Electric Generators

Using a Magnetic Field to

Generate Electricity

Electromagnets, Generators Electromagnets, Generators and Motors, AC & DCand Motors, AC & DC

An electromagnetAn electromagnet: A coil of wire with a current going through it.

ElectromagnetsElectromagnets can be made strongerstronger by:1. Putting an iron or steel bar in the middle of the coil.2.

ElectromagnetsElectromagnets can be made strongerstronger by:1. Putting an iron or steel bar in the middle of the coil.2. Increasing the number ofturns of the coil.

3.

ElectromagnetsElectromagnets can be made strongerstronger by:1. Putting an iron or steel bar in the middle of the coil.2. Increasing the number ofturns of the coil.

3. Increasing the currentthrough the coil.

•Magnet can be turned on/off

Induced Current

� Current can be induced (created) in a wire by a changing magnetic field:

� - moving a wire inside a field

� - changing the magnetic field (increasing, decreasing or moving)

� A galvanometer (very sensitive ammeter) can detect these currents

Ways to Induce Current

� Turn magnetic field off and on

� Rotate magnet in coil

� Rotate coil in magnetic field

� Move magnet in and out of coil

Examples

� It doesn’t matter whether the magnetic field moves, or the conducting wire moves, the result is the same.

Generators

� Devices that turn mechanical energy into electrical energy

� Mechanical energy supplied by spinning wheel, running water, windmill, steam turbine

Types of Generators

Examples

� - bicycle generator

� (spinning wheel turns coil inside a magnet

which generates electricity to run lights etc.)

� - hydroelectric power plant

� (running water turns turbine, which spins a

coil in a magnetic field and generates large

amounts of electricity)

With Alternating CurrentAlternating Current, (AC), the current keeps on changing changing directiondirection.

cycle

FrequencyFrequency is how many cycles cycles per secondper second there are in AC.

1 cycle per second is called one Hertzone Hertz (HzHz).

The current coming into our homes is 60 Hz (or 60 cycles per second)

The current changes direction 120 times each second!

With Direct Direct CurrentCurrent(DC), the current only flows one one wayway through the circuit.

Direct Current (DC)

� Current that goes in only one direction

� The type of current produced by chemical cells.

� Used for flashlights, portable stereos etc.

� Many household devices run on DC that is converted from AC (TV’s, computers)

– Electric Motors:

• An electrical motor is an electromagnetic device

that converts electrical energy into mechanical

energy.

• A motor has two working parts - a stationary

magnet called a field magnet and a cylindrical,

movable electromagnet called an armature.

• The armature is on an axle and rotates in the

magnetic field of the field magnet.

• The axle is used to do work.

Electric Motors

� Electricity is added to make motor turn and get usable mechanical energy

� Reverse of a generator

Electric motor

� Red = North pole

� Green = South

pole

� Opposite

polarities attract

1. A simple DC electric motor. When

the coil is powered, a magnetic field is

generated around the armature. The left

side of the armature is pushed away

from the left magnet and drawn toward

the right, causing rotation.

2. The armature continues to rotate

3. When the armature becomes

horizontally aligned, the commutator

reverses the direction of current

through the coil, reversing the

magnetic field. The process then

repeats.

Transformers

� Allow current flow to be adjusted to any voltage

� Used in electricity transmission and in most electrical devices

� Changes voltage using two linked electromagnets

Transformers

� Step-up transformer:

– more coils on the

secondary coil (right

side)

� Step-down transformer:

-more coils on the

primary coil (left side)

Homework

� Chapterrrrrrrrrrrrr 4 Review

� Jack is an idiot.