Measuring a circuit
Two meters look almost the same. They measure different things.
Read each job. Tap where the meter is joined.
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New words
Potential difference — the push between two points. Its unit is the volt (V). It is the work done to move 1 coulomb: V = W ÷ Q. W is the work done, meaning energy spent, in joules (J). Q is the charge, in coulombs (C). So 1 volt = 1 joule for each coulomb.
The ammeter measures current. It sits in the path, which is called being in series. The whole current must pass through it. The voltmeter measures potential difference. It sits across a part, never in the path. This is called being in parallel.
The rule
An ammeter sits in the path and reads the flow. A voltmeter reaches across a part and reads the push. Each meter tells the truth only in its own place.
| Meter | Placed | Reads |
|---|---|---|
| Ammeter | In the path, in series | Current, in amperes |
| Voltmeter | Across a part, in parallel | Push, in volts |
This loop has a resistor, a part that holds current back. An ammeter reads 0.5 A in the loop. A voltmeter across the resistor reads 3 V.
Resistance is how strongly a part holds current back. An ammeter has a very small resistance. It barely slows the current it measures. A voltmeter has a very large resistance. It draws almost no current for itself.
For example, 5 C move across a push of 9 V. So W = V × Q = 9 V × 5 C = 45 J.