AP® Physics C: Electricity and Magnetism · Unit 11 · Topic 11.5
Ammeters and Voltmeters
An ammeter measures current, so it sits in series inside the current's path; a voltmeter measures a voltage difference, so it sits in parallel across a component. Real meters aren't ideal, so they slightly disturb the circuit they're measuring.
Current is the flow through a path, so to measure it you must become part of that path: the ammeter is wired in series, forced to carry the same charge that the rest of the branch carries. An ideal ammeter has zero resistance, so inserting it adds nothing to the circuit's total resistance and the current it reports is the same current that would have flowed without it. Voltage, by contrast, is a difference between two points, so a voltmeter is wired in parallel across a component, straddling the two points and sampling the potential drop between them without needing to carry the branch's current itself. An ideal voltmeter has infinite resistance, so almost no current sneaks through it and it doesn't change the voltage it's trying to read.
Real meters fall short of ideal, and that's where error creeps in. A real voltmeter has some large but finite resistance, so when it's placed across a resistor it forms a parallel combination with that resistor, pulling the measured resistance down below the true value. A real ammeter has some small but nonzero resistance, so inserting it into a branch adds a bit of extra resistance, lowering the current slightly below what was really there before you measured it.
The worst case is wiring mistake, not meter imperfection: a voltmeter wired in series acts like a near-open switch because its resistance is so large, starving the circuit of current. This sits right next to Kirchhoff's rules and series/parallel resistance combinations, since meter loading is just another resistor to fold into those same equations.
Key terms
- Ideal ammeter
- A current meter with zero internal resistance, so it adds nothing to the circuit it is inserted into.
- Ideal voltmeter
- A voltage meter with infinite internal resistance, so it draws no current and doesn't disturb the voltage it reads.
- Loading error
- The deviation between a true circuit value and the value a real, non-ideal meter reports because the meter itself alters the circuit.
- Series connection
- Components wired end to end so they share the same current, the arrangement used for ammeters.
- Parallel connection
- Components wired across the same two points so they share the same voltage, the arrangement used for voltmeters.
Watch it explained
- How it worksHow does an ideal ammeter's zero resistance keep it from disturbing what it measures?
- Common mistakeWhy doesn't a voltmeter wired in series with a resistor read the voltage across it?
- Worked exampleFind the error a 1000 ohm voltmeter introduces when reading across a 2000 ohm resistor.
- Going furtherWhat happens to the reading when a real ammeter with some resistance goes into the branch?
Check yourself
End of topic.