AP Physics C: Electricity and Magnetism Practice Quiz: Magnetism and Moving Charges

Correct Answer: B

The content explicitly states that 'A single moving charged object produces a magnetic field.' Stationary charges produce electric fields, but not magnetic fields.

Correct Answer: C

A proton is a charged object. All charged objects produce an electric field. Because it is a *moving* charged object, it also produces a magnetic field, as stated in the content.

Correct Answer: B

The magnetic force is given by $\vec{F}_{B}=q(\vec{v}\times\vec{B})$. The magnitude of the cross product is $|q|vB\sin\theta$, where $\theta$ is the angle between the velocity and the magnetic field. If the velocity is parallel to the field, $\theta=0^{\circ}$ or $\theta=180^{\circ}$, and $\sin\theta=0$, resulting in zero force.

Correct Answer: D

The magnetic force is defined by the cross product $\vec{F}_{B}=q(\vec{v}\times\vec{B})$. A key property of the cross product is that the resulting vector ($\vec{F}_{B}$) is always perpendicular to the plane formed by the two input vectors ($\vec{v}$ and $\vec{B}$).

Correct Answer: A

Using the right-hand rule for the cross product $\vec{F}_{B}=q(\vec{v}\times\vec{B})$: point your fingers in the direction of velocity (right), curl them into the direction of the magnetic field (into the page). Your thumb points in the direction of the force, which is upwards.

Correct Answer: B

The content states that 'a moving charged object will experience independent forces from each field.' This implies the principle of superposition, where the total force is the vector sum of the individual electric and magnetic forces.

Correct Answer: C

The magnitude of the magnetic force is given by $F_B = |q|vB\sin\theta$. Since the force is directly proportional to the velocity v, doubling the velocity while keeping q, B, and the angle constant will double the force.

Correct Answer: B

The magnitude of the force is $F_B = |q|vB\sin\theta$. Since an electron and a proton have charges of the same magnitude (|e|), and v, B, and $\theta$ are the same, the magnitude of the force is identical. However, because their charges have opposite signs (q is negative for the electron and positive for the proton), the direction of the force vector $\vec{F}_{B}=q(\vec{v}\times\vec{B})$ will be opposite for each.

Correct Answer: C

The magnitude of the magnetic force is $F_B = |q|vB\sin\theta$. The sine function has a maximum value of 1 when the angle $\theta$ is 90 degrees. Therefore, the force is maximum when the velocity is perpendicular to the magnetic field.

Correct Answer: C

If the velocity is constant, the acceleration is zero. By Newton's second law, the net force on the particle must be zero. The total force is the vector sum of the electric force ($\vec{F}_E$) and the magnetic force ($\vec{F}_B$). For the sum to be zero, the two forces must be equal in magnitude and point in opposite directions, such that $\vec{F}_E + \vec{F}_B = 0$.

Correct Answer: A

The magnetic force equation is $\vec{F}_{B}=q(\vec{v}\times\vec{B})$. If the object is stationary, its velocity $\vec{v}$ is zero. Therefore, the magnetic force is also zero. A magnetic field only exerts a force on *moving* charged objects.

Correct Answer: C

The force is directly proportional to the charge q, as seen in $\vec{F}_{B}=q(\vec{v}\times\vec{B})$. Changing the charge from q to -2q will multiply the force vector by -2. This doubles the magnitude and reverses the direction of the force.

Correct Answer: B

The total force is the sum of the electric and magnetic forces. The electric force is $\vec{F}_E = q\vec{E}$. Since the proton is charged, there is an electric force. The magnetic force is $\vec{F}_{B}=q(\vec{v}\times\vec{B})$. Because the velocity $\vec{v}$ is parallel to the magnetic field $\vec{B}$, their cross product is zero. Therefore, the magnetic force is zero, and only the electric force acts on the proton.

Correct Answer: C

The provided content covers both aspects: 'A single moving charged object produces a magnetic field' and 'A magnetic field will exert a force on a charged object moving within that field'. Therefore, a moving charge is both a source of and is influenced by magnetic fields.