21-5 GeneratorsAn 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 e=NBA w sinwt…where A=lh and w=2pf (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.• Vp Ip =Vs Is

• 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=I2R

• 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.