EXAMPLE
In the figure above, a magnetic field of T is applied locally to one part of an
electric circuit with a 5 resistor and a voltage of 30 V. The length of wire to which the
magnetic field is applied is 2 m. What is the magnetic force acting on that stretch of wire?We are only interested in the stretch of wire on the right, where the current flows in a
downward direction. The direction of current is perpendicular to the magnetic field,
which is directed into the page, so we know the magnetic force will have a magnitude of F
= IlB, and will be directed to the right.
We have been told the magnetic field strength and the length of the wire, but we need to
calculate the current in the wire. We know the circuit has a voltage of 30 V and a
resistance of 5 , so calculating the current is just a matter of applying Ohm’s Law:
Now that we know the current, we can simply plug numbers into the equation for the
force of a magnetic field on a current-carrying wire:
The Magnetic Field Due to a Current
So far we have discussed the effect a magnetic field has on a moving charge, but we have
not discussed the reverse: the fact that a moving charge, or current, can generate a
magnetic field. There’s no time like the present, so let’s get to it.
The magnetic field created by a single moving charge is actually quite complicated, and is
not covered by SAT II Physics. However, the magnetic field created by a long straight wire
carrying a current, I, is relatively simple, and is fair game for SAT II Physics. The
magnetic field strength is given by: