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AP Physics C: Electricity and Magnetism Practice Quiz: Induced Currents and Magnetic Forces

Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026

Test your understanding with short quizzes. This quiz has 9 questions to check your progress.

Question 1 of 9

What fundamental interaction is responsible for the force exerted on a conductor carrying an induced current while it is within an external magnetic field?

All Questions (9)

What fundamental interaction is responsible for the force exerted on a conductor carrying an induced current while it is within an external magnetic field?

A) The interaction between the external magnetic field and the moving charge carriers of the induced current.

B) The gravitational pull on the mass of the charge carriers.

C) The electrostatic repulsion between the conductor and the source of the magnetic field.

D) The nuclear strong force within the atoms of the conductor.

Correct Answer: A

The provided content states that 'the already-present magnetic field will exert a magnetic force on the moving charge carriers within the loop,' which are the constituents of the induced current.

A straight segment of a conducting loop has an induced current I and a length vector $\vec{l}$. It is situated in a uniform external magnetic field $\vec{B}$. Which equation correctly describes the magnetic force, $\vec{F}_{B}$, exerted on this segment?

A) $\vec{F}_{B}=I\vec{B}\times\vec{l}$

B) $\vec{F}_{B}=I\vec{l}\times\vec{B}$

C) $\vec{F}_{B}=q\vec{v}\times\vec{B}$

D) $\vec{F}_{B}=\vec{l}(I \cdot \vec{B})$

Correct Answer: B

The provided content explicitly gives the relevant equation for the magnetic force on a current-carrying conductor as $\vec{F}_{B}=I\vec{l}\times\vec{B}$.

A conducting loop is moved through a magnetic field, creating an induced current. The magnetic force that results from this induced current will:

A) Act in the direction of the loop's motion, causing it to accelerate.

B) Act to oppose the change in magnetic flux that created the current.

C) Be zero if the magnetic field is uniform.

D) Be parallel to the magnetic field lines.

Correct Answer: B

The content describes a force exerted on a conductor due to an induced current. This is a manifestation of Lenz's Law. The induced current creates a magnetic field and experiences a force that opposes the change in flux that induced it. This force will oppose the motion causing the change.

A conducting loop of mass *m* is moving through a region with a magnetic field, which induces a current and a resulting magnetic force $\vec{F}_{B}$. To determine the loop's acceleration, one would most directly apply:

A) Ohm's Law.

B) Gauss's Law for magnetism.

C) Newton's second law.

D) The law of universal gravitation.

Correct Answer: C

The content explicitly states that 'Newton’s second law can be applied to a conducting loop moving in a magnetic field as it experiences an induced emf.' Since the magnetic force $\vec{F}_{B}$ acts on the loop, Newton's second law ($\vec{F}_{net} = m\vec{a}$) is used to relate this force to the loop's motion.

A segment of a conductor is placed in a uniform magnetic field at a fixed angle. If the conditions are changed such that the induced current *I* in the conductor is doubled while its length *l* and the magnetic field *B* remain constant, how does the magnitude of the magnetic force $F_B$ change?

A) It is halved.

B) It remains the same.

C) It is doubled.

D) It is quadrupled.

Correct Answer: C

The magnitude of the force is given by the equation $F_B = IlB\sin\theta$. Since *l*, *B*, and the angle $\theta$ are constant, the force $F_B$ is directly proportional to the current *I*. Therefore, if the current is doubled, the force is also doubled.

For a straight conductor of length *l* carrying an induced current *I* in a uniform magnetic field *B*, the magnitude of the magnetic force is maximized when the angle between the conductor's length vector and the magnetic field vector is:

A) 0°

B) 45°

C) 90°

D) 180°

Correct Answer: C

The magnitude of the force from the cross product $\vec{F}_{B}=I\vec{l}\times\vec{B}$ is given by $F_B = IlB\sin\theta$, where $\theta$ is the angle between $\vec{l}$ and $\vec{B}$. The sine function has its maximum value of 1 when the angle is 90°. Therefore, the force is maximized when the conductor is perpendicular to the magnetic field.

A rectangular conducting loop enters a uniform magnetic field at a constant velocity. Which statement best describes the interaction as the loop enters the field?

A) An induced current is created, but no magnetic force acts on the loop.

B) A magnetic force acts on the loop, but no current is induced.

C) An induced current is created, which results in a magnetic force that opposes the loop's motion.

D) An induced current is created, which results in a magnetic force that accelerates the loop into the field.

Correct Answer: C

As the loop enters the field, the magnetic flux through it changes, inducing a current. The provided content states that this induced current, flowing within the external magnetic field, experiences a magnetic force. This force will oppose the change causing it—in this case, the motion of the loop into the field.

A conductive loop experiences an induced current. On one side of the loop, the current *I* flows vertically upward. This side is in a uniform external magnetic field $\vec{B}$ directed into the page. What is the direction of the magnetic force $\vec{F}_{B}$ on this side of the loop?

A) To the right

B) To the left

C) Upward

D) Downward

Correct Answer: B

Using the right-hand rule for the cross product in $\vec{F}_{B}=I\vec{l}\times\vec{B}$: point the fingers of your right hand in the direction of the current (upward, for $\vec{l}$), then curl them into the direction of the magnetic field (into the page). Your thumb will point to the left, indicating the direction of the force.

According to the equation $\vec{F}_{B}=I\vec{l}\times\vec{B}$, the direction of the magnetic force on a conductor with an induced current is determined by:

A) The algebraic product of the current, length, and magnetic field strength.

B) The direction of the induced current only.

C) The orientation of the magnetic field only.

D) The vector cross product of the conductor's length vector and the magnetic field vector.

Correct Answer: D

The equation $\vec{F}_{B}=I\vec{l}\times\vec{B}$ is a vector cross product. The direction of the resulting force vector $\vec{F}_{B}$ is perpendicular to the plane formed by vectors $\vec{l}$ and $\vec{B}$, as determined by the right-hand rule for cross products.