Electromagnetic Induction

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 produced

7. Current is created in the coil when the magnet is moved towards the coil. The current’s direction always opposes 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 induced emf.

9. Another way to look at it. Changing the area of a coil, in effect, reduces/increases the B field that the coil is subject to. Changing the B field strength experienced by the coil. This will also create a current.

10. Motional EMF The EMF Induced in a Moving Conductor

11. 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?

12. We have been using the term emf, ε, or electro motive force. ε=BLv Potential Difference

13. Magnetic Flux Motional EMF and Magnetic Flux

14. By definition therefore Of course the angle with the field is important

16. It is convenient express emf in terms of area when using induction in motors and generators. E = v BL can be rearranged below to create a new formula: since

17. Faradays Law actually reads Where N is the # of turns in the coil. But what is the negative all about?

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

19. Lenzs’ Law

20. 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”

24. Transformers

25. We need ALTERNATING CURRENT to make this work. It creates a constantly ___________ing magnetic field Basically, this is a transformer!

26. Many devices we plug in don’t need 120 Volts to run. A transformer can change the voltage.It only works with AC current.

27. Under the cover • This transformer came with a rechargeable electric screwdriver. This particular transformer is rated at 3 volts and 240 milliamps.

28. What you can see here are two windings. The purpose of a transformer is to convert one AC voltage to another AC voltage. In this case the transformer converts the normal 120 volt AC current in your house down to three volts.

29. Primary Winding • The 120 volts comes in on the primary winding on the left. Running down the middle of that winding (as well as around the outside) is an iron core. The AC current in the primary winding creates an alternating magnetic field in the iron just as it would in an electromagnet. Iron Core

30. Secondary Winding • The other winding, known as the secondary winding wraps around the same iron core. In the secondary winding the magnetic field in the core creates current. The voltage in the secondary is controlled by the ratio of the number of turns in the two windings. So ifthe primary and secondarywindings have the same number of turns, the primary and secondary voltage will be the same. If the secondary winding has half as many turns as the primary then the voltage in the secondarywill be half that of the voltage in the primary.

31. You can see in the following figure that the primary in this particular transformer uses very fine wire while the secondary uses much thicker wire. To drop down to 3 volts, there needs to be 40 times more turns in the primary than in the secondary.

32. On the other side of the transformer you find two diodes wrapped in rubber insulation. The diodes act as a rectifier, turning the AC current into DC current. Most transformer cubes that you find around the house produce a low-voltage DC current (3 to 12 volts, and less than an amp of current).

33. Turning AC into DC DC current is necessary because rechargeable batteries store DC current, because most electronics require low-voltage DC current and because small DC motors run directly from batteries and are the least expensive motors available.

34. On the other hand, the picture tube in your TV requires 15,000 V to accelerate the electron beam, and a transformer is used to obtain this from a 120 V wall outlet.