Measuring a circuit
Show on a big screenTwo meters look almost the same. They measure different things.
Read each job. Tap where the meter is joined.
Loading lab…
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 . 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.
Try the lab to finish this lesson.