Physics Questions on Magnetic Fields and Forces

A particle of charge q travels with velocity v in the same direction as the magnetic field, B. What is the force exerted on the charge?

• qvB, in the direction of the magnetic field

• qvB, in the direction opposite that of the magnetic field

• qvB, in a direction perpendicular to the magnetic field and the velocity

• Answer: zero

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A particle of positive charge q travels with velocity v towards the top of the screen. The magnetic field present, B, points rightward. The force

• points leftward.

• points into the screen.

• points toward the bottom of the screen.

• points out of the screen.

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Two parallel wires carry currents in the same direction and exert a force on each of magnitude F for each meter of length. One of the currents is now reversed in direction and doubled in magnitude. What are the resulting magnitude for the force per meter of length and its direction relative to the wires?

• 2F, attractive

• 2F, repulsive

• 4F, attractive

• 4F, repulsive

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A charged particle is moving perpendicular to a uniform magnetic field. The radius of its path can be decreased most by: decreasing the magnitude of the magnetic field; increasing its velocity; decreasing its velocity and increasing the magnitude of the magnetic field; increasing its velocity and decreasing the magnitude of the magnetic field.

• decreasing the magnitude of the magnetic field

• increasing its velocity

• decreasing its velocity and increasing the magnitude of the magnetic field

• increasing its velocity and decreasing the magnitude of the magnetic field

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Suppose a long, straight wire carries a current upwards. A short distance directly east of the wire, a negatively charged particle is moving upwards parallel to the wire. In what direction is the magnetic force on the particle?

• The force is upwards.

• The force is eastward.

• The force is westward.

• Either there is no force or its direction is not given among these choices.

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A charged particle moves in a straight line through a region of space. Which of the following answers must be true? (Assume any other fields are negligible.) The magnetic field

• has a magnitude of zero.

• has a zero component perpendicular to the particle's velocity.

• has a zero component parallel to the particle's velocity.

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The north-pole end of a bar magnet is held near a stationary positively charged piece of plastic. The plastic is

• attracted.

• repelled.

• unaffected by the magnet.

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As a charged particle moves freely in a circular path in the presence of a constant magnetic field applied perpendicular to the particle's velocity, the particle’s kinetic energy

• remains constant.

• increases.

• decreases.

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A square and a circular loop with the same area lie in the xy-plane, where there is a uniform magnetic field pointing at some angle q with respect to the positive z-direction. Each loop carries the same current, in the same direction. Which magnetic torque is larger?

• The square loop has a larger magnetic torque.

• The circular loop has a larger magnetic torque.

• Both loops have the same magnetic torque.

• Cannot be determined from the given information.

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Which of the following actions would double the magnitude of the magnetic force per unit length between two parallel current-carrying wires? Choose all correct answers.

(a) Double one of the currents.

(b) Double the distance between them.

(c) Reduce the distance between them by half.

(d) Double both currents.

• (a) Only

• (a) and (c)

• (b)

• (c)

• (d)

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