PHYS 119 - Module Questions

This is a graph of the x-component of the electric field along the x-axis. E(x) = y = -2000x + 2000 for 0<x<1.0 E(x) = y = 0 for x>1.0 The potential is zero at the origin. What is the potential at x = 1 m? a) 0 V b) 1000 V c) -2000 V d) 2000 V e) -1000 V

e) -1000 V (As the potential energy decreases, the kinetic energy increases.)

The plates of a parallel plate capacitor with capacitance C carry a charge Q. What is the capacitance of the capacitor if the charge is increased to 4Q? a) C/2 b) C/4 c) 4C d) 2C e) C

e) C (The capacitance is determined by the geometry of the conductors, adding charge will change the voltage, but not the capacitance.)

Is any work done when moving a charge along a field line? a) No work is required to move a charge along a field line. b) Yes, the field does negative work when a charge moves in either direction along a field line. c) Yes, the field does negative work when a positive charge moves in the direction of the field, and positive work when this positive charge is moved in a direction opposing the field. d) Yes, the field does positive work when a charge moves in either direction along a field line. e) Yes, the field does positive work when a positive charge moves in the direction of the field, and negative work when this positive charge is moved in a direction opposing the field.

e) Yes, the field does positive work when a positive charge moves in the direction of the field, and negative work when this positive charge is moved in a direction opposing the field.

What is the smallest value that a dielectric constant can be? a) 0 b) 1 c) -infinity d) there is no lower limit

b) 1

Can equipotential lines ever cross? a) Yes b) No

b) No

A wire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance? a) It decreases by a factor of 4 b) It decreases by a factor of 2 c) it stays the same d) it increases by a factor of 2 e) it increases by a factor of 4

e) it increases by a factor of 4 (Keeping the volume constant means that if the length is doubled the area is halved. Since R=rho(L/A), this increases the resistance by a factor of four. )

A 3 μF capacitor is connected to a circuit in series with another 6 μF capacitor. The equivalent capacitance is: A. 9 μF B. 6 μF C. 3 μF D. 2 μF E. 1 μF

A. 9 μF (For capacitors in parallel, the capacitances add.)

The charge of sphere 2 is twice that of sphere 1. The force of 1 on 2 points to the right. Which vector below shows the force of 2 on 1? A. The force of 2 on 1 points to the left and has greater magnitude than the force of 1 on 2. B. The force of 2 on 1 points to the left and has the same magnitude as the force of 1 on 2. C. The force of 2 on 1 points to the left and has lesser magnitude than the force of 1 on 2. D. The force of 2 on 1 points to the right and has the same magnitude than the force of 1 on 2. E. The force of 2 on 1 points to the right and has greater magnitude than the force of 1 on 2.

B. The force of 2 on 1 points to the left and has the same magnitude as the force of 1 on 2.

4 charges are positioned in a square formation, the charge on the top right corner has a +2q charge and all the others a +q charge. Which is the direction of the net force on the charge at the lower left? A. The net force points straight to the left B. The net force points down and to the left C. The net force points straight down D. The net force points up and to the right E. The net force = 0.

B. The net force points down and to the left

A parallel plate capacitor with plates of area A and plate separation d is charged so that the potential difference between the plates is V. If the capacitor is then isolated and its plate separation is decreased to d/2, what happens to the potential between the plates? a) It is one half of its original value. b) It is twice its original value. c) It is increased by a factor of 4. d) It is increased by a factor of 8. e) It is unchanged.

a) It is one half of its original value. (The capacitance increases by a factor of 2 when the plate separation decreases by a factor of 2. Since the charge remains the same the potential must also decrease by a factor of 2 since V = q/C. )

Surface A has a radius R and the enclosed charges is Q. Surface B has a radius 2R and the enclosed charges is 2Q. Which spherical Gaussian surface has the larger electric flux? a) Surface B. b) Not enough information to tell. c) Surface A. d) They have the same flux.

a) Surface B. (Flux depends only on the enclosed charge, not the radius.)

Consider two isolated spherical conductors each having net charge Q. The spheres have radii a and b, where b>a . Which sphere has the higher potential? a) The sphere of radius a. b) The sphere of radius b. c) They have the same potential. d) More information is needed to answer.

a) The sphere of radius a (It requires more work to move a test charge in to a smaller sphere as opposed to a larger sphere with the same charge.)

