# Question Video: Calculating a Resistance from Current and Potential Difference

Calculate the effective resistance of a pocket calculator that has a 1.35 V battery if the current through the calculator is 0.200 mA.

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### Video Transcript

Calculate the effective resistance of a pocket calculator that has a 1.35-volt battery if the current through the calculator is 0.200 milliamps.

If we were to take a battery power pocket calculator and unscrew the cover and expose the circuitry beneath, we might find a fairly complicated electrical circuit. But since we’re solving for the effective resistance of the pocket calculator, we don’t need to understand most of that complexity in this case. All we need to know is that the circuit has a certain potential difference, 1.35 volts, across it and that it overall carries a current of 0.200 milliamps.

We can use Ohm’s law which tells us that the potential difference in the circuit is equal to the current in it multiplied by its effective resistance and rearrange this equation to solve for that effective resistance which we’ll call 𝑅 sub eff. That resistance is equal to 1.35 volts divided by 0.200 times 10 to the negative third amps. And that fraction is equal to 6750 ohms. However, many resistors are part of this calculator circuit. That’s the effective resistance of all of them combined.