How Electricity Works

Every electrical problem you will ever meet comes down to the same few ideas. A dead headlight, a furnace that will not call for heat, a tripped breaker and a motor that runs hot all follow the same rules. Learn those rules once and you can reason about a circuit you have never seen, instead of guessing and swapping parts.

What You Will Be Able to Do

• Explain voltage, current, resistance and power in plain words.
• Use Ohm's law to work out an unknown value in a circuit.
• Tell an open circuit from a short circuit and a ground fault.
• Explain why a bad connection gets hot and starves the load.
• Tell AC from DC and say where you will meet each one.

The Four Quantities

Electricity is the movement of electric charge through a conductor. A conductor is a material such as copper or aluminum that lets charge move easily. An insulator, such as rubber or plastic, resists that movement.

A water hose is a useful picture. Voltage is like the pressure in the hose. Current is like the amount of water moving through it. Resistance is like a narrow spot or a kink. Pressure with the nozzle closed moves no water, and voltage with no complete path moves no current. That is why you can measure voltage on a wire that is carrying no current at all.

A Circuit Needs a Complete Path

Current only flows in a closed loop. It leaves the source, passes through the load, and returns to the source. A load is whatever does the work, such as a lamp, a heater, a motor or a relay coil.

Every working circuit has four parts:

1. A source, such as a battery, a transformer or a generator.
2. Conductors to carry the current out and back.
3. A load that uses the energy.
4. A way to control and protect it, such as a switch and a fuse or breaker.

The return side matters as much as the supply side. In building wiring, the normal return is usually a conductor called the neutral. In most vehicles, the metal chassis is the return, and a loose ground strap can stop a circuit as surely as a cut power wire. In a 24-volt control circuit, the return is the side of the transformer labeled common. Many faults that look like "no power" are really a broken return.

Ohm's Law

Voltage, current and resistance are tied together by one equation:

Voltage = Current x Resistance (V = I x R)

Rearranged, it gives you the other two:

• Current = Voltage / Resistance (I = V / R)
• Resistance = Voltage / Current (R = V / I)

In words: more voltage pushes more current, and more resistance allows less current.

Worked Example: A 12-Volt Lamp

A lamp on a 12-volt DC system has a resistance of 3 ohms.

1. Write what you know: V = 12, R = 3.
2. Pick the form you need: I = V / R.
3. Work it out: 12 / 3 = 4 amps.

Now imagine the system voltage sags to 9 volts. The current falls to 9 / 3 = 3 amps, and the lamp dims. Nothing about the lamp changed. The source did.

Power

Power tells you how fast energy is being used or turned into heat, light or motion.

Power = Voltage x Current (P = V x I)

Rearranged:

• Current = Power / Voltage (I = P / V)
• Voltage = Power / Current (V = P / I)

Worked Example: A 1,500-Watt Heater

A portable heater is rated 1,500 watts on a 120-volt circuit.

1. Find the current: I = P / V = 1,500 / 120 = 12.5 amps.
2. Find the resistance: R = V / I = 120 / 12.5 = 9.6 ohms.

Plug the same two facts into a 15-amp circuit and you can see why a second heater on that circuit trips the breaker. Two heaters would draw 25 amps.

Worked Example: A Control Transformer

A 24-volt transformer rated 40 VA can supply about 40 / 24 = 1.67 amps. VA means volt-amps, which is voltage times current. Transformers are rated this way because not all of that power does useful work in an AC circuit. If the coils and relays on that transformer add up to more than 1.67 amps, the transformer overheats or its voltage sags.

Series and Parallel

Components can be connected in two basic ways, and they behave very differently.

Loads are normally wired in parallel so each one gets full voltage and can work on its own. Headlights, wall receptacles and the lamps on a control panel are all parallel loads.

Switches, fuses and safety contacts are wired in series with the load they control, because opening any one of them must stop the current. A chain of safety switches in a furnace or a machine is a series circuit. If any one of them opens, the load drops out. That is why finding the open switch finds the fault.

AC and DC

Direct current (DC) flows in one direction. Batteries, solar panels, vehicle electrical systems, most electronics and many control circuits run on DC. Polarity matters, so positive and negative are marked.

Alternating current (AC) reverses direction many times a second. In the United States it completes 60 cycles per second, written as 60 hertz (Hz). Utility power is AC because transformers can raise it for long-distance transmission and lower it for use.

You will meet these common voltages across the trades:

A low voltage is not automatically a harmless one. A 12-volt battery will not usually shock you through dry skin, but it can deliver hundreds of amps into a short, enough to melt a wrench and burn you badly.

Open, Short and Ground Fault

Most electrical failures are one of three things.

Ohm's law explains the short circuit. With almost no resistance in the path, the same voltage drives an enormous current. A 120-volt circuit with 0.1 ohm of resistance would try to carry 1,200 amps. That is why fuses and breakers exist. They protect the wire from overheating before it starts a fire.

The ground path deserves a separate word. In a building, the bare or green equipment grounding conductor carries no current in normal operation. It exists so that a fault to a metal case has an easy path back to the source, which trips the breaker quickly. The neutral, by contrast, carries normal current all the time. They are different conductors with different jobs, even though they are connected at one point in the system.

Why Bad Connections Get Hot

A loose or corroded connection adds resistance in series with the load. That resistance does two kinds of damage. It uses up voltage, and it turns power into heat in a small spot.

Worked Example: A Corroded Connection

A 12-volt circuit carries 10 amps, and a corroded terminal has 0.5 ohm of resistance.

1. Voltage lost at the terminal: V = I x R = 10 x 0.5 = 5 volts.
2. Voltage left for the load: 12 - 5 = 7 volts.
3. Heat at the terminal: P = V x I = 5 x 10 = 50 watts.

Fifty watts concentrated in one terminal is enough to melt plastic. In practice the current would fall somewhat as the resistance rises, but the lesson holds. A bad connection starves the load and creates heat in exactly the place you least want it. It is the same failure on a battery cable, a receptacle, a contactor or a service lug.

The current through the conductor also matters. Heat rises with the square of the current, so doubling the current through a resistance makes four times the heat. That is why undersized wire on a heavy load runs hot, and why wire sizes and breaker sizes are matched.

Common Mistakes

Check Your Understanding

1. A trailer light on a 12-volt system draws 2 amps. What is its resistance?

R = V / I = 12 / 2 = 6 ohms.

2. A 240-volt water heater element is rated 4,800 watts. How much current does it draw?

I = P / V = 4,800 / 240 = 20 amps.

3. A switch in a circuit is open and the load is off. Is there still voltage at the supply side of the switch?

Yes. The push is still there even though nothing is flowing. A conductor can be at full voltage while carrying no current, which is why you test before you touch.

4. A motor circuit's fuse blows the instant power is applied, every time. Is this more likely an open or a short?

A short or a ground fault. An open circuit stops current, so it would never blow a fuse. A fuse opens because the current became too high.

5. A lamp is dim, and you find the wire connector beside it is warm. What is happening?

The connection has too much resistance. It is using up voltage that should reach the lamp and turning it into heat at the connector.

Key Takeaways

• Voltage is the push, current is the flow, resistance opposes the flow, and power is the rate of work.
• Ohm's law is V = I x R, and power is P = V x I. These two equations solve most field problems.
• Current needs a complete path, and the return side fails as often as the supply side.
• Loads are usually in parallel. Switches, fuses and safety devices are in series.
• A short drives huge current, an open stops current, and a ground fault sends current where it should not go.
• Bad connections steal voltage and create heat, on any system at any voltage.