unit vector from one charge to another to computing with the actual vector in the same direction.
The Lorenz magnetic force is not part of Maxwell's equations but provides a way to define the B field. A true statement about it: It is always mutually perpendicular to the velocity vector of a charge and to the direction of the magnetic field. It is proportional to the inverse square of the distance between the particle and the magnetic field. In the absence of other fields, it implies a spiral path for a charged particle. It can arise from a static electric field.
It is always mutually perpendicular to the velocity vector of a charge and to the direction of the magnetic field.
Factoid: Automobiles in the United States have their fuel efficiency specified in miles driven per gallon of fuel consumed. In Europe, the specification is usually in litres consumed per 100 kilometers driven. A liter is a cubic decimeter, so in SI units, one litre is 10-3 cubic meters (the SI unit of volume). A kilometer, of course, is 1000 meters, so 100 kilometers is 105 meters. Now, 30 MPG works out to about 8 L/100 km. Litre measures volume; km measures distance; so the ratio is area. 8 L/100 km represents an area of about a square millimeter. What is that area physically? Explain your logic.
It represents the slice (two-dimensional) needed to propel the car a small distance dx
As you apply more voltage to the terminals of a DC motor, the motor's speed increases. Torque is the amount of rotational force a motor can apply. The [stall current] is the maximum current a motor can draw when it encounters so much resistance that it stops spinning; it indicates the motor's potential power. You can modify the motor's torque and speed with mechanical advantage by using a gearbox. A slower gearbox provides more torque than a fast one, and vice versa.
It was fill in the bank
When power is applied to the motor's terminals , it travels across the brushes to the commutator. The brushes got their name because they literally brush across the commutator as it spins freely between them. The [commutator] is made out of separate pieces of conductive material attached around the motor's shaft. Each piece of material is attached to a different wire in the motor's winding. As the shaft spins, and the brushes rub against them, they make and break electrical contact with each one and power the different windings, which are basically inductor coils. These create a(n) magnetic field through the rotor. The field that is created in the rotor reacts to the magnetic field of the magnets which are permanently fixed in place inside of the stator (metal enclosure). These competing magnetic forces causes the rotor, which is attached to the motor shaft, to spin into alignment as it is repelled / attracted to the fixed magnets.
It was fill in the bank
You can reverse the direction of a DC motor by "reversing polarity" . You can also attach "linkages" to the motor shaft to produce different kinds of motion. Linear motion, or "motion across a flat plane" , as you might see in a "conveyor belt" may be produced with a "rotary cog", "piston" and a "pulley" around the motor shaft. Many linkage options are available if you want to produce a "reciprocating" motion, where something moves back and forth along a line, as you might see in a "piston" . If you want to create an oscillating motion back and forth along an "arc" , like you would find in a metronome, you can use a fixed pivot being moved back and forth.
It was fill in the bank
Microelectronic Matching _ENABLE N/C (no connection) M0 LOW M1 LOW M2 HIGH RESET HIGH SLEEP HIGH STEP Function generator output DIR Either HIGH or LOW !VMOT VCC GND GND B2 D D B A1 A A2 C FAULT N/C (no connection) GND GND
It's matching
A voltage sinusoid source is attached to the generator end of a transmission line. The line is terminated in its characteristic impedance. With reference to the generator end, the signal's phase along the length of the transmission line Lags in proportion to the length along the line at a spatial rate dependent on signal frequency. Leads in proportion to the length along the line at a spatial rate dependent on signal frequency. Has a phase lag along the length of the line that is independent of signal frequency Has a phase lead along the length of the line that is independent of signal frequency
Lags in proportion to the length along the line at a spatial rate dependent on signal frequency.
