3.4.1Electricity
Measuring Current, Potential Difference and Resistance
5 min read 5 parts Lesson 9 of 16
Section 1 of 5The Three Meters
Each meter is connected in a different way because it measures a different quantity.
Ammeter
- Measures current in amperes and is connected in series so that the whole current passes through it.
Voltmeter
- Measures potential difference in volts and is connected in parallel across the component.
Ohmmeter
- Measures resistance in ohms and is used on a component that has been disconnected from the circuit.
Multimeter
- One instrument with a dial or buttons can act as any of these three meters.
Digital and analogue
- Digital meters show the value as numbers, while analogue meters use a pointer, so the scale must be read carefully.
| Meter | Measures | Connection |
|---|---|---|
| Ammeter | Current (A) | In series |
| Voltmeter | Potential difference (V) | In parallel |
| Ohmmeter | Resistance (Ω) | Across a component removed from the circuit |
Section 2 of 5Meter Properties
Good meters change the circuit as little as possible.
Meter Properties
Ideal ammeter
- Has very low resistance so that adding it does not reduce the current in the circuit.
Ideal voltmeter
- Has very high resistance so that hardly any current is diverted through it.
Polarity
- Connect the positive terminal of a meter towards the positive side of the supply.
Ranges
- Start on the highest range and move down to avoid overloading the meter.
Zero error
- Check that each meter reads zero before use; otherwise every reading will be wrong by the same amount.
Section 3 of 5Finding Resistance Indirectly
Resistance can be calculated from two readings instead of being measured directly.
1
Connect the component in a circuit with an ammeter in series with it.
2
Connect a voltmeter in parallel across the component only.
3
Close the switch and read the current and the potential difference at the same moment.
4
Calculate the resistance using R = V ÷ I.
5
Repeat at several settings and average the results to improve reliability.
Section 4 of 5Worked Example
Question
- An ammeter in series with a resistor reads 0.25 A and a voltmeter across the resistor reads 3.0 V.
- Calculate the resistance.
- Write down I = 0.25 A and V = 3.0 V.
- Use R = V ÷ I.
- R = 3.0 ÷ 0.25 = 12 Ω.
Answer The resistance is 12 Ω.
Section 5 of 5Exam Checklist & Tips
3.4.1 – Key Points for the Exam
- Ammeter A meter, connected in series, that measures current.
- Voltmeter A meter, connected in parallel, that measures potential difference.
- Ohmmeter A meter that measures the resistance of a component.
- Multimeter A meter that can be switched between measuring current, voltage and resistance.
Common Mistakes
- Connecting a voltmeter in series, which blocks the current and gives a wrong reading.
- Measuring resistance with the component still connected to a live circuit.
- Reading the wrong scale on a multimeter, for example mA instead of A.
- Using the highest range only, which gives readings too coarse to be useful.