Real ammeters and voltmeters are close to perfect but not ideal. On this page you will find out how ammeters and voltmeters are optimised for use.
Essentials
Ammeters
Ammeters measure current and are connected in series. Therefore, the resistance of a perfect ammeter should be as low as possible (ideal: zero).
Voltmeters
Voltmeters measure potential difference and are connected in parallel. Therefore, the resistance of a perfect voltmeter should be as high as possible (ideal: infinte).
What are the effects of non-ideal voltmeters in a circuit?
A voltmeter of resistance 10 kΩ is used to measure the pd across A. What result will it give?
The combined resistance of A plus voltmeter wil be 5 kΩ so th the pd across A will be 1/2 the pd across the other resistor. The total is 9 V so that makes 6 V across one and 3 V across the other.
A 9V battery is connected to two 10 kΩ resistors.
A voltmeter of resistance 10 MΩ is used to measure the pd across A. What result will it give?
The voltmeter has a very high resistance so will not draw any current
Exam-style Questions
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