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conantee
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« on: December 31, 2009, 01:58:56 PM »

29. A negative test charge is moving near a long straight wire in which there is a current. A force will act on the test charge in a direction parallel to the direction of the current if the motion of the charge is in a direction.
(A) toward the wire
(B) away from the wire
(C) the same as that of the current
(D) opposite to that of the current
(E) perpendicular to both the direction of the current and the direction toward the wire
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conantee
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« Reply #1 on: December 31, 2009, 02:06:42 PM »

We know the the magnetic force is \vec{F} = q \vec{v} \times \vec{B}

Now, suppose the current points upward. \vec{B} will swirl around the current by right hand rule.

If we want the force to be in the same direction as the current, which this upward as we suppose, then (C), (D) and (E) are wrong, since these directions of motion are not perpendicular to both force and magnetic field at the same time.

Now, using right hand rule (and don't forget that charge is negative), you should find that the correct direction is toward the wire.

The correct answer is (A). 38 out of 100 pepole got the right answer.
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