Physics 20, 21, 22
How could you tell whether moving electrons in a certain region of space are being deflected by an electric field or by a magnetic field?
-If the moving electrons are changing speed as they are being deflected, then an electric field is present. -If the moving electrons are being deflected but they are not changing speed, then only a magnetic field is present.
In a particular region of space there is a uniform magnetic field B. Outside this region, B=0. Can you inject an electron from outside into the field perpendicularly so that it will move in a closed circular path in the field? What if the electron is injected near the center?
-No, the electron would travel in a path that is only half of a circle and at that point it would come back out of the magnetic field and head back in the direction it came. -Yes, if an electron were injected into the middle of a large region of space occupied by a perpendicular magnetic field, the electron could travel in a closed circular path.
Can you set a resting electron into motion with a magnetic field? With an electric field? Explain.
-No, you cannot set a resting electron into motion with a magnetic field; a magnetic field can only put a force on a moving charge. -You can set a resting electron into motion with an electric field. An electric field will put a force on any charged particle moving or not.
Explain why, exactly the lights may dim briefly when a refrigerator motor starts up. When an electric heater is turned on, the lights may stay dimmed as long as the heater is on. Explain the difference.
-There is a small back emf in the circuit which allows for a large amount of current to flow to the fridge. The large current overtaxes the source and diminishes the current to all devices on circuit. As the fridge speeds up to normal speed, the back emf increases to its normal level and current delivered to fridge is now limited to its usual amount. -A heater draws a large amount of current at all times. The source is continually overtaxed and the other devices on the same circuit remain dimmed all the while that the heater is operating.
Can EM waves travel through a perfect vacuum? Can sound waves?
-Yes em waves can because no medium is required -No sound waves cannot because a medium is needed to carry energy of mechanical wave
Discuss how cordless telephones make use of EM waves. What about cell phones?
-cordless phones use em waves when sending info back and forth between the phone and its base; usually very weak -cell phones use em waves when sending info back and forth between the phone and nearest tower; need to be much stronger
What would be the effect on B inside a long solenoid if A) the diameter of all the loops was doubled, B) the spacing between loops was doubled, C) the solenoid's length was doubled along with a doubling in the total number of loops?
A-Nothing would happen to B B-B would decrease by a factor of 2 C-Nothing would happen to B
A lost person may signal by flashing a flashlight on and off using morse code. this is actually a modulated em wave. is it am or fm? what is the frequency of the carrier?
AM wave; frequency is visible light aka 10^14 Hz.
An enclosed transformer has four wire leads coming from it. How could you determine the ratio of turns on the two coils without taking the transformer apart? How would you know which wires paired with which?
Apply a known ac input source voltage to one pair of leads and measure the output voltage across the other two. Vsource/Voutput=Nsource/Noutput which provides us with the ratio. Then you could use an ohmmeter to determine the pairs since the two source wires are in no way electrically connected to the two output wires. if resistance is small, two wires are a pair.
A metal bar, pivoted at one end, oscillates freely in the absence of a magnetic field; but in a magnetic field, its oscillations are quickly damped out. Explain
Areas of the metal bar experience a change in magnetic flux which induces eddy currents with the free conduction electrons in the metal bar which are then acted on by the magnetic field and the resulting force opposes the motion of the swinging metal bar. The opposing force acts on the bar no matter which direction it is swinging through the magnetic field, thus damping the motion during both the to and fro portions of the swing
If an aluminum sheet is held between the poles of a large bar magnet, it requires some force to pull it out of the magnetic field even though the sheet is not ferromagnetic and does not toouch the pole faces. explain.
Areas of the sheet that used to be in B = 0 regions start to gain magnetic flux. This changing flux will induce currents in the "free" conduction electrons of the aluminum to oppose the change. These eddy currents are then acted on by the magnetic field and the resulting force opposes the motion of the aluminum sheet. Thus, it requires some amount of force to remove the sheet from between the poles.
A bar magnet falling inside a vertical metal tube reaches a terminal velocity even if the tube is evacuated so that there is no air resistance. Explain
As a magnet falls through a metal tube, an increase in the magnetic flux is created in the areas ahead of it in the tube. This flux change induces a current to flow around the tube walls to create an opposing magnetic field in the tube (Lenz's law). This induced magnetic field pushes against the falling magnet and causes it to slow down. The opposing magnetic field cannot cause the magnet to actually come to a stop. Thus, the magnet reaches a state of equilibrium and falls at a constant terminal velocity.
Suppose you have three iron rods, two of which are magnetized but the third is not. How would you determine which two are the magnets without using any additional objects?
