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Problem 11: The voltmeter in the circuit to the right may be considered to ideal. The values are =15.0 V internal resistance r = 5.00 Ω R1 = 100 Ω and R2 = 300 Ω. Calculate the current through R1.

Solution

(An ideal voltmeter means infinite resistance, so that no current flows through it. )

Strategy: We apply Ohm’s law to solve the problem. An equivalent resistance has to be found, then the current in the circuit. As R1 and R2 are in parallel, the current in the required resistor would be found accordingly.
Now the equivalent resistance R3 of the parallel resistors is

Substitute the values,

Similarly, R3 and r are in series and the equivalent resistance of the circuit is

To find the current in the circuit we apply Ohm’s law

Therefore, the voltage across R3 will be

The same potential difference would be across R1. Therefore, current in the said resistor

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