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A potentiometer measures the emf of an old cell as 1.50 V…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A potentiometer measures the emf of an old cell as . A voltmeter of resistance joined across the same cell reads . Find the internal resistance of the cell and the current that the voltmeter draws. Why does the potentiometer give the true emf?
Need a reminder? The key ideaShow
The gradient is , and the potential at a distance is . At balance , so . A balance with the cell open and with across it gives . A voltmeter of resistance across a cell reads .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The internal resistance. From , we get .
Step 2. The current. .
Step 3. The lost volts. , which is the missing from the reading.
Why the potentiometer is true. At balance no current flows through the cell, so and the cell's full emf is balanced against the wire.
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