Check off the advantages of brushless DC motors over brushed DC motors. Less overall maintenance due to lack of brushes Low overall construction costs Operates effectively at all speeds with rated load Controller not needed for fixed speed Higher speed range and lower electric noise generation High efficiency and high output power to size ratio Ideal for extreme operating environments
Less overall maintenance due to lack of brushes Operates effectively at all speeds with rated load Higher speed range and lower electric noise generation High efficiency and high output power to size ratio
The magnetic field lines of a finite-length helix of wire (i.e., a solenoid) with current flowing in it: Loop back on themselves and all pass through the inside of the helix Originate and terminate at the ends of the coil Are curl-free Have divergence
Loop back on themselves and all pass through the inside of the helix
What is the name of the mechanical force experienced by a current-carrying conductor in the magnetic field of the permanent magnet? Front EMF Correct! Lorentz Force Friction Entropy
Lorentz Force
Maxwell's version of Ampere's law says that Magnetic fields have curl. Magnetic fields can have divergence. Moving charges always produce changing magnetic fields. Static electric fields produce static magnetic fields.
Magnetic fields have curl.
Coulomb's law for magnetism, the second of Maxwell's equations, indicates that Magnetic flux lines originate from and terminate on magnetic charges. Magnetic flux lines have divergence. Magnetic monopoles do not exist. Magnetic field lines of a magnetic dipole have the same overall shape as the electric field lines of an electric dipole.
Magnetic monopoles do not exist.
Describe verbally and with Maxwell's equations the problem with Ampere's Law that Maxwell solved by defining the displacement current.
Maxwell identified is that the changing electric field creates the magnetic field. This changing electric field he termed the displacement current.
Induction motors, which are named by the induced current in the rotor's squirrel cage, present what sort of load to their electric power source? Purely resistive electrical load, regardless of mechanical load Mixed capacitive-resistive electrical load when delivering no mechanical power Varying capacitive or inductive electrical load depending on the mechanical load, with purely resistive electrical load at the motor's rated shaft output power Correct! Mixed inductive-resistive electrical load regardless of mechanical load on the shaft
Mixed inductive-resistive electrical load regardless of mechanical load on the shaft
Drift (or conduction) Current
Motion of charge carriers in a metallic conductor or semiconductor. The charge carriers are generally moving quite slowly compared with the motion of the corresponding electric field
Convection Current
Motion of charge carriers in free space, not in a metallic conductor or semiconductor. The carriers may be moving slowly, or at relativistic speeds
In the solenoid-plus-core problem of the text on page 588, section 7.3.4, Figure 7.17, note the constant DC current source supplying current to the coil. If the core is ferrous and is centered in the coil, and we're staying on topic, Motion of the core doesn't change voltage across the current source, since current sources don't have voltage across their terminals Correct! Moving the core out of the coil either direction will cause the left side of the current source to become negative relative to the right side during the motion Currants don't work that way, they get jammed up with pectin Moving the core out of the coil to the left will cause the left side of the current source to become positive relative to the right side during the motion
Moving the core out of the coil either direction will cause the left side of the current source to become negative relative to the right side during the motion
(Note: NEMA standards are not specific to stepper motors alone. This just seemed like a logical place to stick this question in terms of the pacing of the quizzes.) The National Electrical Manufacturers Association (NEMA) maintains a recognized standard for motor size, and a labeling system for motor specifications. Look up NEMA standards online to answer this question.Which of the following is the NEMA code that describes a 3.4" diameter stepper motor, with a flange, that is 1.6" long, has a phase current of 1.6 A, class B insulation, 5.3 V phase, 200 steps per revolution, and type A winding connection? NEMA A200-1.6-3.4F-B 34C016-1.6 B 53 200SR A NEMA 34D16053BA NEMA 34D016-016B053200A
NEMA 34D016-016B053200A
In this museum display of a Van de Graaff generator, is the girl provided with an earth ground? No. If she were grounded, the demonstration wouldn't work. Yes, but just for safety--the demonstration would work either way Yes, because no grounding her could be dangerous to others nearby the demonstration. No, the grounding could be dangerous to the machine.
No. If she were grounded, the demonstration wouldn't work.
See Example 8.16. We talked about the earth's static electric field, from charges deposited by thunderstorms. It's about 100 volts per meter. Its effect on the electric field component of solar radiation is: Nothing. The fields exhibit superposition and one doesn't affect the other. Significant at higher light frequencies, and accounts for the blue shade of the daytime cloudless sky. Significant, and accounts for the red shade of the setting sun as the light rays pass through long, low atmosphere. Significant, and we would all have sunburn and retinal burn all the time without it.
Nothing. The fields exhibit superposition and one doesn't affect the other.
