Question 1

Momentum is not conserved in an inelastic collision.

Accepted Answer

In an inelastic collision, total momentum is conserved, but kinetic energy is not conserved.

Question 2

Imagine a neutral atom near an infinite charged plane. Will the atom be attracted to the plate (A), repelled by it (B) or experience no force from it (C)?
(A) A
(B) B
(C) $\mathrm{C}$

Accepted Answer

The answer of Imagine a neutral atom near an infinite...

Question 3

A closed metal can (in static equilibrium) will perfectly shield a point in its interior from the static field of any external charged particle(s).

Accepted Answer

This is due to the electrostatic shielding effect, where the electric field inside a conductor in electrostatic equilibrium is zero, effectively shielding the interior from external static electric fields.

Question 4

Every fundamental charged particle has an antiparticle such that when the particle and antiparticle collide, they
annihilate each other to create two (uncharged) photons. What does conservation of charge imply about how a
particle's charge compares to that of its antiparticle? The particle's and antiparticle's charges
A. must be identical in sign and magnitude.
B. must have opposite signs and the same magnitude.
C. must have opposite signs, maybe different magnitudes.
D. must each be zero.
E. are not constrained at all by charge conservation.

Accepted Answer

The answer of Every fundamental charged particle has an antiparticle...

Question 5

Because of electrostatic polarization, a sufficiently charged rubber balloon will attract a neutral horizontal insulating plate strongly enough so that the balloon sticks firmly to the plate's bottom. If the horizontal plate is a conductor instead of an insulator, the charged balloon will
A) stick only if the plate is connected to the earth.
B) stick only if the plate is insulated from the earth.
C) stick in either case.
D) not stick at all.
E) repel the plate.

Accepted Answer

When a charged balloon approaches a conductive plate, whether the plate is earthed or not, the electric field of the balloon induces opposite charges on the near side of the plate and like charges on the far side. This induced charge distribution creates an attractive force between the balloon and the plate, allowing the balloon to stick to the plate in either case.

Question 6

Imagine that we have two identical metal spheres on insulating stands that we give the same negative charge. If we slowly bring the spheres closer and closer together, the charge per unit area $\boldsymbol{\sigma}$ on each sphere will at the point nearest the other sphere
A) become increasingly negative.
B) become less negative.
C) remain constant.

Accepted Answer

As the two negatively charged spheres are brought closer together, the repulsive forces between the like charges cause the charges to redistribute. This results in a decrease in charge density ($\sigma$) at the points nearest each other, making it less negative in those areas.

Question 7

Two metal spheres on insulating stands have the same positive charge $Q$. If we reduce the distance between the spheres' centers by a factor of two, the magnitude of the repulsive force between the spheres increases by
A) a bit less than a factor of two.
B) exactly a factor of two.
C) a bit more than a factor of two.
D) a bit less than a factor of four.
E) exactly a factor of four.
F) a bit more than a factor of four.

Accepted Answer

The answer of Two metal spheres on insulating stands have...

Question 8

Suppose we suspend a negatively charged ball like a pendulum inside a hollow but completely enclosing metal box. Assume that the ball hangs a few inches from the box's left vertical face. Imagine that we now bring a very strong positively charged object to a point just to the left of the box's left face, so that it is only a few inches away from the suspended ball (though the box's left face is between them). The ball will be deflected
A) strongly toward the external object.
B) strongly away from the external object.
C) weakly toward the external object.
D) weakly away from the external object.
E) not the least little bit.

Accepted Answer

The metal box acts as a Faraday cage, shielding the inside from external electric fields. Thus, the charged object outside the box has no effect on the suspended ball inside.

Question 9

Suppose the two plates of a parallel-plate capacitor have fixed charges $Q$ and $-Q$. If we double the gap between the plates, the:
-(a) potential difference between the plates,
A) quadruples.
B) doubles.
C) remains the same.
D) is reduced by a factor of two.
E) is reduced by a factor of four.
F) some other result (specify).

Accepted Answer

The potential difference (V) between the plates of a capacitor is directly proportional to the distance (d) between the plates when the charge (Q) on the plates and the permittivity of the medium between the plates are constant. Mathematically, V = Qd/(εA), where ε is the permittivity of the medium and A is the area of the plates. Doubling the distance between the plates doubles the potential difference.

