3rd Quarter Test MCQs

A very small, isolated sphere with charge +Q exists in an empty region of space. A second very small sphere is moved from far away to a short distance from the first sphere The second sphere has a charge of +2.0 x 10-9 C. As it is moved closer to the first sphere at a constant speed, the second sphere passes through the circular equipotential lines due to the first sphere. Two of these lines are separated by a distance of 0.020 m and have potentials of 100 V and 150 V. What is the magnitude of the average force needed to move the second sphere between the two equipotential lines?

5.0 x 10-6 N

The magnetic flux for each turn of a 30-turn coil increases from zero to 0.60 T∙m2 in 3 s. The average emf induced in the coil is

6V

A circular wire loop is placed near a long, straight, current-carrying wire in which the current is either increasing or decreasing. In which of the following situations is the induced current in the loop counterclockwise?

I decreasing to the left above the circle

In the circuit above, the current through the battery is 15A . Which of the following correctly describes the currents I into and out of point P ? (I1 is connected to battery and I2,I3,I4 are parallel)

I1= 15A (same as battery) I2=I3=I4 = 5 (15/3)

A 100-turn wire coil has a resistance of 400 Ω and a cross-sectional area of 0.01 m2. The coil is placed in a magnetic field directed parallel to the axis of the coil. If the field changes at a rate of -0.4 T/s, what is the current induced in the coil?

1*10^-3 A

A simple circuit has a 12.0V battery and three resistors of 6.0Ω connected in parallel. If the current through the battery is 4.0A , what is the internal resistance of the battery?

1.0Ω

The graph shows the electric potential along the x−axis in a region of space. What is the magnitude of the average electric field between x=2m and x=4m?

30V/m find slope of line

A circuit containing only a lightbulb is placed in a uniform magnetic field B⃗ �→ directed into the page, as shown above on the left. The variation of the magnetic field as a function of time is shown in the graph. During which of the following intervals will the bulb be brightest?

2-4s (on graph the negative sloped line)

The horizontal segment of wire of length 50 cm and mass 0.010 kg shown is connected to a circuit (not shown) and carries a current of 0.50 A to the right. The wire is released from rest in a uniform magnetic field of magnitude 0.50 T. The force diagram for the wire shows the directions of the forces but not their magnitudes. (force diagram - Fb pointing up, mg pointing down) What is the magnitude of the acceleration of the wire?

2.5 m/s2 Fb-mg=ma IℓB−mg=ma a = (IℓB-mg)/m a = ((0.5A)(0.5m)(0.5T) - (10)(.01))/.01

The circuit shown above contains a capacitor, a battery, and four lightbulbs, A, B. C. D, with the same resistance. The circuit has been connected for a long time. The capacitor is now removed from the circuit and replaced with a connecting wire. Which bulbs are dimmer at equilibrium in the second circuit compared to the first circuit?

B when capacitor on, C and D have no brightness, with capacitor off, B splits its charge with them

The figure above on the left shows two particles of equal and opposite charge that form an electric dipole. The figure on the right shows a bar magnet, which is a magnetic dipole. At which corresponding pair of labeled points do the associated fields (electric or magnetic) point in opposite directions?

B1 and B2 ( the center of each) For the charged particles, the field points from the positive to the negative charge. The magnetic field lines for the magnet are continuous and point from the south to the north pole.

A rectangular loop of wire and a resistor are attached to a cart by an insulating rod. The cart is initially moving to the right along a track with negligible friction at constant speed v0. There is a region of uniform magnetic field with magnitude B0 perpendicular to the plane of the loop and directed into the page as shown above. Current flowing through the loop of wire generates a region of magnetic field Bloop within the wire loop. Which of the following statements describes the changes, if any, in Bloop from the instant the center of the cart is over point A until the instant the center of the cart is over point C ?

Bloop is initially directed out of the page when the center of the cart is over position A, is zero when it is over position B, and is directed into the page when the center of the cart is over position C.

