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Lectures for Conceptual Physics, 8th Ed. 1 Electromagnetic Induction p436 1820 Hans Oersted showed that current affected a magnet. 1831 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 2 Here is a critical concept! Magnet being moved. Coil of insulated wire The magnet resists being moved! 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). 3 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. The repelling magnetic fields explain why you have to push. That’s it! Tricky but true. Picture from Serway, vol 2 p990 4 By the way, when you push current goes one way. Then, when you pull, current goes the other way. Generators and Alternating Current Starting and stopping is hard. p439 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. 5 + 0 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. 6 Turbogenerator Power Power in. p441 Fig 24-9 Power out. This is the “turbo” part. This is the generator part. 7 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. 8 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 Secondary voltage = Number of primary turns Number of secondary turns 9 The answers are on p 445. Study the answers…hint, hint! 10 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”. 11 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 Transformers “step-up” the voltage at the production plant. V And, transformers “stepdown” the voltage at the customer end. 12