Circuit Measurements
Physics: Cambridge International A Level, topic 1.2.3
Measurements in Circuits
Several different properties of currents can be measured.
Ammeters
- An ammeter measures the flow of current that passes through it.
- Ammeters have to be connected in series (in the same loop of the circuit) with the electrical component whose current you are measuring. For example component X above.
Voltmeters
- Voltmeters measure the potential difference (voltage) between two points in a circuit. For example between two points either side of component X above.
- Voltmeters must always be connected in parallel (on a separate branch of the circuit) with the two points being measured.
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Current vs potential difference
- The current is a flow of charge. Current is measured through a component.
- Potential difference is the energy used between two points in a circuit, therefore it is measured between two points either side of a component.
- We describe this as the potential difference measured across a component.
Galvanometers
- An analogue meter uses a galvanometer, which is essentially a coil of wire with small resistance, in a magnetic field, with a pointer attached that points to a scale.
- Current flows through the coil, causing the coil to rotate. To use the galvanometer as an ammeter, a small resistance is placed in parallel with the coil.
- For a voltmeter, a large resistance is placed in series with the coil.
Test yourself on Circuit Measurements
What instrument is used to measure current?
Which of the following are true of measurements of a circuit?