### Units of Measure and Measuring Devices

Different aspects of electricity are measured using different units of measure. Special measuring devices are used. The following table shows the different units and devices.

### Ohm's Law

*Ohm's law* describes the relationship among electrical pressure (voltage), current strength (amperage), and resistance (ohms) in any circuit:

If you know two of these three quantities, you can always calculate the third.

Here's an easy way to remember how to use Ohm's law to find the third quantity if you know two already. On the circle (below), place your finger over the quantity you want to find. Look at the remaining two quantities to see how to calculate the third quantity.

Memorize the equations on the circle; you'll need them on the ASVAB. Let's look at a couple of examples of how to use the Ohm's law circle.

Find the amperes if a 120-volt current runs through 6 ohms.

Amperes = volts/ohms

Amperes = 120/6 = 20 amperes

Or cover ohms in the circle. Notice that what remains is volts divided by ohms (the same result you would get by remembering the three formulas above).

Amperes = 120/6 = 20 amperes

In a 6-volt circuit with 24 amperes flowing, what is the resistance?

Ohms = volts/amperes

Ohms = 6/24 = 1/4 ohms

### 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-up*transformer, and the output voltage will be greater than the input voltage. If there are more turns on the input side, it's a *step-down*transformer, 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
*generator*changes rotating (kinetic) energy into electric energy. - A
*motor*changes 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 *rotor*that 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.

### ELECTRICAL DEVICES AND SYMBOLS

Electrical devices are the guts of many electrical and electronic systems. You'll need a basic acquaintance with these devices. You should also know the symbols used for them in electrical diagrams.

A *capacitor*is a device that can briefly store electricity.

A *resistor*creates resistance to the flow of electrons. If a circuit does not have any resistance, it's called a short circuit. Excess current will cause such a circuit to heat up, which may cause a fire.

A *transformer*changes the voltage and amperage of a current. Transformers work by changing electricity into magnetism, then back into electricity. As you can see from the diagram, transformers have two coils. One gets current (usually AC) from a source; the other supplies the output. The first coil creates a magnetic field, which rises and falls as the current alternates. The second coil is inside a changing electric field, and any wire in a changing magnetic field will pick up current from that magnetism.

A *battery*stores electricity as chemical energy that can be readily converted into electric current. Batteries may be wet (like the lead-acid storage batteries used in cars) or dry (like the nickel-cadmium or metal hydride batteries used in flashlights, computers, and the like). Batteries always make direct current. Depending on their chemistry, some batteries can be recharged.

In the diagram, the electrochemical reactions between the cathode and the carbon electrode at the center result in free electrons, which can travel through a circuit to make an electric current.