Put one end of one rod close to one end of another rod. Keep doing this until two ends repel each other and those two are the magnets.
in the electromagnetic spectrum, what type of EM wave would have a wavelength of 10^3 km? 1 km? 1 m? 1 cm? 1mm? 1um?
10^3 km-sub radio waves 1 km-radio waves 1 m-tv signals and microwaves 1 cm-microwaves and satellite tv signals 1 mm-microwaves and infrared waves 1 um-infrared waves
Describe electric and/or magnetic fields that surround a moving electric charge.
A moving electric charge creates both electric and magnetic fields around it. The electric field is radial. The magnetic field circles the charge in a plane that is perpendicular to the direction of motion. Both of these fields decrease in strength as you go farther away from the moving charge in any direction.
Does the emf of the battery affect the time needed for the LR circuit to reach A) a given fraction of its maximum possible current B) a given value of current?
A-No; independent B-Yes; dependent
A charged particle is moving in a circle under the influence of a uniform magnetic field. If an electric field that points in the same direction as the magnetic field is turned on, describe the path the charged particle will take.
The charged particle will now travel in a helix. For as long as the electric field stays on, the particle will move in a constantly elongating helix.
Two ions have the same mass, but one is singly ionized and the other is doubly ionized. How will their positions on the film of a mass spectrometer differ?
The doubly ionized ion will hit the film at half the radius as the singly ionized ion.
An electric field is applied perpendicular to a blood vessel and to a magnetic field. Explain how ions are caused to move. Do positive and negative ions feel a force in the same direction?
The electric field puts a force on the charged particles and causes them to accelerate in a vertical direction from rest. The magnetic field now also puts a force on the charged particles. Using right hand rule, both positive and negative particles experience a force in the same direction.
in what direction are the magnetic field lines surrounding a straight wire carrying a current that is moving directly away from you?
clockwise circles centered on the wire
Are the wavelengths of radio and television signals longer or shorter than those detectable by the human eye?
The wave lengths of radio and tv signals are much longer than visible light
The use of higher-voltage lines in homes-say 600 V or 1200 V-would reduce energy waste. Why are they not used.
They would be dangerous. The large potential different between wires and anything grounded would more easily cause electrical breakdown and much more sparking would occur. Increases possibility of short circuits and accidental electrocutions
Two long wires carrying equal currents I are at right angles to each other, but don't quite touch. Describe the magnetic force on exerts on the other.
Thus, the lower wire experiences a counter-clockwise torque about the vertical direction due to the magnetic forces from the upper wire. This torque is attempting to rotate the two wires so their currents are parallel.
How can you make a compass without using iron or other ferromagnetic material?
Use a small loop of current - carrying wire that is able to pivot freely along a vertical axis (the faces of the loop should be pointing horizontally). Use the right-hand rule to determine which face of the loop has a magnetic north pole pointing out of it. The north and magnetic south poles created by the loop will attempt to pivot to align with Earth's magnetic field. The north face of the loop will point toward the magnetic south pole of Earth (which is near the geographic North Pole).
What is the advantage of placing the insulated electric wires carrying ac close together or even twisted about each other?
Whatever magnetic fields are created by the changing current moving one way in one wire is approximately cancelled out by the magnetic field created by the current moving in the opposite direction in the second wire. The effective area of the current loop is decreased and the induced current is minimized.
When you connect two loudspeakers to the output of a stereo amplifier, should you be sure the lead-in wires are equal in length so that there will not be a time lag between speakers?
It is not necessary to make them the exact same length. Energy travels nearly at the speed of light so the difference in time between getting to the different speakers would be too small for your ears to detect
What is the difference between magnetic flux and magnetic field?
Magnetic flux is proportional to the total number of magnetic field lines passing through an enclosed loop area: (equation) and so the flux is proportional to the magnitude of the magnetic field. -different units -M field is a vector -M flux is a scalar
If a moving charged particle is deflected sideways in some region of space, can we conclude for certain that B does not equal 0 in that region? explain.
No, a moving charged particle can be deflected sideways with an electric field that points perpendicular to the direction of the velocity, even when the magnetic field in the region is zero.
Is sound an electromagnetic wave?
No, it isa mechanical (pressure) wave
When you flop a light switch on, does the light go on immediately?