Students perform an experiment using the setup shown in the figure. A sphere with uniform charge distribution on an insulating stand is placed a distance from parallel uncharged conducting plates separated by a fixed distance d. Points A and B are labeled. Which of the following best explains how and why the charges will distribute in the plates?

C The sphere will induce charge separation in the right plate, which will in turn induce charge separation in the left plate, and the net charges of the plates will remain zero. (There will be charge separation in both plates, and because both plates are isolated and initially uncharged, they will remain uncharged.)

If the distance between the capacitor plates is halved while the charge on the capacitor remains the same, which of the following is true?

C will be doubled and V will be halved.

A circular conducting loop is in a region of magnetic field B directed out of the page, as shown above. The magnitude of the magnetic field is decreasing. The direction of the induced current in the loop is

CCW

A student has an isolated system of charged particles. The student wants to calculate the change ΔUE in the electric potential energy of the system between two instants in time but cannot do that directly, since the student does not have any information about the magnitude of the charges. Which of the following correctly describes a mathematical routine, if any, that can be used to determine ΔUE? Gravitational effects are negligible.

Calculate the change ΔK in the total kinetic energy of the particles. This will be equal to −ΔUE− The total energy of the system is constant because it isolated. So, any change in kinetic energy will correspond to an equal and opposite change in the electric potential energy.

A student wants to investigate how different values of capacitance affect the potential difference across capacitors in series. Which of the following circuits will allow the student to do this?

Capacitors are in series, voltmeter parallel to capacitor

Four horizontal wires are arranged on vertical wooden poles, as shown in the figure above. The wires are equally spaced and have equal currents I in the directions indicated in the figure. The dots represent four charged dust particles moving in the plane of the wires. The sign of the charge on each particle and its direction of motion at a particular instant are shown. ( I points left -dust moving right (A) I points left + dust moving left (B) I points right - dust moving up (C) I points right + dust moves left (D) Which of the dust particles has a magnetic force exerted on it in the downward direction at this instant?

D

The student wants to experimentally determine the effect of plate area on capacitance. The student connects each capacitor in the circuit shown above. Which of the following indicates quantities to be measured that will provide useful information and explains why the information is useful?

D Potential difference across the capacitor and charge on it a short time after the switch is closed, because that will allow the capacitance to be determined.

A loop of wire is connected to an ammeter, as shown above, and held in place. The ammeter has a scale with zero in the middle and a needle that moves away from zero in either direction to measure the magnitude and direction of any current in the loop. A bar magnet is held above the center of the loop and released from rest. Which of the following best describes the action of the ammeter needle as the magnet falls through the loop?

D The ammeter needle deflects in one direction as the north end of the magnet approaches the loop, returns to zero when the center of the magnet is in the loop, and deflects in the other direction when the south end of the magnet is moving away from the loop.

The figure above shows a long, straight wire that has a steady current I in the +y-direction. A small object with charge +q hangs from a thread near the wire. A student wants to investigate the magnetic force on the object due to the current but is not able to observe or measure changes in the tension in the string. Of the following actions that the student can take, which will allow the student to observe a reaction of the object due to the magnetic force on it?

Moving the object in the +y-direction

A student is performing an experiment in which protons with a horizontal speed of 106 m/s enter a region with a uniform magnetic field of magnitude 10−2 T, as shown above. The student observes the deflection of the protons by the magnetic field, and in analyzing the data ignores the effects of gravity on the protons. Does the student need to revise the analysis? Why or why not?

No, because the force of gravity on the protons is negligible compared to the magnetic force.

A student takes a capacitor of known capacitance and inserts a material of unknown dielectric constant between the plates. The student connects the capacitor in the circuit shown above and records the current in the circuit and the potential difference across the capacitor immediately after the switch is closed. Will the measurements allow the student to determine the dielectric constant, and why or why not?

No. Since V=Q/C, information about potential difference across the capacitor and charge on the capacitor at some point in time is needed to determine the new capacitance.

A student plans to determine the resistivity of a specific type of metal. Which should be kept constant?

