1. CH 23-2

2. Review Changing the magnetic flux in a coil induces an emf around the coil. (As long as the coil is connected in a complete circuit, a current is induced in the coil.) This effect is called electromagnetic induction. Two typical ways to induce a current in a pickup coil is: (1)change the magnetic field in the plane of the pickup coil (a)by moving a magnet back and forth (b)by changing the current in a primary coil (2)change the orientation of the pickup coil in a uniform magnetic field (or alternatively, rotate a magnet and keep the coil fixed)

3. Example A primary coil has a current of 0.5 A, radius 0.02 m, and 20 loops of wire. It creates a magnetic field in the plane of a pickup coil of 0.001 T. If the current in the primary coil decreases to zero in 0.1 s, what is the magnitude of the emf induced in the pickup coil if its radius is 0.01 m and it is made of 100 loops of wire? (Assume that the two coils are aligned along the same axis.)

4. Direction of the induced current -- energy must be conserved

5. Lenz’s Law The induced current will flow so as to produce a magnetic field opposing the change in flux. If the flux in the pickup coil is increasing, then the induced current produces a magnetic field that is in the opposite direction as the applied magnetic field. If the flux in the pickup coil is decreasing, then the induced current produces a magnetic field that is in the same direction as the applied magnetic field.

6. Example move the magnet toward the coil N S

7. Example move the magnet away from the coil N S

8. Example move the magnet toward the coil S N

9. Example move the magnet away from the coil S N

10. Example increase the current in the primary coil I (+z) I (  z)

11. Example decrease the current in the primary coil I (+z) I (  z)

12. The coils in these two circuits are wound around a common core, but the circuits are electrically insulated from each other. According to Lenz’s law, the current induced in Circuit 1 flows ________ when the switch is closed, and then, with the switch kept closed, it flows ________ as the sliding contact of the variable resistor in Circuit 2 is moved from point c to point d. A. from a to b … from b to a B. from b to a … from a to b C. from a to b … from a to b D. from b to a … from b to a a bc d CIRCUIT 1 CIRCUIT 2

13. A battery is connected to a wire loop, with a switch in the circuit. When the switch is closing, what direction does current flow in the pickup coil?

14. A battery is connected to a wire loop, with a switch in the circuit. When the switch is opening, what direction does current flow in the pickup coil?