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# Electronics Information Study Guide for McGraw-Hill's ASVAB (page 3)

By Dr. Janet E. Wall
McGraw-Hill Professional
Updated on Jun 26, 2011

### The Law of Electric Power

The amount of power consumed by an electrical or electronic device can be calculated using the following formula:

Watts = volts = amperes

You can use this equation to find any one of the three factors as long as you know the other two. Here are two examples.

How much power is consumed by a lamp that draws 10 amperes of current at 120 volts?

Watts = volts = amperes
Watts = 120 = 10 = 1,200 watts

A clothes dryer is rated at 2,400 watts. At 120 volts, how much current does it draw?

Amperes = watts/volts
Amperes = 2400/120 = 20 amperes

### ELECTRICITY AND MAGNETISM

Electricity and magnetism are tightly connected: It's easy to change from an electric current to a magnetic field and back again. This close relationship explains electromagnets, transformers, motors, and generators. Let's start with electromagnets.

A current passing through a conductor creates a magnetic field around it. In most electromagnets, the conductor (wire) is wrapped around an iron core.

A transformer is like two electromagnets placed next to each other. If the transformer has more turns of wire on the output side, it is a step-uptransformer, and the output voltage will be greater than the input voltage. If there are more turns on the input side, it's a step-downtransformer, and the output voltage is smaller than the input voltage.

### Motors and Generators

Electromagnetism also explains motors and generators. In fact, motors and generators are really the same machines, operating backwards.

• A generatorchanges rotating (kinetic) energy into electric energy.
• A motorchanges electric energy into kinetic energy.

Here's how a motor works. Each magnet has two poles: north and south. Opposite poles attract, and like poles repel: North attracts south, but repels north. A motor has two magnets: a rotorthat spins inside a stator, a fancy name for a stationary magnet. One of these magnets, usually the rotor, is an electromagnet. It is wired so that the magnetic field changes twice per rotation. When the rotor starts rotating, the rotor and the stator repel each other, forcing the rotor to start turning. At just about the point where the magnets would stop repelling, the rotor changes polarity, and it again repels the stator. This change in polarity is what drives the motor. We call them electric motors, but motors are all about magnetism.

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