Temperature because resistivity changes with temperature

A student wants to determine the direction of the magnetic field close to one particular straight, horizontal section of wire in the circuit. The capacitor is removed from the circuit so there is a steady current in the wire. The student uses a device that emits a visible beam of electrons and plans to observe the initial deflection of the beam to determine the direction of the magnetic field. How should the student position the beam of electrons so that the beam is immediately deflected by magnetic field of the wire?

The beam should be horizontal and positioned parallel to and next to the horizontal wire. (Since the magnetic field that causes the deflection forms concentric circles around the wire, the electrons in the beam will move perpendicular to the magnetic field and a deflection will occur.)

A student has a nonideal 10 V10 V battery, some resistors, a voltmeter, and an ohmmeter and wants to determine the internal resistance of the battery and whether the resistors are ohmic. Which of the following indicates what can be concluded about the battery and resistors?

The internal resistance of the battery is about 1Ω, and the ohmic nature of the resistors cannot be confirmed using only one potential difference measurement each.

An unmagnetized iron bar has a pivot through its center. The iron bar is held in place as the south pole of a bar magnet is brought near one end of the iron bar, as shown in the figure. If the iron bar is released so it is free to rotate about the pivot, in which direction, if any, will it rotate, and why?

The iron bar will rotate clockwise, because the magnetic domains in the iron bar will become aligned with the magnetic field from the bar magnet and the iron bar will behave like a bar magnet with a north pole on the left.

Two wires in the same circuit are very close to each other and oriented parallel to one another, with currents traveling in the same direction. Each wire exerts a force on the other wire. Which of the following correctly describes the fundamental cause of these macroscopic forces?

The moving charges in each wire create magnetic fields that exert forces on the moving charges in the other wire.

The figure above shows a bar magnet with two metal paper clips hanging from it. Which of the following explains why the bottom paper clip is attracted to the top paper clip? (south of mag facing up, clips attached to north side facing down)

The top paper clip's magnetic domains are temporarily aligned, making the clip magnetic while it touches the bar magnet.

The figure above shows a metal bar that is supported by two sections of a fixed, conducting U-shaped horizontal rail. The rod and rail are located in a region of magnetic field B directed into the page. The bar is moving to the right at a constant speed. Which of the following is true of the horizontal forces acting on the bar?

There is both a magnetic force and an external applied force.

A student is given a box of identical lightbulbs and is asked to design a procedure to determine if the lightbulbs are ohmic or nonohmic. The student plans to connect the circuit shown above and use the meters to measure a potential difference and current. Which of the following is the best procedure to use to determine if the lightbulbs are ohmic or nonohmic?

Use one lightbulb and take measurements for a number of different potential difference settings on the power source.

If Q is doubled and C is not changed, how are V (voltage) and U (energy) of capacitor affected?

V - doubled U- Quadrupled

To estimate the resistivity of the material using only the slope of a graph of the data, which of the following should be graphed as a function of L/A?

V/I (ur french, remember ur french side ;)

A positively charged particle is moving horizontally when it enters the uniform electric field between two parallel charged plates, as shown in the figure above. Which of the following correctly shows the x- and y-components of the velocity v as a function of time t ?

Vx is postive side horizontal line Vy is diagonal line with negative slope from 0.

A sphere with net charge +q+� hangs vertically from a spring scale, as shown above. There is a vertical wire to the left of the sphere. The current in the wire can be switched on or off. Students want to determine the direction of the current in the wire when it is switched on using only the apparatus shown above. Which of the following measurements could be used to determine the direction of the current?

With the current on, move the apparatus with the charged sphere toward the wire and look for any change in the spring scale reading.

The table above shows experimental data for the capacitance C of a parallel-plate capacitor as a function of the plate separation S. The students who took the data neglected to record their units. Do the data support the accepted relationship between capacitance and plate separation? Why or why not?

Yes - data shows C is proportional to 1/S (if seperation is halved - capacitance is doubled)

A student connects the circuit shown above and measures the values shown for three currents. The student indicates that each value has a measurement error of ±0.2 A. Can the information be used as evidence for conservation of charge, and why or why not?

Yes, because the sum of the currents in the two branches equals the current through the battery, within measurement error.