What will happen if the positive and negative connections on the voltmeter are reversed? a) The voltage readings will simply appear as negative instead of positive. b) This will have no effect on the voltage readings. c) The voltmeter may malfunction.

a) The voltage readings will simply appear as negative instead of positive.

Can there be an electric field at a point where there is no charge? a) Yes b) No c) It depends on the situation/geometry.

a) Yes

All other things being equal, current will be larger in a wire that has a larger value of (I am looking for a quantity that describes how well a material conducts ): a) conductivity. b) resistivity. c) the coefficient of current d) net charge. e) potential.

a) conductivity.

Experimenter A uses a test charge q0 and experimenter B uses a test charge -2q0 to measure an electric field produced by stationary charges. A finds a field that is: a) the same in both magnitude and direction as the field found by B b) greater in magnitude than the field found by B c) less in magnitude than the field found by B d) opposite in direction to the field found by B e) either greater or less than the field found by B, depending on the accelerations of the test charges

a) the same in both magnitude and direction as the field found by B (The field is the force on the test charge divided by the magnitude of the test charge. It is independent of the magnitude and sign of the test charge.)

Two protons, A and B, are in an electric field. The electric field points to the right. Proton A is situated on a field line where the line are parallel. Proton B is situated on a field line where the line are branching out. Which proton has the larger acceleration? Neglect interactions between the protons. a) Proton B b) Proton A c) Both have the same acceleration

b) Proton A (The field is stronger where the field lines are closer together and weaker where the field lines are farther apart.)

Imagine two fixed charges on the x axis. Charge one is +q and is located to the left of charge two which is equal to -4q. Where on the axis (other than at an infinite distance) is there a point at which their net electric field is zero? a) Between the charges b) To the left of the charges c) To the right of the charges

b) To the left of the charges

Stray capacitance is: a) negligible in this lab, I don't have to worry about it b) something I can try to minimize with my experimental setup c) a systematic constant error I'll just have to deal with in my final analysis

b) something I can try to minimize with my experimental setup

Conduction electrons move to the right in a certain wire. This indicates that a) the current density and electric field both point right b) the current density and electric field both point left c) the current density points right and the electric field points left d) the current density points left and the electric field points right e) the current density points left but the direction of the electric field is unknown

b) the current density and electric field both point left (Conventional current moves in the direction of the electric field and electrons move in the opposite direction. )

A cylindrical Gaussian surface surrounds an infinite line of charge. r is the radius and L the length of the cylinder. The flux Φe through the two flat ends of the cylinder is: a) 2 × 2π rE. b) It will require an integration to find out. c) 0. d) 2 ×rLE. e) 2 × π r2 E.

c) 0.

An electron is pushed into an electric field where it acquires a 1-V electrical potential. Suppose instead that two electrons are pushed the same distance into the same electric field. What is the electrical potential of one of the electrons now? a) 0.25V b) 0.5 V c) 1 V d) 2 V e) 4 V

c) 1 V (The electrical potential of a charged particle is dependent on the position of the charged particle and not its charge.)

In the lab, reasonable step sizes to take for the distance between parallel plates would be: a) 1 cm b) 10 cm c) 1 mm

c) 1 mm

V(x) = y; V(0)=0 x(A) < x(B) The function grows parabolically. At which point is the electric field stronger? a) At x(B). b) There's not enough information to tell. c) At x(A). d) The field is the same strength at both.

c) At x(A). (|E| = the slope of the potential graph.)

Two protons, A and B, are next to an infinite plane of positive charge. Proton B is twice as far from the plane as proton A. Which proton has the larger acceleration? Neglect interactions between the protons. a) Proton B b) Proton A c) Both have the same acceleration

c) Both have the same acceleration

A charge +q, sits 1 cm from a second charge -3q. How does the magnitude of the force the first charge (+q) feels compare to the magnitude of the force the second charge (-3q) feels ? a) It's 2 times as large b) It's 1/3 as large c) It's the same magnitude d) Not enough information

c) It's the same magnitude

Capacitor B has one-half the capacitance of capacitor A. How does the charge on capacitor A compare to that on B when the two are connected in series to a battery for a long time? a) The charge on cap A is one-fourth the charge on cap B. b) The charge on cap A is one-half the charge on cap B. c) The charge on cap A is the same as the charge on cap B. d) The charge on cap A is twice the charge on cap B. e) The charge on cap A is four times the charge on cap B.

c) The charge on cap A is the same as the charge on cap B. (Capacitors in series have the same charge)

