Practical Circuits (Physics Question)

This is a Physics question but there tends to be faster replies on the Math forum so here goes if anyone can help and show how you got to the answer I'd appreciate it.

Question . When a high resistance voltmeter is placed across an isolated battery, the reading is 3V. When a 10 ohm resistor is connected across the terminals of the battery, the voltmeter reading drops to 2.8V. What is the Internal Resistance of the battery.

Reply 1

DFranklin

Just as a one-off, I won't make an issue of this, but intentionally posting in the wrong forum is actually a 2pt warning offence.

As far as your question goes, model the battery as an 'ideal' battery supplying a voltage V, with a resistor of resistance R in series.

Then the reading across the isolated battery gives you the value of V, and then you can consider the scenario with the 10 ohm resistor as your resistance R and a 10ohm resistance in series. Then the current I through each resistor is the same, and since "V=IR" for each resistance, you can work out the voltage across each resistor (as a function of R). Since the sums of those voltages has to equal the value of V, you can find I and from there you can find R.