21-5 Generators An electric generator, or dynamo, converts mechanical energy to electrical energy. An ac generator consists of many loops of wire wrapped around an armature that can rotate in a magnetic field… it produces an ac current. The emf formula is  =NBA  sin  t…where A=lh and  =2  f (See page 629) Over 99% of the electricity in the USA is produced by generators. See Example 21-6 p630.

21-5 Generators (cont’d) DC generators are much like ac generators, but the slip-ring is replaced by a split-ring commutator. The AC current is reversed every ½ turn to give a + only current. (see p 630) A capacitor can be placed in series with the generator, or wrap many more windings to smooth the current. In an alternator, dc current is produced by semiconductor diodes.

21-6 Counter EMF and Torque; Eddy Currents. Because of Lenz’s law, a generator produces a counter emf or back emf to oppose mottion. See Example 21-7 p 632 Torque that opposes the motion of a generator is called counter torque. Smaller counter-currents in a conductor moving in a magnetic field are called eddy currents.

21-7 Transformers; Transmission of Power A transformer is a device for increasing or decreasing an ac voltage. A transformer consists of 2 coils a primary coil and a secondary coil. Iron transformers are about 99% efficient. A step-up transformer increases voltage… a step-down transformer decreases voltage. V p I p =V s I s See Example 21-9 on p 635

21-7 (cont’d) There is always some power loss in transmission of electricity, but it is minimized by transmission at high voltage and low current. P=VI and P=I 2 R Voltage is stepped-up to 2,400 V and then stepped-down to 240V or 120V. See p636 Example 21-10

HOMEWORK P655 pr#19-23, and 24-26, and pr#30-35 due Wednesday BOP. Packet 3B due Monday BOP.