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AP Physics C: Electricity and Magnetism Practice Quiz: Electric Current

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

Test your understanding with short quizzes. This quiz has 10 questions to check your progress.

Question 1 of 10

Which of the following physical quantities is defined as the rate at which electric charge passes through a cross-sectional area of a conductor?

All Questions (10)

Which of the following physical quantities is defined as the rate at which electric charge passes through a cross-sectional area of a conductor?

A) Electric Potential Difference

B) Electromotive Force

C) Electric Current

D) Current Density

Correct Answer: C

The provided content explicitly states, 'Current is the rate at which charge passes through a cross-sectional area of a wire.' The relevant equation is I = dq/dt.

According to the provided information, what is the primary cause for the movement of electric charges within a circuit?

A) The drift velocity of the charge carriers

B) The density of the charge carriers

C) An electric potential difference

D) The cross-sectional area of the wire

Correct Answer: C

The content states, 'Electric charge moves in a circuit in response to an electric potential difference, sometimes referred to as electromotive force, or emf (E).'

In the equation for current density, J = nq(v_d), what does the term 'n' represent?

A) The total number of charges in the conductor

B) The net charge of the conductor

C) The number of charge carriers per unit volume

D) The number of newtons of force on the charges

Correct Answer: C

The content states that current density is related to the motion of charge carriers. In the standard equation J = nq(v_d), 'n' represents the charge carrier density, which is the number of mobile charge carriers per unit volume within the conductor.

If a total charge of 30 Coulombs passes through a wire's cross-section in 6 seconds at a constant rate, what is the electric current in the wire?

A) 0.2 A

B) 5 A

C) 36 A

D) 180 A

Correct Answer: B

Using the definition of current, I = dq/dt. For a constant rate of flow, this can be calculated as I = Δq / Δt. Given Δq = 30 C and Δt = 6 s, the current is I = 30 C / 6 s = 5 A.

The term electromotive force, or emf (E), is another name for which of the following quantities that causes charge to move?

A) Current density

B) Drift velocity

C) Electric potential difference

D) Charge carrier density

Correct Answer: C

The provided text explicitly states that electric charge moves in response to an 'electric potential difference, sometimes referred to as electromotive force, or emf (E).'

In a specific conductor, the drift velocity of the charge carriers is doubled, and the number of charge carriers per unit volume is also doubled. How does the new current density magnitude compare to the original?

A) It remains the same.

B) It is doubled.

C) It is quadrupled.

D) It is halved.

Correct Answer: C

Current density is given by J = nq(v_d). If 'n' is doubled (2n) and 'v_d' is doubled (2v_d), the new current density J' will be J' = (2n)q(2v_d) = 4(nq(v_d)) = 4J. The current density is quadrupled.

Based on the definitional equation I = dq/dt, the unit of electric current, the Ampere (A), is equivalent to which combination of fundamental units?

A) Coulomb per second (C/s)

B) Joule per Coulomb (J/C)

C) Coulomb-second (C·s)

D) Volt per meter (V/m)

Correct Answer: A

The equation I = dq/dt defines current as charge (dq) per unit time (dt). The unit for charge is the Coulomb (C) and the unit for time is the second (s). Therefore, the unit for current is Coulombs per second (C/s).

The equation for current density is given as a vector equation: J = nq(v_d). What does the vector nature of this equation imply about the direction of current density?

A) It is always perpendicular to the direction of charge movement.

B) It is a scalar quantity and has no direction.

C) It is in the same direction as the drift velocity for positive charge carriers.

D) It is always in the opposite direction of the drift velocity.

Correct Answer: C

The vector equation indicates that the direction of the current density vector J is determined by the direction of the drift velocity vector v_d. Since n and q (for positive charges) are scalars, J points in the same direction as v_d. By convention, current is the direction of positive charge flow.

Two wires are made of the same material and are subjected to the same electric potential difference. Wire A has twice the drift velocity of the charge carriers as Wire B. Assuming the charge (q) and charge carrier density (n) are the same, how does the current density in Wire A (J_A) compare to that in Wire B (J_B)?

A) J_A = 1/2 J_B

B) J_A = J_B

C) J_A = 2 J_B

D) J_A = 4 J_B

Correct Answer: C

Current density is given by J = nq(v_d). Since n and q are the same for both wires, the current density is directly proportional to the drift velocity (v_d). If Wire A has twice the drift velocity of Wire B (v_dA = 2v_dB), then its current density will also be twice as large: J_A = nq(v_dA) = nq(2v_dB) = 2(nq(v_dB)) = 2J_B.

Which statement best distinguishes electric current (I) from current density (J)?

A) Current is a vector describing charge flow at a point, while current density is a scalar describing total flow.

B) Current is a scalar quantity representing the total rate of charge flow through an area, while current density is a vector describing the flow per unit area at a point.

C) Both are identical concepts, but current is used for circuits and current density is used for free space.

D) Current is caused by an electromotive force, while current density is independent of any external forces.

Correct Answer: B

Current (I = dq/dt) is a scalar that measures the total rate of charge passing through a cross-sectional area. Current density (J = nq(v_d)) is a vector that describes the concentration of current, or the flow of charge per unit area, at a specific point within a conductor. Current is a macroscopic property, while current density is a microscopic, local property.