Voltage, Current and Resistance

Three quantities describe everything electrical, and beginners routinely confuse the first two. Getting them straight makes the rest of the course easier and the meter readings meaningful.

The water analogy, and where it stops

Imagine water in a pipe.

Voltage is the pressure — how hard the water is being pushed. Measured in volts. It exists between two points; you always measure it across something.

Current is the flow — how much water actually moves past a point. Measured in amps. It travels through things, so measuring it means putting the meter in the path.

Resistance is opposition — how much the pipe restricts the flow. Measured in ohms.

The analogy is good enough to build on and breaks down eventually. What it gets right is the most important thing for a beginner: pressure and flow are not the same. You can have plenty of pressure and almost no flow, if something is restricting the path.

That single idea explains the most common diagnostic mistake in the trade, and the whole of module three.

Ohm's law

The three are tied together by one equation:

Voltage = current × resistance

Rearranged the two useful ways:

Current = voltage ÷ resistance Resistance = voltage ÷ current

You will use this more than any other piece of theory. A few worked examples make it concrete.

Example 1 — how much current will this draw?

A 12-volt supply feeding a load of 4 ohms.

Current = 12 ÷ 4 = 3 amps

Useful for sizing a fuse or judging whether a circuit is behaving.

Example 2 — is this bulb faulty?

A bulb should draw about 5 amps at 12 volts. That implies a resistance of:

Resistance = 12 ÷ 5 = 2.4 ohms

Measure it cold and find several hundred ohms, and it is open. Measure near zero and it is shorted. Neither requires guessing.

Example 3 — how much is this bad joint costing?

A connection has 2 volts dropped across it while 10 amps flow.

Resistance = 2 ÷ 10 = 0.2 ohms

That sounds tiny, and it is enough to stop a starter motor turning properly. Small resistances matter enormously in high-current circuits, which is why battery and starter connections get so much attention.

Power, and why bad connections get hot

A fourth quantity follows from the first three.

Power = voltage × current

Measured in watts, and it is what becomes heat, light or movement.

The practical consequence is worth understanding rather than memorising. A resistive connection drops voltage across itself, and current is flowing through it, so power is being dissipated right at that point. Not in the load where you want it — in the joint.

Take the example above: 2 volts dropped, 10 amps flowing.

Power = 2 × 10 = 20 watts

Twenty watts is a soldering iron. Concentrated in a connector the size of a fingernail, that is why bad joints discolour, melt their housings and eventually burn.

So a connector that is hot, discoloured or smells is not a curiosity. It is a fault reporting itself, and it will get worse rather than better.

Series and parallel, briefly

Two arrangements you will meet constantly.

In series — components one after another, in a single loop. The same current flows through all of them, and the supply voltage divides between them according to their resistance. Old Christmas lights are the classic example: one fails, the loop breaks, all go out.

The important automotive consequence: every connection, wire and switch in a circuit is in series with the load. So each one that has resistance takes a share of the available voltage, leaving less for the thing you actually want to work.

In parallel — components side by side, each with its own path. Each gets the full supply voltage, and the total current is the sum of what each draws. Nearly everything in a vehicle is wired in parallel to the supply, which is why one failed bulb does not take the rest with it.

Measuring, and what each measurement tells you

Full technique is module three. What matters now is which quantity answers which question.

Voltage — measured across two points, with the circuit live. Answers is pressure available here?

Resistance — measured on a circuit with no power applied, otherwise the reading is meaningless and you may damage the meter. Answers is this path continuous?

Current — measured by putting the meter in the path, so the current flows through it. Answers how much is actually flowing?

The trap already worth knowing: measuring voltage at a point tells you pressure is available, not that current can flow. A corroded joint can show a perfect reading with nothing connected and collapse completely under load.

"I checked it and there was power there" is the most common dead end in automotive diagnosis. The fix is a technique called the voltage drop test, and it is the single most useful thing in this course.