No. the electrons in the filament wire need to move to light the bulb, and these electrons need to receive energy to begin to move so there is a delay but it is very small
An unmagnetized nail will not attract an unmagnetized paperclip. However, if one end of the nail is contact with a magnet, the other end will attract a paper clip. Explain
Once the nail is in contact with a magnet (let's say the north pole), though, many of the nail's domains will align in such a way that the end touching the magnet becomes a south pole, due to the strong attraction, and the opposite end of the nail then becomes a north pole. There are mainly north poles of nail domains close to the paper clip, which causes some of the domains in the paper clip to align in such a way that the end near the nail becomes a south pole, which is exactly the same procedure that the nail was "magnetized."
a compass needle is not always balanced parallel to the Earth's surface, but one end may dip downward. explain.
the earth's magnetic field is not always parallel to the surface of the earth-it may have a component perpindicular to the earths surface. the compass will tend to line up with the local direction of the magnetic field, so one end if the compass will dip downward.
two iron bars attract each other no matter which ends are placed close together. are both magnets? explain.
the bars cannot both be magnets or they would repel each other.
What would be the advantage, in Faraday's experiments, of using coils with many turns?
the emf and induced current are proportional to N, which makes it easier to experimentally measure those quantities.
Suppose you are holding a circular ring of wire and suddenly thrust a magnet, south pole first, away from you toward the center of the circle. Is a current induced in the wire? Is a current induced when the magnet is held steady within the ring? Is a current induced when you withdraw the magnet?
1. Yes, current is induced in ring when you bring south pole towards ring (changing magnetic flux); direction is clockwise. 2. No a current is not induced in the ring when the magnet is held steady with the ring. 3. Yes, a current is induced in the ring when you withdraw the magnet (changing magnetic flux); counter clockwise.
A horizontal current-carrying wire, free to move in Earth's gravitation field, is suspended directly above a second, parallel, current-carrying wire. A) in what direction is the current in the lower wire? B) Can the upper wire be held in stable equilibrium due to the magnetic force of the lower wire?
A-The current in the lower wire is pointing in the opposite direction of the current in the upper wire The current of the upper wire must be toward the south so that the magnetic force created on it is upward. B-Yes, the upper wire is vertically stable.
In an LRC circuit can the rms voltage across A) an inductor B) a capacitor be greater than the rms voltage of the ac source?
A-Yes B-Yes The different voltages are out of phase with each other. At any given instant, the voltage across either the inductor or capacitor could be negative (for example), allowing for a very large positive voltage on the other device
Suppose you are looking along a line through the centers of two circular wire loops, one behind the other. A battery is suddenly connected to the front loop, establishing a clockwise current. A) will a current be induced in the second loop? B) if so, when does this current start? C) when does it stop? D) in what direction is this current? E) is there a force between the two loops? F) if so, in what direction?
A-Yes, once connected to the front loop, it will create increasing magnetic field that points away from you. The second loop will try to oppose this increase in magnetic flux through it and an emf and current will be induced B-Immediately upon connection of the battery to front loop C-As soon as the current in the front loop becomes steady D-Counterclockwise E-Yes. Both loops carry currents and create magnetic fields while the current in the front loop is increasing from the battery F-Force between two loops will repel each other. the front creates magnetic field toward the second loop. Second loop opposes the magnetic field and the induced current creates a magnetic field pointing toward the front loop. The two fields act like north poles.
Describe briefly how the frequency of the source emf affects the impedance of A) a pure resistance B) a pure capacitance C) a oure inductance D) an LRC circuit near resonance E) an LRC circuit far from resonance
A-does not affect B-varies inversely with the frequency of the source emf C-varies directly with the frequency of the source emf D-if frequency is set at resonance exactly, the LRC circuit's impedance is very small. Impedance increases rapidly as the source frequency is either increased or decreased a small amount from resonance E-if source frequency is higher than resonance, impedance is directly proportional to frequency of source emf; if source frequency is much lower than resonance, impedance is inversely proportional to frequency of source emf
How would you arrange two flat circular coils so that their mutual inductance was A) greatest B) least
A-place them face to face and very close; almost all magnetic flux from one coil goes to other B-place them with their faces at right angles; almost no magnetic flux is transferred
It has been proposed that eddy currents be used to help sort solid waste for recycling. The waste is first ground into tiny pieces and iron removed with a dc magnet. The waste then is allowed to slide down an incline over permanent magnets. How will this aid in the separating of nonferrous metals from nonmetallic materials?
As the pieces of conducting materials slide down the incline past permanent magnets, the "free" conduction electrons in the pieces are forced to create eddy currents. These eddy currents are not created in the non-metallic pieces and will experience a magnetic force opposing the original motion of the pieces of metal. Thus, the metallic pieces will be slowed as they slide down the incline, while the non-metallic pieces will continue to accelerate the entire time. Since the metallic pieces have been slowed, they will basically just drop off of the end of the incline and you would put a "metal bin" directly below the end to catch the metal pieces. The non-metallic pieces, will come off the end of the incline with a high speed and overshoot the "metal bin" and be caught in a "non-metal bin" sitting farther away from the end.