A very small, isolated sphere with charge +Q exists in an empty region of space. A second very small sphere is moved from far away to a short distance from the first sphere In the figures below, the central dot represents the first sphere. The dashed circles are isolines with the same potential difference between adjacent circles. Which of the figures best represents the isolines and electric field vectors around the first sphere while the second sphere is still far away?

all arrows facing outward, circles are closer together near center

A negatively charged sphere of mass ms swings at the end of a light string in a magnetic field directed into the plane of the page. The string makes an angle θ with the vertical, as shown above left. The tension T in the string, the magnetic force FB on the sphere, and the weight W of the sphere are represented in the figure above on the right. Which of the following is a correct equation for the horizontal acceleration ax of the sphere at the position indicated in the figures above?

ax=(Tsinθ-FBsinθ)/ms the acceleration is equal to the vector sum of the horizontal forces divided by the mass

A student has a small disc of unmagnetized iron and places it close to the north pole of a magnet. In the figures below, the arrows represent the alignment of the magnetic fields of domains in the disc. Which of the figures best represents the alignment of the domains before and after the disc is placed near the magnet?

before - pointed in random directions after - all pointed down

The figure above shows a small compass C that is equidistant from the poles of two identical bar magnets. Which of the following best represents the direction that the needle of the compass will point?

diagonal, south end of compass facing north end of magnet and vice versa

The magnet shown above is initially far away from the conducting loop and is moved at constant speed toward and completely through the loop. If T is the time at which the magnet is halfway through the loop, which of the following graphs best represents the induced current I in the loop as a function of time t ?

graph is like sin graph positive curve to 0 then negative curve back to 0

Each of the figures below shows the path of a charged particle moving in the plane of the page in a magnetic field that is perpendicular to the page. If the mass, speed, and charge of the particles are the same, in which case does the field have the greatest magnitude?

half -circle in field (most curved answer)

A circular loop of wire in a uniform magnetic field rotates at a constant rotational speed about the axis shown. At time t = 0 the plane of the loop is parallel to the field with point P at the top as shown above left. At time t= t1 the loop has rotated 180° so that point P is at the bottom as shown above right. Which of the following graphs best represents the magnetic flux through the loop as a function of time t from 0 to t1 ?

half circle facing down

The bar magnet is removed, and the loop is placed in a region with a uniform constant magnetic field, as shown above. The loop then undergoes simple harmonic motion, moving up and down vertically. Which of the following statements correctly describes the resulting induced current, if any?

induced current is zero

An electron moves horizontally toward a charged parallel-plate capacitor, as shown in the figure above (positive on top). In which direction should a magnetic field be generated in the region of the capacitor so that the electron can continue traveling horizontally between the capacitor plates?

into the page (The electron will be attracted toward the positive charges and repelled by the negative charges, RHR - point thumb to positive side, fingers pointing into the page)

In an experiment, a sample of iron and a sample of glass are each initially in a region that does not contain a net magnetic field. The iron sample is placed and secured on the tray inside the loop, and the switch on the loop is then closed. The iron sample is left on the tray for a long time. The switch is then opened. In an identical experiment, the glass sample is placed and secured on the tray. The switch is closed and then reopened after a long time. Which of the following accurately represents the direction of the net magnetic field produced by each sample after the experiments have been conducted?

iron is gray box with xs glass is clear

What can be concluded about the dielectric constants κ� of the materials? distance of x is smallest, distance of Z is largest

kx<ky<kz

Two conducting wire loops move near a very long, straight conducting wire that carries a current I. When the loops are in the positions shown above, they are moving in the directions shown with the same constant speed v. Assume that the loops are far enough apart that they do not affect each other. Which of the following is true about the induced electric currents, if any,in the loops?

loop 1 - clockwise loop 2 - no current

A metal spring has its ends attached so that it forms a circle. It is placed in a uniform magnetic field, as shown above. Which of the following will NOT cause a current to be induced in the spring?

moving the spring parallel to the magnetic field

A student uses iron filings in the spaces between three bar magnets so that the student can create the sketch of the magnetic field lines around the magnets, as shown above. Which of the indicated poles in the sketch are the opposite of pole P4 ?

