E.M.F and internal resistance

1. The problem statement, all variables and given/known data

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

2. Relevant equations

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

3. 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.


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