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Problem 30.8: Find the internal resistance of voltmeter and ammeter

Circuits A and B are different configurations of the same circuit elements. Assume the battery is ideal (no internal resistance). Restart. Pick an animation to show the voltage and current on the meters (voltage is given in volts and current is given in milliamperes).

Use circuits A and B to determine the internal resistance of the ammeter and the voltmeter that are used in both circuits. You can vary the resistor in circuits A and B (and see the resistor value).

1. Which circuit should you use to find the resistance of the ammeter? Which circuit to find the resistance of the voltmeter? Why?

Once you determine which circuit you will use to find the resistance of the ammeter, you should keep in mind the ideal resistance of an ammeter (ideally 0 Ω; why is this the ideal resistance of an ammeter?) and pick your variable resistance appropriately (e.g., if a small resistance is in series with a very large resistor, the voltage drop across the big one will not be measurably different than the voltage drop across both of them, etc.). The same is true for your determination of the resistance of a voltmeter.

1. What is the resistance of both the ammeter and the voltmeter?
2. If you don't know the internal resistance of the meters or the value of the variable resistor (which is often the case), and you simply want to divide the voltmeter reading by the ammeter reading to determine the unknown resistance, which circuit, A or B, is the best for measuring small resistances?
3. Which circuit, A or B, is the best for measuring large resistances? Explain.

Problem authored by Anne J. Cox.

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