p2 and p5 (Magnetic field lines from P4 have a curved path into P2. This means that these two poles must be opposite. Field lines go from P4 to P3 and P6 to P5 but not P4 to P6 meaning P4=P6 and P3=P5)

Which Circuits create the same brightness in bulbs?

parallel and a straight loop circuit

A compass is placed above a current-carrying wire so that the compass needle completely lines up along the magnetic field created by the wire. How will the compass needle be aligned?

perpendicular to the wire

The gray rectangle in the figure above represents a region of uniform magnetic field directed out of the page. A square loop of wire of side s is in the plane of the page and is pulled at constant speed v through the field. Which of the following could show the current I in the loop as a function of the position of the right edge of the loop?

positive box until s, contant at 0 until 2s, negative box until 3s

A battery with emf 6.0 V and an internal resistance r is connected to a resistor of resistance R, as shown in Figure 1 above. The current in the circuit is 2.0 A. When an identical battery is added to the circuit in series, as shown in Figure 2, the current in the circuit is 3.0 A. Which of the following statements about the resistances in the circuit is true?

r and R are comparable in magnitude, but they are not equal.

The figure above shows a circuit containing four resistors, a battery, and two ammeters. The current in ammeter A1 is 1.0 A, and the current in ammeter A2 is 0.6 A. The two resistors labeled R are identical. How do you find the currents in R2 and in each of the two resistors labeled R?

subtract the amneters for the middle resistor, split the amneter for the parallel resistors

A particle of charge +Q moving with speed vo enters a region of constant magnetic field B directed into the page, as shown above. (moving to the right into field of X's) The initial direction and magnitude of the acceleration of the particle as it enters the magnetic field is toward the

top of page and proportional to both B and vo (RHR - point fingers into page, thumb is pointed up)

A square loop is located in a region containing a magnetic field of magnitude B. The following figures show five possible directions of the magnetic field relative to the loop. For which of the directions is the magnetic flux through the loop greatest?

up through plane B

If C is doubled and Q is not changed, how are V and U affected?

v - halved u- halved

A rigid circular loop of wire contains a lightweight battery and a switch. The loop is suspended from the ceiling over a desk by a light insulated string. Hanging within the loop on two other insulated strings is a plastic tray shaped like an arrow onto which objects may be placed and secured. The switch is initially open, as shown in the figure above, and the loop is initially at rest. There are initially no objects on the tray. When the switch is closed, current initially flows clockwise and the ring begins to turn. With the switch still closed, the ring eventually comes to rest. In what direction will the arrow of the tray be pointing when the ring comes to rest?

west the rotating ring will come to rest so that the magnetic field caused by current in the loop faces north, lining up with Earth's magnetic field. If the magnetic field inside the ring points north, then the arrow will point west.

A student is analyzing results from several experiments where electrons were projected at various orientations near wires that could carry current. Which of the following, where the heavy line is the wire, shows a path for the electrons that would correspond to an experiment where the electrons were projected parallel to a current-carrying wire?

wire is horizontal, curve upwards above the wire (Using the right-hand rule for current shows that the field forms a circle around the wire. The electrons will experience a force, since the field is perpendicular to their motion. the force will be perpendicular to the electrons' velocity, which will make the electrons move in a curved path)

The figure shows a proton p and an electron e that are at rest on a line in the plane of the page. They are between two large uncharged parallel plates (not shown). Assume the particles are far apart so that their mutual gravitational and electrostatic forces are negligible. A potential difference is now established between the plates, and as a result an electric field directed toward the top of the page is created. Which of the following describes the velocity of the center of mass of the particles before either of them hits one of the plates?

zero (the net force on proton-electron system is zero, so the center of mass does not move)

A uniform magnetic field B that is perpendicular to the plane of the page now passes through the loops, as shown above. The field is confined to a region of radius a, where a < b, and is changing at a constant rate. The induced emf in the wire loop of radius b is ε. What is the induced emf in the wire loop of radius 2b ?

ε