Question 10

Suppose the two plates of a parallel-plate capacitor have fixed charges $Q$ and $-Q$. If we double the gap between the plates, the:
-(b) the capacitor's capacitance,
A) quadruples.
B) doubles.
C) remains the same.
D) is reduced by a factor of two.
E) is reduced by a factor of four.
F) some other result (specify).

Accepted Answer

The capacitance of a parallel-plate capacitor is inversely proportional to the distance between the plates. Therefore, if the gap is doubled, the capacitance is halved.

Question 11

Suppose the two plates of a parallel-plate capacitor have fixed charges $Q$ and $-Q$. If we double the gap between the plates, the:
-(c) the energy stored in the capacitor
A) quadruples.
B) doubles.
C) remains the same.
D) is reduced by a factor of two.
E) is reduced by a factor of four.
F) some other result (specify).

Accepted Answer

The energy stored in a capacitor is given by $ U = \frac{Q^2}{2C} $. The capacitance $ C $ of a parallel-plate capacitor is inversely proportional to the distance $ d $ between the plates, $ C = \frac{\varepsilon A}{d} $. Doubling the gap halves the capacitance, thus doubling the energy stored.

Question 12

Assume that the Drude model accurately describes a certain metal. If we increase the metal's temperature, its conductivity
A) should increase.
B) should decrease.
C) should remain the same.
D) changes in a way that depends on the specific metal.

Accepted Answer

According to the Drude model, increasing the temperature of a metal increases the phonon scattering of free electrons, which in turn reduces the metal's conductivity.

Question 13

If each copper atom in a wire were to contribute two conduction electrons instead of just one, its conductivity would
A) increase by a factor of 4 .
B) increase by a factor of 2.
C) remain the same.
D) decrease by a factor of 2 .
E) decrease by a factor of 4 .

Accepted Answer

The conductivity of a material is directly proportional to the number of charge carriers (in this case, conduction electrons) available. Doubling the number of conduction electrons would double the conductivity.

Question 14

Two wires have a nearly equal electron number density. Assume that the electric field in each wire points directly along the wire, that electric fields in the two wires have the same magnitude, but that the conductivity in wire $A$ is twice that in wire $B$. This means that the drift velocity in wire $A$ is $b$ times that in wire $B$, where $b$ is
A) 4 .
B) 2.
C) 1 .
D) $1 / 2$.
E) $1 / 4$.
F) Impossible to determine from the given information.

Accepted Answer

The drift velocity ($v_d$) in a conductor is directly proportional to the electric field ($E$) and the conductivity ($\sigma$) of the material, as given by the relation $v_d = \frac{E}{\rho}$ or $v_d = \sigma E$ where $\rho$ is the resistivity. Since the conductivity of wire $A$ is twice that of wire $B$ and the electric fields have the same magnitude, the drift velocity in wire $A$ will be twice that in wire $B$.

Question 15

Why do you think air is such a poor conductor com-pared to a metal (at normal field strengths)?
A) Air is much less dense than a metal.
B) Electrons can travel much farther in a metal before col-liding with an atom than they can in air.
C) Air molecules are much bigger than metal atoms.
D) Electrons move much more slowly in air.
E) Air normally has very few free charge carriers.

Accepted Answer

Air is a poor conductor because it has very few free charge carriers compared to metals, which have a high density of free electrons that facilitate conduction.

Question 16

Imagine that at a certain place in a river, water moves horizontally to the left. Consider an oriented sur-face facing upward. The flux of water velocity $\vec{v}$ through this surface is
A) positive.
B) negative.
C) zero.
D) Impossible to determine from the given information.

Accepted Answer

The flux of water velocity through a surface oriented upward (perpendicular to the direction of water flow) is zero because the direction of flow is horizontal and does not pass through the upward-facing surface.

Question 17

Imagine that at a certain place in a river, water moves horizontally to the left. Consider an oriented sur-face that faces in a direction that is $30^{\circ}$ upward of right. The flux of water velocity $\vec{v}$ through this surface is
A) positive.
B) negative.
C) zero.
D) Impossible to determine from the given information.

Accepted Answer

The flux is negative because the water is moving to the left, while the surface is oriented upward to the right. The angle between the velocity vector and the surface normal is greater than $90^{\circ}$, resulting in a negative dot product and thus a negative flux.