As the lightbulb is moved along the lecher line, it illuminates at certain points. The distance between consecutive points is: One wavelength apart. One half-wavelength apart. One meter apart. Variable; they are not equally spaced.
One half-wavelength apart.
One of the constitutive relationships (Links to an external site.) is between the E field and the D field. A simplified version of it is Equation 4.31 in the text. The true statements about these fields: One represents force the field places on a charge and is material dependent; the other represents a charge equivalence at a distance and is not material-dependent. They have the same units and represent the difference between SI and cgs measurement systems. They always point in the same direction. The E field accounts for bound charges within the material medium, such as those in a crystal lattice.
One represents force the field places on a charge and is material dependent; the other represents a charge equivalence at a distance and is not material-dependent.
In the elementary alternating current generator of Example 7.6, and considering that a load might be put across the two slip rings, Increasing the number of windings in the loop would increase the magnitude output voltage but not shaft torque, everything else being equal Increasing the number of windings in the loop would increase the output voltage's frequency The magnitude open-circuit output voltage at the commutator rings is independent of shaft rotational velocity (RPMs) Correct Answer Replacing the slip rings with commutator rings would convert this into a direct-current generator.
Replacing the slip rings with commutator rings would convert this into a direct-current generator.
Steady, direct currents in a room-temperature wire Lose energy only to the surrounding magnetic field, not to heat. Correct! Require a closed circuit. Can be maintained without an external energy source. Can occur in an open circuit such as a radiating antenna.
Require a closed circuit.
An inductor (i.e., the ideal circuit element that has inductance): Is inherently self-shielding in the sense that its magnetic field lines are contained within it. Stores energy in its electric field Stores energy in its magnetic field Can couple energy (and therefore signal) to another circuit only by touching the other circuit
Stores energy in its magnetic field
Which of these will flip the direction of a three-phase motor? Turn the motor off. Disconnect one of the phases. Correct! Swap two of the phases. It is not possible to reverse the direction of a three phase motor.
Swap two of the phases.
If powered up without a starter winding, an induction motor will not turn until an external force is applied. Correct! True False
T
In equation 7.1, what is meant by the induced voltage and by the flux linkage? In the accompanying footnote, what is meant by total flux linkage? Remember the topology of a helix for understanding of why multiple turns in the coil multiply the flux linkage:
TFL describes the amount of B field lines that cup through a helix. Total linkage is proportional to number of turns
The magnetic field B may be expressed as a field with curl because The B field is always divergence free. Because it never has gradient. Because it has divergence and is therefore the curl of another field (the current field) Because it has divergence from magnetic monopoles and therefore has curl.
The B field is always divergence free.
What's the difference between the stator of a permanent magnet DC motor and the stator of a universal motor? The stator of the universal motor is larger. There is no difference. The PMDC motor has permanent magnets in the stator, whereas the universal motor also has a winding that acts as an electromagnet. The PMDC motor has permanent magnets in the stator, whereas the universal motor has only a winding that acts as an electromagnet.
The PMDC motor has permanent magnets in the stator, whereas the universal motor also has a winding that acts as an electromagnet.
[Consider a topographical map of Vermont] Gravitational fields are conservative, as are static electric fields. What is the analog here to electric potential surfaces? The brown lines of constant elevation, called the contour lines. The local peaks, shown with elevation numbers. The black dots, representing areas of local high mass density, similar to charges. The red and white line radiating from the "Ch" symbol, representing field lines from charge.
The brown lines of constant elevation, called the contour lines.
A number of charges are arrayed in space and you have worked out the electric field. You place a test charge (remember, test charges are a limiting case of infinitesimal charge with infinitesimal mass) at a point that has nonzero field strength. The charge moves along the field lines, gaining energy. The charge moves along electric potential contours, gaining energy. The charge moves along electric potential contours without an energy change.. The charge moves along electric field lines without energy change.
The charge moves along the field lines, gaining energy.
