1 Lectures for Conceptual Physics, 8 th Ed.

2 Electromagnetic Induction p Hans Oersted showed that current affected a magnet Michael Faraday and Joseph Henry made electricity from magnets. Made it possible to light up cities at night and ruined the sleep habits of the new era. It was simple…just rotate (move) a loop of wire in a magnetic field and electricity was produced. OR AND

3 Here is a critical concept! Magnet being moved. Coil of insulated wire Resistor The resistor is converting electric power into heat (work). The source of that power is the person. She has to forcibly push the magnet into the coil. The power she puts into making current comes out as heat (work). The electricity just connects her work (input) to the work done (output). The magnet resists being moved!

4 More on Lenz’s Law: Moving the magnet to the right increases the magnetic field lines in the loop. The induced current creates a magnetic field which opposes this increase. That’s it! Tricky but true. The repelling magnetic fields explain why you have to push. Picture from Serway, vol 2 p990

5 Generators and Alternating Current p439 By the way, when you push current goes one way. Then, when you pull, current goes the other way. Starting and stopping is hard.It’s easier to make something rotate…the loop. (fig 24-5) Rotating the loop induces current in the loop. This is the input. The heat and light from the bulb is the output.

Maximum current in one direction. Maximum current in the other direction. Nikola Tesla, the scientist, along with George Westinghouse, the astute business person, provided the world with abundant electrical power.

7 Turbogenerator Power p441 This is the “turbo” part. This is the generator part. Power in. Power out. Fig 24-9

8 Transformers p442 Transform voltage from low to high or high to low. A galvanometer (voltmeter) Putting an iron core through the coils focuses the magnetic field lines.

9 More on Transformers On the left side (the primary side) a 1 VAC input is operating. It is critical that the input voltage is alternating, changing directions. The magnetic field produced by the primary coil causes an opposing magnetic field in the secondary coil. (Lenz’s Law) The fluctuating secondary magnetic field creates an alternating current in the secondary coil. Primary voltage Number of primary turns = Secondary voltage Number of secondary turns

10 The answers are on p 445. Study the answers…hint, hint!

11 Self-Induction p445 Imagine a large coil with a current running through it. Things are normal. The current is steady and creating a large magnetic field. Did I mention that this is called an electromagnet? Then you suddenly open the switch. (Cut off the current.) The magnetic field with all its energy collapses quickly producing a giant current in the other direction which zaps across the switch. This phenomenon is also called “back emf”, or “back electromotive force”.

12 Power Transmission p446 Large currents in wire create heat and loss of power. So “electricity” is sent long distances at high voltage and small currents. I V = I V Transformers “step-up” the voltage at the production plant. And, transformers “step- down” the voltage at the customer end.