1. Electromagnetic Induction
Chapter 20

2. If a current can create a magnetic field, can a magnetic field create a current?
YES, but not all the time.

3. How do we know when a current will be induced?
Induced current in a loop of wire occurs only when the magnetic field through that loop CHANGES!
Called electromagnetic induction.
Instead of talking about the current, we often talk about the emf (electromotive force, voltage)

4. Demonstration
As the magnet gets closer to the loop of wire, the magnetic field lines through the loop increase and current is induced in the wire

5. The current alternates direction, so as you move the magnet back and forth, you produce alternating current!

6. What does the induced current depend on?
Number of turns in the loop
Relative velocity between the magnet & loop
Magnetic field strength

7. Can we solve the ENERGY CRISIS?
How might we be able to generate electrical power using a magnet and a loop of wire?
Swinging loop demo

8. Lenz’s Law
An induced emf in a wire loop or coil has a direction such that the current it creates produces its own magnetic field that opposes the change in the magnetic flux through that loop or coil.

9. An example: Which way is current induced in the loop?

10. Why can’t we solve the energy crisis with a pendulum?

11. What are some different ways of inducing current?
Bring a magnet closer and farther from a stationary loop of wire.
Bring a loop of wire closer and farther from a stationary magnet.
Change the magnetic field strength.
Change the shape of the loop of wire.
Rotate the wire.

12. Is there a current in the loop?X
Is there a CHANGING magnetic field?
What is the direction of change?
What direction opposes change?
What direction is induced current?
Yes, there is a current in the loop – SOMETIMES!
But when is there a current and in what direction? X X

13. Emf induced in a moving conductor

14. We can create a circuit and utilize this motional emf.

15. But wait… can’t I just make a current since once I get the bar going, it should keep going by Newton’s 1st Law, right?!

16. How can we utilize induction?
Even though we can’t create energy (can’t get somethin’ for nothin’), we can still use induction to our benefit.
We can change the voltage using a transformer.

18. Power lost in the transport of power
We learned earlier that less power is lost in transmission with a higher voltage.
We use transformers to change the voltage several times from the power plant to your home.
200, ,000V
20,000V
120V (US)

19. What the heck is this?