Electricity Basics

General Science asks the basics of electricity and Electronics Information asks them again in more depth. This lesson is the shared foundation, and getting the three quantities straight here makes both subtests easier.

The three quantities

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Voltage is not a flow. It is a difference in electrical pressure between two points, which is why it is sometimes called potential difference. A battery does not contain current; it supplies the push that makes current flow when a path exists.

Current is the flow itself, measured as charge passing a point per second.

Resistance converts electrical energy to heat, which is how a heater and a filament bulb work. It is not purely a nuisance.

Charge

Like charges repel; opposite charges attract. Two negatives push apart, and a positive and a negative pull together.

Electrons carry negative charge and are what move in a wire. Protons are locked in the nucleus and do not flow.

Static electricity is charge built up and not moving, which is what makes a balloon stick to a wall after being rubbed. Current electricity is charge in motion through a conductor.

Ohm's law

V = I x R. Voltage equals current times resistance.

Rearranged:

• I = V / R - current is voltage divided by resistance.
• R = V / I - resistance is voltage divided by current.

A 12-volt battery drives current through a 4-ohm resistor. What is the current?

12 divided by 4 is 3 amperes.

A current of 2 amperes flows through a resistance of 25 ohms. What voltage is across it?

2 times 25 is 50 volts.

The relationships worth stating in words, because that is how questions are phrased:

• At fixed voltage, more resistance means less current.
• At fixed resistance, more voltage means more current.

Power

Power = voltage x current, measured in watts.

A device draws 4 amperes at 120 volts. What power does it use?

4 times 120 is 480 watts.

Electrical energy is power times time, which is what a kilowatt-hour measures and what an electricity bill charges for.

Power rises with the square of voltage or current. Combine power = voltage x current with Ohm's law and you get power = current squared x resistance, so with the resistance fixed, doubling the current or the voltage quadruples the power. That is asked directly.

Devices are energy converters, and the questions name the conversion: an incandescent light bulb turns electrical energy into light and a great deal of heat, a motor into motion, a speaker into sound, a heater into heat.

Circuits

A circuit is a complete path. Current flows only in a closed loop from the source, through the load, and back.

A short circuit is dangerous because low resistance means high current, by Ohm's law, and high current means heat. That is what fuses and breakers exist to interrupt.

A switch works by opening and closing the circuit, which is why a single switch controls the whole loop.

Series and parallel

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Two 6-ohm resistors in series total 12 ohms. The same two in parallel total 3 ohms.

That parallel result surprises people, and the reason is worth holding: adding another branch adds another path, and more paths means easier flow overall. Total resistance in parallel is always below the smallest individual branch, which is the check that catches an arithmetic error.

Household wiring is parallel, which is why switching off one lamp does not darken the room and why every outlet gets the full supply voltage.

Old-style holiday lights were series, which is why one failed bulb killed the whole string. That contrast is the question.

Conductors and insulators

Conductors let charge move easily, because their outer electrons are loosely held. Metals are the conductors, with silver best, then copper, then gold, then aluminum. Copper is what wiring is made of, because it is nearly as good as silver and far cheaper.

Insulators resist charge movement: rubber, glass, plastic, dry wood, air. Wire insulation is the applied case.

Semiconductors sit between the two - silicon and germanium - and conduct under some conditions and not others, which is what makes every transistor and diode possible.

Water conducts because of dissolved minerals, not because pure water is a good conductor. That distinction is occasionally asked and it is why wet hands are dangerous around electricity.

AC and DC

Direct current (DC) flows one way. Batteries supply DC.

Alternating current (AC) reverses direction periodically. Household supply is AC, at 60 hertz in the United States.

AC is used for distribution because its voltage can be changed by a transformer, which allows high-voltage transmission with low losses. The reason is the squared term above: power lines lose energy as current squared x resistance, so sending the same power at high voltage and low current cuts the loss sharply. DC cannot be transformed so simply, which is the reason the grid is AC.

What you can skip

Across all 2,104 General Science questions in our bank:

• Kirchhoff's laws and impedance never appear on General Science. Ohm's law and the series and parallel rules cover every circuit question here; impedance belongs to Electronics Information, in the AC and DC lesson.
• Semiconductors and transistors come up once on General Science. They matter for Electronics Information, where the Electronics lessons teach them.

Where people lose points

Saying current is used up around a circuit. It is not; voltage drops.

Adding parallel resistances. They combine to less than the smallest.

Saying voltage flows. Current flows; voltage is a difference.

Reversing Ohm's law. Write V = IR and rearrange from it.

Thinking a short circuit means no current. It means too much.

Saying protons move in a wire. Electrons do.

Work one in under a minute

Two 10-ohm resistors are connected in parallel across a 20-volt supply. What is the total resistance, and what current does the supply deliver?

Two equal resistors in parallel give half of one of them, so 5 ohms.

Check it against the rule: 5 is less than 10, which it must be.

Current: 20 divided by 5 is 4 amperes.

For two equal resistors in parallel, halve one. That shortcut covers a surprising share of parallel questions.

Where this leads

This is the foundation for the whole of Electronics Information, where Ohm's law and the series-parallel contrast are asked in more depth.