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## Homework Statement

Power supply of EMF = E and internal resistance of R. Need E and R.

This is connected to resistor of 10'000 Ohm. The current drawn from the power supply is 0.91A.

A voltmeter of internal resistance 10'000 Ohm is connected in parallel to the resistor. The voltmeter reads 8.3 Volts.

Original question (number 2) can be found here:

https://www.dropbox.com/s/n842i3ojnanhtxc/2013-01-13 14.37.43.jpg

## Homework Equations

V = IR and Terminal PD = E - IR, presumably.

## The Attempt at a Solution

Overall plan: Use the two situations to set up two simultaneous equations, then solve for E and R.

I'm assuming the questions is saying that the drawing of the current was independent of connecting the voltmeter.

From 1st part:

V = IR,

R = 10'000 Ohm

I = 0.91A

So V = 9'100 V

V = terminal PD so:

9'100 = E - 0.91×R (Call this 1)

From 2nd part:

Total resistance of Voltmeter and resistor is:

1/10000 + 1/10000 = 1/5000 so R_tot = 5000 Ohm.

Voltage measured is terminal voltage (8.3V)

Current is I = V/R = 8.3/5000 = (C 0.00166 A

So:

8.3 = E - 0.00166×R (Call this 2).

HOWEVER: combining 1 and 2 gives a negative internal resistance!

Any ideas what I've done wrong?

Thanks guys!

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