1. You just have to keep motion between the magnets and wires Electromagnetic Induction Magnetism can induce electrical currents in wires

2. Michael Faraday • 1791 – 1867 • Great experimental scientist • Invented electric motor, generator and transformers • Discovered electromagnetic induction • Discovered laws of electrolysis Section 20.1

3. Faraday’s Experiment – Set Up • A current can be produced by a changing magnetic field. • First shown in an experiment by Michael Faraday • A primary coil is connected to a battery. • A secondary coil is connected to an ammeter. Section 20.1

4. Faraday’s Experiment • There is no battery in the • secondary circuit. • When the switch is closed, the ammeter reads a current and then returns to zero. • When the switch is opened, the ammeter reads a current in the opposite direction and then returns to zero. • When there is a steady current in the primary circuit, the ammeter reads zero. Section 20.1

5. Faraday’s Conclusions • An electrical current is produced by a changing magnetic field. • The secondary circuit acts as if a source of electromotive force (emf) were connected to it for a short time. • It is customary to say that an induced emf is produced in the secondary circuit by the changing magnetic field. • EMF is another word for VOLTAGE Section 20.1

6. When there is no relative motion between the coils of wire and the magnet there is no current produces

7. Current is created in the coil when the magnet is moved towards the coil. The currents direction is always to oppose the change in the magnetic field Note: here conventional current (+) with RIGHT hand rule is used. The same result for electron flow would come from the left hand rule.

8. Current also exists when you pull it away from the coil, just in the opposite direction. The current in the coil is called an induced current. The coil itself acts as a source of emf known as an induced emf.

9. Motional EMF The EMF Induced in a Moving Conductor

10. A rod is being pushed to the right with constant speed v. Suddenly the bulb lights. Why? Where is the current coming from ? Where is this opposing force coming from?

11. We have been using the term emf, ε, or electromotive force. It is a synonym for the voltage induced. ε=BLv B = magnetic field strength L = length of wire in field V=- velocity wire moves perpendicular to the field. Potential Difference Example: Find the voltage generated by moving 50 cm of wire at 4 m/s perpendicularly through a 3 Tesla magnetic field. E = BLv= (3T)(0.5m)(4m/s) = 6 Volts

12. Consider the field created by the counterclockjwise loop in our previous problem. What is the direction of its field?

13. Lenzs’ Law The direction of induced current flow is always in the direction that OPPOSES the change in the magnetic flux

14. By RHR for + conventional current, a counter clockwise current is induced on entering the field……….. Field is unchanging in the middle By RHR for + conventional current, a clockwise current is induced on leaving the field. The induced emf resulting from a changing magnetic field will produce a current in such a way that the induced magnetic field will oppose the original change in flux. Like “magnetic inertia”