Consider Example 4.23 and Example 4.26. In the first one, a voltage source is applied to the capacitor and enough time has passed that the system is in equilibrium. The voltage source is left connected. In the second, that has happened and then the voltage source is removed. (machines exist that do the second version cyclically, called Wimshurst generators (Links to an external site.). There's a paper on them in the Files section, and please watch this video (Links to an external site.) to verify your understanding of the process. This one (Links to an external site.) will give a different perspective on their operation and show you how to use one as a steampunk bicycle headlight) If the distance between the plates is increased in each example, The electric field in the constant-voltage one is reduced and the mechanical work goes to charging the battery. In the second the electric field between the two plates stays the same, and the mechanical work goes to increasing the electric potential of the charges. In both, the potential energy of the system is increased. In the second one, the voltage between the plates increases. All of these answers are correct
The electric field in the constant-voltage one is reduced and the mechanical work goes to charging the battery.
Run this simulation: https://nationalmaglab.org/education/magnet-academy/watch-play/interactive/faraday-s-ice-pail (Links to an external site.) Set the charge at zero, at a medium level, and at maximum while you lower the metal sphere into the insulated metal bucket, and watch the electrometer. The pivot angle of the electrometer is an indication of the charge on it and therefore on the bucket. What would happen if you ran the experiment with charge >0, lowered the ball, and then grounded the bucket for a few seconds--say, if you stood barefoot on the earth and touched your finger to the bucket, then took your finger away? The electrometer leaves would close and stay closed. The electrometer leaves would close and reopen fully when you took your finger away. The electrometer would close, then reopen partially when you took your finger away. The behavior of the electrometer leaves would depend on whether they were made of the same metal as the bucket.
The electrometer leaves would close and stay closed.
Electrons in a wire made of a highly-conductive metal such as copper and with the wire connected to a current source that puts an electric field along the length of the wire and with the wire at room temperature (we're not talking about superconductors here) The electrons move toward the anode, maintaining charge balance with protons moving toward the cathode. The electron count in the wire drops as electrons are drained off the positive end of the wire. Correct! The electrons have an average velocity that is the statistical representation of the current through the wire. The electrons undergo continuous acceleration along the length of the wire from the applied electric field.
The electrons have an average velocity that is the statistical representation of the current through the wire.
At 4:10, the construction and use of a gold-leaf electroscope is shown. Household aluminum foil is used as a substitute for gold leaf, it works well (cheaper, thinner foil works better than name-brand. They are both elemental aluminum and the thinner foil is lighter per unit area). Note that the leaves separate before the charged rod is brought near them. This is because Humidity in the air around the leaves caused them to become charged. The electroscope is so sensitive that even the small field of the rod far away separated the leaves. The bright video lights transferred charge to the leaves. The metal parts of the electroscope were not grounded first and therefore had net charge.
The electroscope is so sensitive that even the small field of the rod far away separated the leaves.
Magnetic field strength can be measured by The force on a charged object held stationary in the field By how much field it takes to cause a spark in air. The force on a charging moving through the field The direction that a compass needle points when held in the field.
The force on a charging moving through the field
What's one difference between the stator of a brushed DC motor and the stator of an induction motor? The stator of the induction motor is longer. The induction motor has no brushes. The PMDC stator is a different color. The stator of the induction motor has magnets.
The induction motor has no brushes.
A rubber balloon has been rubbed on a cat (Bella, come over here, kitty, kitty) and acquired some electrons from the cat's fur. The rubber balloon is now held in the vicinity of a metal bar that is insulated from ground. The metal bar remains without net charge. The metal bar stays neutral everywhere on its surface. The metal bar experiences a force away from the balloon. The cat returns when called a second time.
The metal bar remains without net charge.
Define "commutation" in the context of motors. To send messages through a motor. To pardon a death sentence. To drive to work. The process of regulating or reversing the direction of an alternating electric current, especially to make it a direct current.
The process of regulating or reversing the direction of an alternating electric current, especially to make it a direct current.
Consider the arrangement of a charge and a concentric metal shell in Example 4.21. An electron is brought from infinitely far away to the outer surface of the metal shell. The work done is The same as if the shell weren't there. Less than it would be if the shell weren't there, because of the charge rearrangement in the shell. More than it would be if the shell weren't there, because of the work that the center charge's field did to separate the charges in the shell. It depends on the metal used to make the shell, because metal resistivities vary.
The same as if the shell weren't there.
