1. Physics 12 Mr. Jean December 14 th, 2012

2. The plan: Video Clip of the day Facts about Magnets Lenz’s Law E=mc 2 Video part #2

3. Facts about Magnets: Permanent magnets are made of ALNICO. Aluminum, nickel, and cobalt. There are also rare earth elements, neodymium, that produce extremely strong permanent magnets for their size.

4. About Magnets: Permanent magnets are fragile. Dropping them will cause the domains to disorient. Heating them weakens the magnet. Once cooled, it will regain its strength unless heated past the “Currie point”. The magnet will be totally destroyed. – (p. 755 Currie point table)

5. Galvanometer Galvanometer - device to measure very small currents. – Consists of a small coil of wire placed in the strong magnetic field of a permanent magnet. Each turn of the wire is a loop. –The current passing through the loop goes in one side of the loop and out the other side. –One side of the loop is forced down while the other side is forced up (third RH rule). –The loop has torque and rotates.

7. Producing Current One way to produce an electric current through a conductor is to move a conducting rod through a magnetic field.

8. When there is a voltage in the rod, it becomes part of an electric circuit.

9. Lenz’s Law (1834): For motion, Lenz’s law states that if a loop of wire moves while inside an external magnetic field then a current is produced in the moving wire that will produce its own magnetic field.

10. This magnetic field will interfere with the external magnetic field in such a way that the two fields will try to oppose the motion (change in position of the moving loop of wire).

11. DC Electric Motor Has a coil with an iron core ⇒ ARMATURE It is surrounded by electromagnets

12. Problems with DC Power: 1. The magnetic forces are aligned directly opposite each other and will no longer experience a torque. –If you could change the direction of the current, the coil would again experience a torque. 2. If the coil keeps turning, the leads will twist and eventually break.

15. AC Electric Motor Uses slip rings as commulator. Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave. The magnetic field is sinusoidally varying, just as the current in the coil varies.

17. AC vs. DC Motors: Electric motors are mostly AC because our electric energy for industry and home is transmitted as AC. DC motor – starter motor on a car.

18. Magnetic Field Demonstration: Video Demonstration of Ferrell Fluid Rare Earth Magnets AC & DC Motors