Cracking the SAT Physics Subject Test

4. What’s the induced emf in the loop?


5. In which direction will current be induced to flow?

Here’s How to Crack It

4. As we found in Questions 1−3, the magnetic flux through the
   loop changes when the loop rotates. Using the values we determined
   earlier, Faraday’s law gives


5. The original magnetic flux was 6 × 10−4 Wb upward, and was
   decreased to 3 × 10−4 Wb. So the change in magnetic flux is −3 ×
   10 −4 Wb upward, or, equivalently, ∆ɸB = 3 × 10−4 Wb, downward.
   To oppose this change, we would need to create some magnetic flux
   upward. The current would be induced in the counterclockwise
   direction (looking down on the loop) because the right-hand rule
   tells us that then the current would produce a magnetic field that
   would point up.