Question 18

Consider two vector fields $\vec{U}$ and $\vec{W}$. At every point in space, $\vec{U}$ points vertically upward, while $\vec{W}$ points radially away from some central point. But $|\vec{U}|=|\vec{W}|=$ a fixed constant everywhere. Assume that we calculate the flux of $\vec{U}$ and $\vec{W}$ through hemispherical surfaces having the same size and orientation, as shown below. Assume that the tile vectors on each surface point outward. Also assume that in the right-hand case, the central point from which the field $\vec{W}$ radiates is at the center of the flat plane at the bottom of the hemisphere. Do not assume that these fields necessarily represent particle flows.
  
How do the fluxes of these fields through the two identical hemispherical surfaces compare?
A. Field $\vec{U}$ has the larger flux.
B. Field $\vec{W}$ has the larger flux.
C. Both fields have the same flux.
D. The result depends on the surface's radius.
E. We need more information to compare the fluxes.

Accepted Answer

The answer of Consider two vector fields $\vec{U}$ and $\vec{W}$....

Question 19

Consider the segment of cylindrical metal wire shown below. Note that the drift speed of electrons is increasing in the wire as we move to the right.

Assume that charge cannot move off the wire. The total charge between the two cross-sectional areas $A$ and $B$ is
A) Increasing.
B) Decreasing.
C) Remaining the same.

Accepted Answer

The answer of Consider the segment of cylindrical metal wire...

Question 20

Consider the segment of cylindrical metal wire shown below. Assume that the drift speed of electrons is everywhere the same in the segment.

Note that area $B$ is tilted so that its nose vector makes an angle of $\theta<90^{\circ}$ with the drift velocity direction but still spans the wire's entire cross section. How do the current density fluxes through areas $A$ and $B$ compare?
A) The flux through $A$ is greater than the flux through $B$.
B) The flux through $B$ is greater than the flux through $A$.
C) The fluxes through $A$ and $B$ are the same.

Accepted Answer

The answer of Consider the segment of cylindrical metal wire...

Quiz 4: Electricity and Magnetism Are Unified

Momentum is not conserved in an inelastic collision.

Imagine a neutral atom near an infinite charged plane. Will the atom be attracted to the plate (A), repelled by it (B) or experience no force from it (C)? (A) A (B) B (C) $\mathrm{C}$

A closed metal can (in static equilibrium) will perfectly shield a point in its interior from the static field of any external charged particle(s).

Because of electrostatic polarization, a sufficiently charged rubber balloon will attract a neutral horizontal insulating plate strongly enough so that the balloon sticks firmly to the plate's bottom. If the horizontal plate is a conductor instead of an insulator, the charged balloon will

Imagine that we have two identical metal spheres on insulating stands that we give the same negative charge. If we slowly bring the spheres closer and closer together, the charge per unit area $\boldsymbol{\sigma}$ on each sphere will at the point nearest the other sphere

Two metal spheres on insulating stands have the same positive charge $Q$ . If we reduce the distance between the spheres' centers by a factor of two, the magnitude of the repulsive force between the spheres increases by

Suppose the two plates of a parallel-plate capacitor have fixed charges $Q$ and $-Q$ . If we double the gap between the plates, the: -(a) potential difference between the plates,

Suppose the two plates of a parallel-plate capacitor have fixed charges $Q$ and $-Q$ . If we double the gap between the plates, the: -(b) the capacitor's capacitance,

Suppose the two plates of a parallel-plate capacitor have fixed charges $Q$ and $-Q$ . If we double the gap between the plates, the: -(c) the energy stored in the capacitor

Assume that the Drude model accurately describes a certain metal. If we increase the metal's temperature, its conductivity

If each copper atom in a wire were to contribute two conduction electrons instead of just one, its conductivity would

Why do you think air is such a poor conductor com-pared to a metal (at normal field strengths) ?

Imagine that at a certain place in a river, water moves horizontally to the left. Consider an oriented sur-face facing upward. The flux of water velocity $\vec{v}$ through this surface is

Imagine that at a certain place in a river, water moves horizontally to the left. Consider an oriented sur-face that faces in a direction that is $30^{\circ}$ upward of right. The flux of water velocity $\vec{v}$ through this surface is

Consider the segment of cylindrical metal wire shown below. Note that the drift speed of electrons is increasing in the wire as we move to the right. Assume that charge cannot move off the wire. The total charge between the two cross-sectional areas $A$ and $B$ is

Quiz 4: Electricity and Magnetism Are Unified

Momentum is not conserved in an inelastic collision.

