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Measuring a circuit

Electricity and magnetism · level 4, lesson 4 of 6

A question to start

You want to know the current through a bulb and the voltage across it. Where do you put the meters?

The idea

  • An ammeter measures current. It goes in the loop (in series), so the current flows through it.
  • A voltmeter measures voltage. It goes across a part (in parallel), with one lead on each side, so it compares the two ends.

A multimeter can do both; you choose the setting and which sockets to use.

In a single loop, the voltages across the parts add up to the battery's voltage. The energy each coulomb picks up in the battery is all handed out to the parts on its way round.

AV ammeter: in the loopvoltmeter: across the bulbcell

Worked examples

1. Two bulbs in a loop. A 6 V battery drives two identical bulbs in one loop. A voltmeter reads 3 V across each: \(3 + 3 = 6\) V.

2. Unequal parts. A 9 V battery, a bulb and a motor in one loop. The bulb has 4 V across it, so the motor has \(9 - 4 = 5\) V.

3. The ammeter. An ammeter before the bulb and one after both read the same current, say 0.3 A.

Try it

  1. Where does an ammeter go in a circuit?
  2. Where does a voltmeter go?
  3. A 12 V battery runs three identical bulbs in one loop. What is the voltage across each?
  4. A 4.5 V battery drives a buzzer and a bulb in one loop. The buzzer has 3 V across it. What is across the bulb?
Answers
  1. In the loop, so the current flows through it.
  2. Across the part, one lead on each side.
  3. \(12/3 = 4\) V.
  4. \(4.5 - 3 = 1.5\) V.

Common mistake

Putting an ammeter across a part. An ammeter lets current through very easily, so across a battery it makes a short circuit: a huge current that can blow the meter's fuse. It must go in the loop.

Where it leads