Introduction to SAT II Physics

CHARGES MOVING IN A MAGNETIC FIELD create an electric field, just as charges
moving in an electric field create a magnetic field. This is called electromagnetic
induction. Induction provides the basis of everyday technology like transformers on
power lines and electric generators.
On average, SAT II Physics asks only one question about electromagnetic induction.
However, less than half of the test takers usually get this question right, so if you get the
hang of this material, you’ll be separating yourself from the crowd. On the whole, this
question will be qualitative, with only a minimum of calculation involved.

Motional Emf

Consider the bar in the figure below. It has length l and moves at speed v to the right in
magnetic field B, which is directed into the page.

The field exerts a magnetic force on the free electrons in the bar. That force is

: using the right-hand rule, you will find that the vector is directed

upward along the bar, but since electrons are negatively charged, the magnetic force
acting upon them is directed downward. As a result, electrons flow to the bottom of the
bar, and the bottom becomes negatively charged while the top becomes positively
charged.
The separation of charge in the rod creates an electric field within the bar in the
downward direction, since the top of the bar is positively charged and the bottom of the

bar is negatively charged. The force from the electric field, , pulls negative

charges upward while the force from the magnetic field pulls negative charges downward.
Initially, the magnetic field is much stronger than the electric field, but as more electrons
are drawn to the bottom of the bar, the electric field becomes increasingly stronger. When
the two fields are of equal strength, the forces balance one another out, halting the flow of
electrons in the bar. This takes place when:

Induced Current and Motional Emf

The electric field in the metal bar causes a potential difference of V = El = vBl. If the bar
slides along metal rails, as in the figure below, a closed circuit is set up with current