In situations where a small signal must travel over a distance, a "shielded cable" is used in which the signal wire is surrounded by an insulator and then enclosed by a cylindrical conductor carrying the return current. Why is a shield necessary?
As the signal in the wire varies in time, it creates changing magnetic fields that emanate from the wire. If there were external magnetic fields nearby, these could interact with the signal wire's magnetic fields and cause interference or noise in the signal. With the "shield" in place, the net current in the wire, would be zero and, thus, the wire would not emanate a magnetic field. the interference and noise in the signal would be reduced.
A transformer designed for a 120-v ac input will ofter "burn out" if connected to a 120-v dc source. Explain
When 120 V dc is applied to the transformer, there is no induced back emf that would usually occur with 120 V ac. 120 v dc encounters much less resistance which would cause too much current. This would overheat the coils and could melt the insulation and burn out or short out the transformer
Will an eddy current brake work on a copper or aluminum wheel, or must the wheel be ferromagnetic?
Yes eddy current brakes will work on metallic wheels. The external magnetic field of the brake needs to interact with "free" conduction electrons in the metal wheels to make it work.
Can two radio or tv stations broadcast on the same carrier frequency?
Yes, but if both signals are of similar strength in the same locality, the signals will be "scrambled"
A charged particle moves in a straight line through a particular region of space. Could there be a nonzero magnetic field in this region? Give two possible situations.
Yes. 1. The charged particle is traveling in a direction parallel to the magnetic field lines. 2. The charged particle is traveling through "crossed" electric and magnetic fields in such a way that the electric force and the magnetic force are canceling each other out.
Why will either pole of a magnet attract an unmagnetized piece of iron?
Yes. The magnetic domains in the unmagnetized piece of iron are initially pointing in random directions. When the south pole of a strong external magnet is brought close to the random magnetic domains of the iron, many of the domains will rotate slightly so that their north poles are closer to the external south pole. When the north pole of a strong external magnet is brought close to the random magnetic domains of the iron, many of the domains will rotate slightly so that their south poles are closer to the external north pole.
Can radio waves have the same frequencies as sound waves?
Yes. These em waves would have extremely long wavelengths
can an iron rod attract a magnet? can a magnet attract an iron rod? what must you consider to answer these questions?
a magnet can attract an iron bar, so by newtons 3 law, an iron bar can attract a magnet. consider that iron has domains that can be made slightly magnetic by an external magnetic field.
will a magnet attract any metallic object, or only those made if iron? why is this so?
a magnet will not attract just any metallic object-only ferromagnetic materials. the ferromagnetic materials contain domains that can temporarily align when a strong magnet is near. because of the alignment, the north pole of domain points to south pole of magnet, which creates the attraction
The carrier frequencies of fm broadcasts are much higher than am broadcasts. on the basis of what you learned about diffraction in chapter 11, explain why am signals can be detected more readily than fm signals behind low hills or buildings
diffraction effects are only evident when the size of the wavelength is larger than the size of the object. Buildings and hills are much larger than fm waves, so fm waves will not diffract around the buildings and hills and the fm signal will not be received. AM waves will diffract around them easily
a horsehie magnet is held vertically with the north pole on the left and south pole on the right. a wire passing between the poles, equidistant from them, carries a current directly away from you. in what direction is the force on the wire?
downward
the magnetic field due to current in wires in your home can affect a compass. discuss the effect in terms of currents.
due to the mass of the compass needle, its reaction to 60 hz (the normal current in a house circuit) will not be noticeable. a dc current in a single wire can affect the compass depending on the relative orientation of the wire and the compass, the magnitude of the current, and the distance from the wire to the compass. but a dc current being carried by twi close wires in opposite direction would not affect the compass
Describe how to make impedance in LRC circuit a minimum
make resistance very small and make the reactance of capacitor equal to the reactance of inductor
The electric field in an EM wave traveling north oscillates in an east-west place. Describe the direction of the magnetic field vector in this wave
the magnetic field must oscillate up and down; the electric and magnetic fields must all be perpendicular to each other
explain why a strong magnet held near a crt tv screen causes the picture to become distorted. also explain why the picture goes black when field is strongest.
the magnetic field puts a force on the moving electrons, which causes them to bend away from where they were supposed to hit the screen. when the picture goes black, all of the electrons that were supposed to hit that particular spot have been deflected by the strong field.
If a radio transmitter has a vertical antenna, should a receiver's antenna be vertical or horizontal to obtain best reception?
veritcal; the oscillating carrier electric field is up and down so antenna would pick up that signal better