Calculating E.M.F and Internal Resistance of a Cell in Series Circuit

[Scottie]

Homework Statement

A cell of unknown e.m.f. , ε , and internal resistance, r, is connected in series with a
variable resistor. A voltmeter is then connected across the terminals of the resistor.
When the resistor has a value of 16.0 Ω the voltmeter reads 1.20 V. When the
resistance is reduced to 8.0 Ω the voltmeter reading falls to 1.00 V.

Calculate the e.m.f. and internal resistance of the cell

Homework Equations

V=IR
emf = v + vr
emf = v + ir
(r = internal resistance)

The Attempt at a Solution

I (perhaps wrongly) worked out each current for the given voltages and resistances as

1.2/16= 0.075A
1/8 = 0.125A

I then drew a graph of voltage against current, (I think I may be wrong here also) to find that the e.m.f is 1.5v and the internal resistance(gradient with the two points of (current , voltage)) as -0.25.

But I cannot seem to get calculatiosn with these to add up for instance if the emf is 1.5 then v at either point 1.2 or 1 added to current x internal resistance should equate to 1.5.

Anyhelp would be much appreciated.

 

[rl.bhat]

Currents you have calculated can be written as I = ε /( r + R ). Write two equations for two currents and solve for r.

 

[Scottie]

Currents you have calculated can be written as I = ε /( r + R ). Write two equations for two currents and solve for r.

Hi, Thanks a lot.

I saw the other thread in which you helped https://www.physicsforums.com/showthread.php?t=352782

I was a little unsure if the currents were correct due to the location of the voltmeter but I have discovered and you have confirmed the location doesn't matter in this case.

Thanks again!