The plates of an isolated parallel plate capactitor with a capacitance C carry a charge Q. The plate separation is d. Initially, the space between the plates contains only air. Then, a Teflon (kappa=2.1) sheet of thickness 0.5d is inserted between, but not touching, the plates. How does the electric field and the stored energy of the capacitor change as a result of inserting the Teflon sheet? a) The electric field will decrease and the energy stored will increase. b) The electric field will increase and the energy stored will also increase. c) The electric field will decrease and the energy stored will decrease. d) The electric field will increase and the energy stored will decrease. e) The electric field will not change and the energy stored will increase.

c) The electric field will decrease and the energy stored will decrease. (The capacitor is isolated - this means the charge must stay constant.)

Why is no work done in moving a charge along an equipotential line? a) The force acting on the charged particle is parallel to the direction of motion. b) No force is acting on the charged particle. c) The force acting on the charged particle is perpendicular to the direction of motion. d) The work done is not zero.

c) The force acting on the charged particle is perpendicular to the direction of motion.

How can you tell from the pattern of electric field lines where the magnitude of the electric field is the greatest? The magnitude is greatest: a) where the density of electric field lines is least. b) where the electric field lines cross over one another. c) where the density of the electric field lines is greatest

c) where the density of the electric field lines is greatest

10C of charge are placed on a spherical conducting shell. A particle with a charge of -3C is placed at the center of the cavity. The net charge on the inner surface of the shell is: a) -7C b) -3C c) 0C d) +3C e) +7

d) +3C (Inside the conductor the E field is zero and a Gaussian surface in the conductor encloses no net charge. )

At a distance of one centimeter from an electron, the electric field strength has a value E. At what distance is the electric field strength equal to E/2? a) 3.2 cm b) 4.0 cm c) 2.0 cm d) 1.4 cm

d) 1.4 cm

Wire 2 is twice the length and twice the diameter of wire 1. What is the ratio R2 / R1 of their resistances? a) 1 b) 2 c) 1/4 d) 1/2 e) 4

d) 1/2 (R = ρL/A. L is increased by 2. A is proportional to the diameter squared, so A increases by 4)

Capacitor B has one-half the capacitance of capacitor A. How does the charge on capacitor A compare to that on B when the two are connected in parallel with a battery for a long time? a) The charge on cap A is one-fourth the charge on cap B. b) The charge on cap A is one-half the charge on cap B. c) The charge on cap A is the same as the charge on cap B. d) The charge on cap A is twice the charge on cap B. e) The charge on cap A is four times the charge on cap B.

d) The charge on cap A is twice the charge on cap B. (The capacitors are in parallel and must have the same voltage.)

A charged point particle is placed at the center of a spherical Gaussian surface. The electric flux is changed if: a) the sphere is replaced by a cube of the same volume b) the sphere is replaced by a cube of one- tenth the volume c) the point charge is moved off center (but still inside the original sphere) d) The point charge is moved to just outside the sphere e) a second point charge is placed just outside the sphere

d) The point charge is moved to just outside the sphere. (The charge must be within the Gaussian surface to affect the net electric flux through the surface. )

The size of the current in an electrical conductor is a function of which of the following? a) velocity of charge carriers b) conductor thickness c) density of charge carriers d) all of the above choices

d) all of the above choices

The outer surface of the cardboard center of a paper towel roll: a) is a possible Gaussian surface b) cannot be a Gaussian surface because it encloses no charge c) cannot be a Gaussian surface since it is an insulator d) cannot be a Gaussian surface because it is not a closed surface

d) cannot be a Gaussian surface because it is not a closed surface

A solid spherical conductor is given a net nonzero charge. The electrostatic potential of the conductor is: a) largest at the center b) largest on the surface c) largest somewhere between center and surface d) constant throughout the volume

d) constant throughout the volume (An excess charge on the surface of a conductor distributes itself so that all points inside or on the conductor are at the same potential. )

How can you tell where E=0? a) everywhere the electric potential (voltage) is instantaneously zero. b) everywhere the electric potential (voltage) is negative. c) whenever the electric potential (voltage) is increasing steadily. d) whenever the electric potential (voltage) is constant.

d) whenever the electric potential (voltage) is constant

Light from the sun allows a solar cell to move electrons from the positive to the negative terminal, doing 1.6×10−19J of work per electron. What is the emf of this solar cell?

emf = -1.0 V