Now you're moving an electron from the inner surface of the shell to near the charge. The work is The same as if the shell weren't there. Greater than if the shell weren't there, because the induced charges in the shell hold the electron toward the metal. Less than if the shell weren't there, because the induced charges in the shell hold the electron toward the metal. It depends on the polarity of that electron.
The same as if the shell weren't there.
Which of these is used to control torque and speed in a three-phase motor? Correct! The torque-speed curve The torque-speed equation A speed-torque converter The breakdown torque
The torque-speed curve
Circular polarization represents two waves traveling together with a phase difference. The two waves must be of exactly the same frequency. The two waves have to be in a harmonic frequency relationship. The relative frequencies of the two waves is not part of the computation. For the polarization to be circular rather than elliptical, the two waves' amplitudes must differ.
The two waves must be of exactly the same frequency.
The fourth scenario is a charge by induction, but no metal conductor is involved. There is induction because The glass plate under the bubble has charge induced. The air inside the soap bubble is what has the charge induced. The water/soap combination is an adequate conductor for induction to take place. The glass plate is grounded.
The water/soap combination is an adequate conductor for induction to take place.
A square, flat loop of wire is connected to a current source. The wire segments have force on them that causes the loop to expand The wire segments have force on them that causes the loop to contract The wire loop will tend to twist into a three-dimensional shape The wire segments have no force on them.
The wire segments have force on them that causes the loop to expand
What's the difference between the rotor of a permanent magnet DC motor and the rotor of a universal motor? The rotor of a DC motor is commutated, and the rotor of a universal motor is not. The rotor of a universal motor is smaller. There is no difference. The rotor of a DC motor is ferromagnetic, and the rotor of a universal motor is paramagnetic.
There is no difference.
Every real wave medium ends with a termination at the load end. For a light wave terminating at a mirror, the free-space E&M wave of the light is short-circuited by the metal film and is completely reflected. In a transmission line, the termination is a two-port device attached to the end. The termination may be nothing more than an open-circuit or a short-circuit, or it may be a circuit of some sort. If a transmission line's two-port termination is a resistor with resistance the same as the characteristic impedance of the transmission line, The reflection is noninverted. The reflection is delayed by the load resistor's energy storage. Correct! There is no reflection, and the load resistor turns the signal into heat. The reflection is inverted.
There is no reflection, and the load resistor turns the signal into heat.
In the text's Example 7.9 (p. 583), two solenoids are wound, one on top of the other. DC current is flowing in the two coils, so the time-change of the current is zero. Without the current changing, the back EMF is zero, so where does the stored energy discussed in the problem come from? Correct! There was back EMF while the current was increasing, and the magnetic field energy is the integrated product of the voltage and the current It's from the combination of the magnetic field and the electric field between the turns of the coil It's from the ratio of turns in the two coils, depending on whether the ratio is less than or greater than one It's from the heat being dissipated by the coil's resistance
There was back EMF while the current was increasing, and the magnetic field energy is the integrated product of the voltage and the current
Also in the electroscope demonstration... The coil of wire outside the glass jar served to Increase the wire's inductance. To help dissipate the charge before the next experiment is conducted. To intercept more electrical field lines from the charged plastic rod. To facilitate charging by contact, a different experiment from what is shown.
To facilitate charging by contact, a different experiment from what is shown.
From the Greek word for rubbing or friction, triboelectricity is Transfer of charge by rubbing together materials that have different affinity for weakly-bound electrons Rubbing together conductors to put them at the same potential (voltage) Holding two insulators near each other to check the force between them. Doesn't cause enough voltage for air breakdown at 1 millimeter (doorknob distance)
Transfer of charge by rubbing together materials that have different affinity for weakly-bound electrons
These cartoons illustrate static electric field generation from what sort of charge transfer? [Rubbing socks on rug] Triboelectric transfer Induced transfer Grounding Overlimit charging
Triboelectric transfer
Electrons in an X-ray tube (you've had an X-ray, no?) start by being boiled off a heated filament of wire. The filament is put at the negative end of a voltage source and is called the cathode; the positive anode end is a distance away and so there is an electric field established. The electrons in this field Establish an equilibrium velocity during their travel from the cathode to the anode Become positively charged as they accelerate. Have their energy reduced because they are "falling" through a potential field. Correct! Undergo continuous acceleration until they are past the anode and in a zone without field
Undergo continuous acceleration until they are past the anode and in a zone without field
Assume that mercury is incompressible. Will the lab's ohmmeters be able to measure the change in resistance directly? Select all statements below that are true. Using Kelvin (four-wire) measurement would be a good idea here. Any old ohmmeter will do. The meters in the Sears lab would be able to measure this difference. The more common bonded strain gauge will require a Wheatstone bridge for measurements.