Imagine a neutral atom near an infinite charged plane. Will the atom be attracted to the plate (A), repelled by it (B) or experience no force from it (C)? (A) A (B) B (C) C\mathrm{C}C

A closed metal can (in static equilibrium) will perfectly shield a point in its interior from the static field of any external charged particle(s).

Because of electrostatic polarization, a sufficiently charged rubber balloon will attract a neutral horizontal insulating plate strongly enough so that the balloon sticks firmly to the plate's bottom. If the horizontal plate is a conductor instead of an insulator, the charged balloon will

Imagine that we have two identical metal spheres on insulating stands that we give the same negative charge. If we slowly bring the spheres closer and closer together, the charge per unit area σ\boldsymbol{\sigma}σ on each sphere will at the point nearest the other sphere

Two metal spheres on insulating stands have the same positive charge QQQ . If we reduce the distance between the spheres' centers by a factor of two, the magnitude of the repulsive force between the spheres increases by

Suppose the two plates of a parallel-plate capacitor have fixed charges QQQ and −Q-Q−Q . If we double the gap between the plates, the: -(a) potential difference between the plates,

Suppose the two plates of a parallel-plate capacitor have fixed charges QQQ and −Q-Q−Q . If we double the gap between the plates, the: -(b) the capacitor's capacitance,

Suppose the two plates of a parallel-plate capacitor have fixed charges QQQ and −Q-Q−Q . If we double the gap between the plates, the: -(c) the energy stored in the capacitor

Assume that the Drude model accurately describes a certain metal. If we increase the metal's temperature, its conductivity

If each copper atom in a wire were to contribute two conduction electrons instead of just one, its conductivity would

Why do you think air is such a poor conductor com-pared to a metal (at normal field strengths) ?

Imagine that at a certain place in a river, water moves horizontally to the left. Consider an oriented sur-face facing upward. The flux of water velocity v⃗\vec{v}v through this surface is

Imagine that at a certain place in a river, water moves horizontally to the left. Consider an oriented sur-face that faces in a direction that is 30∘30^{\circ}30∘ upward of right. The flux of water velocity v⃗\vec{v}v through this surface is

Consider the segment of cylindrical metal wire shown below. Note that the drift speed of electrons is increasing in the wire as we move to the right. Assume that charge cannot move off the wire. The total charge between the two cross-sectional areas AAA and BBB is

Imagine a neutral atom near an infinite charged plane. Will the atom be attracted to the plate (A), repelled by it (B) or experience no force from it (C)? (A) A (B) B (C) $\mathrm{C}$

Imagine that we have two identical metal spheres on insulating stands that we give the same negative charge. If we slowly bring the spheres closer and closer together, the charge per unit area $\boldsymbol{\sigma}$ on each sphere will at the point nearest the other sphere

Two metal spheres on insulating stands have the same positive charge $Q$ . If we reduce the distance between the spheres' centers by a factor of two, the magnitude of the repulsive force between the spheres increases by

Suppose the two plates of a parallel-plate capacitor have fixed charges $Q$ and $-Q$ . If we double the gap between the plates, the: -(a) potential difference between the plates,

Suppose the two plates of a parallel-plate capacitor have fixed charges $Q$ and $-Q$ . If we double the gap between the plates, the: -(b) the capacitor's capacitance,

Suppose the two plates of a parallel-plate capacitor have fixed charges $Q$ and $-Q$ . If we double the gap between the plates, the: -(c) the energy stored in the capacitor

Imagine that at a certain place in a river, water moves horizontally to the left. Consider an oriented sur-face facing upward. The flux of water velocity $\vec{v}$ through this surface is

Imagine that at a certain place in a river, water moves horizontally to the left. Consider an oriented sur-face that faces in a direction that is $30^{\circ}$ upward of right. The flux of water velocity $\vec{v}$ through this surface is

Consider the segment of cylindrical metal wire shown below. Note that the drift speed of electrons is increasing in the wire as we move to the right. Assume that charge cannot move off the wire. The total charge between the two cross-sectional areas $A$ and $B$ is