A current runs through a long straight wire which is oriented along the x-axis,
ID: 2017497 • Letter: A
Question
A current runs through a long straight wire which is oriented along the x-axis, and passes through the origin. Conventional
current runs in the +x direction. An electron located at , which is moving with velocity ,
experiences a magnetic force of magnitude due to the current in the wire.
A. What is the direction of the magnetic force on the electron? Briefly explain your reasoning.
B. What is the magnitude of the current in the wire? Show your work.
C. A compass is placed under the wire (the compass lies in the xz plane). On the diagram below, which is a top view of
the compass (you are at a location on the +y axis, looking toward the origin), draw the position of the compass needle.
Explanation / Answer
a)The wire creates a magnetic field that goes into the page at the bottom of the x-axis using the RHR. The velocity of the is upwards. Again, using the RHR, the electron experiences a force that is to the right.
b)The current in the wire is calculated from the equation for a magnetic field for a straight line.
B = UoI/2r
Therefore, we get I = B2r/Uo
c)Since the current is into the page at the xy plane. The compass will be pointing in a north direction and we can see that is towards the y plane.