Using Kelvin (four-wire) measurement would be a good idea here. The meters in the Sears lab would be able to measure this difference. The more common bonded strain gauge will require a Wheatstone bridge for measurements.
Displacement Current
Virtual current produced by a changing electric field. It has no charge carriers associated with it, but produces a changing magnetic field as if it did
Two different ways of winding an inductor are a solenoid (helix) and a toroid (helix wrapped back around on itself); see Examples. 6.14 and 6.15, and Figure 6.41. Noting that 1) infinitely long solenoids are hard to manufacture and 2) toroids are much more expensive to manufacture than finite length solenoids, toroids are used in circuits When avoiding magnetic fields outside the inductor is important. When miniaturization is important. When weight is important, because toroids generally use less metal in their wires. When distributed capacitance along the inductor is important.
When avoiding magnetic fields outside the inductor is important.
In the first scenario, the experimenter rubs the plastic plate with dry cloth, then separates them. Does she expend work in doing so? Yes, rubbing the two together transfers charge and requires work. No, the forces involved are small and there is no work. No, one has higher affinity for electrons than the other and work is released by facilitating the transfer. Yes, the plate and the cloth become oppositely charged and separating them requires work.
Yes, the plate and the cloth become oppositely charged and separating them requires work.
Similar setup to Question 4, but you find that no current is flowing. You check the transmitter, it is generating the voltage you thought it was. Hmm, no current, infinite impedance...the termination must be Zero ohms (short-circuit) Open-circuit Something reactive, the value and sign can't be determined from this Something resistive, the value cannot be determined from this.
Zero ohms (short-circuit)
The simplest transmission line is two parallel wires held a constant distance apart by an insulator. A lossless line is made of wires that have no resistance, and insulating material that has no conductance. You're in the circuits lab with a 30 meter piece of line marked "450 ohm." You put an ohm-meter on one of the wires at one end and the same wire at the other end. It reads... Zero ohms (short-circuit) 225 ohms, since you're measuring only one wire. 13,500 ohms, since it's 30 meters times 450 ohms per meter and the "per meter" is left out in the manufacturer'slabelling. 450 ohms, since that's the line's impedance
Zero ohms (short-circuit)
The complex propagation constant γγ is composed of a real part that indicates signal attenuation by distance; and an imaginary part that indicates wavelength. a real part that indicates material conductivity and an imaginary part that indicates material permeability a real part that indicates decibels of attenuation in the material, and an imaginary part that indicates material reactance. A real part that distinguishes magnitudes of the forward from backward traveling waves, and an imaginary part that indicates the relative phase of those two signals.
a real part that indicates signal attenuation by distance; and an imaginary part that indicates wavelength.
Lightning is a transfer of charge from clouds to ground or from cloud to cloud. A lightning bolt represents air that has turned into plasma and is conducting the electricity. air that has turned into packed red cells and is relatively insulating. electicity propagating through vacuum as air is displaced by the lightning about the same energy release as a spark on a doorknob on a winter day.
air that has turned into plasma and is conducting the electricity.
At the surface of a perfect conductor, static field lines such as we are studying in this section are always perpendicular. are always tangential. take an angle to the surface that depends on the surrounding insulator. take an angle to the surface that depends on the metal's magnetic properties, even though it is a perfect conductor.
are always perpendicular.
You have a plastic insulator with a net negative charge. You bring it near (but not touching) one end of a metal rod that is suspended by an insulating thread. The metal rod becomes polarized: One end has negative charges, the other positive. takes on a net charge. is grounded. warms noticeably.
becomes polarized: One end has negative charges, the other positive.
In Figure 7.1, what are the time-derivatives of current in the primary and secondary coils? (This is expanded upon in figure 7.5, please examine that and its graphs.)
d_I1 / dt = (-V / L1) *e^(-R1 * t/L1) d_I2 / dt = (-V* / L2) *e^(-R2 * t/L2) V* = -d(phi)/dt L and R are properties of imperfect wire V* created by Faraday's Law
The frequency of the generator may be determined from the distance from one lightbulb illumination to the next. To do this: divide speed of light by the distance between illumination points. divide speed of light by the distance between illumination points, then divide by two.. multiply the distance by the speed of light. divide the distance between the points by the distance between the two wires of the lecher line, then multiply by speed of light.
divide speed of light by the distance between illumination points, then divide by two..
When you're building an electronic circuit, every component lead has inductance Proto-board capacitance from one line to the next is so low as to be negligible, that's how they're designed. The difference between audio frequencies and radio frequencies doesn't matter in circuit layout for the same type of circuit. Capacitors have no current rating because they don't dissipate power.
every component lead has inductance
A transmitted signal at 146 MHz Correct! has a phase number of approximately 3 rad/meter and in what is called the 2 meter band has a phase number of approximately 3 rad/meter and in what is called the 20 meter band has a phase number of approximately 2 meters per radian and in what is called the 2 meter band has a phase number of approximately 2 meters per radian and in what is called the 6 meter band
has a phase number of approximately 3 rad/meter and in what is called the 2 meter band
In the 210 lab (as opposed to in lecture, where an R is just an R and a ψψ is just a ψψ), a resistor has resistance, capacitance, and inductance. has a resistance and a power rating, that's it. has capacitance but no inductance has inductance but no capacitance.
has resistance, capacitance, and inductance.
The Poynting vector (spelled that way and not Pointing!) Correct! is mutually perpendicular to the electric and magnetic fields, pointing in the direction of wave propagation. indicates the flow of momentum from E&M waves is zero in lossless materials, since no power is transferred. cannot be computed at frequencies as high as that of visible light.
is mutually perpendicular to the electric and magnetic fields, pointing in the direction of wave propagation.
The electric field inside a metallic conductor such as copper or silver is zero when the conductor is in a static electric field. is always zero after the very short time required for the electrons to come to equilibrium. is kept at zero by continuous current. is zero because the electrons are always evenly distributed in the mass of the metal.
is zero when the conductor is in a static electric field.
Same setup as Question 1, except this time, ground the bucket, release the ground (take your finger away), and remove the ball from the bucket. The electrometer leaves move from closed to open. move from open to closed. get jammed because the charge is now of opposite polarity to before, and the electrometer only measures one polarity. will have different behavior for different metals in this experiment.
move from closed to open.
The Lorenz force law is not part of Maxwell's equations. It gives the force a charge experiences independently from electric fields and magnetic fields, and is computed as F⃗ =q(E⃗ +v⃗ ×B⃗ )F→=q(E→+v→×B→) Determine from this the SI units for B⃗ B→ (hint: Set E⃗ E→ to zero and examine the dimensions of the equation) newtons per meter per ampere volts per ampere volts per meter newtons per ampere
newtons per meter per ampere
The two-terminal device from Circuits called a capacitor stores energy in the form of an electric field between its plates. Has a net electric charge when it has nonzero voltage across its leads (i.e., when it's charged).. emits electromagnetic radiation when it has voltage across its leads but no current through them. Has semiconductor between its parallel plates.
stores energy in the form of an electric field between its plates.
The argument of the cosine here: s(t)=cos(2πft−kz)s(t)=cos(2πft−kz) indicates that the signal is varying in time and traveling in space (along the z axis) Has a phase term k has a negative frequency is changing in time no matter how an observer moves alongside it in space.
the signal is varying in time and traveling in space (along the z axis)
Waves far from their radiator (say, more than 10 wavelengths) are referred to as plane waves. This means that Correct! the wave's surface of constant phase is planar, or at least approximately so. the wave's electric field exists in a plane. the wave's magnetic field exists in a plane. the wave exists in the plane of the radiating antenna.
the wave's surface of constant phase is planar, or at least approximately so.
