a/s emf physics questions stumped me, appreciate any help

3. The pd across the terminals of a cell was 1.1v, and the current from the cell was 0.2A, and 1.3V when the current was 0.10amps. Calculate :

The internal resistance of the cell
b. The cells EMF

4. A battery of unknown emf and internal resistance r is connected in series with an ammeter and a resistance box R. The current was 2.0A when R = 4.0ohms, and 1.5A when r = 6.0 ohms, calculate the internal resistance, and emf.

You have two unknown values, EMF and internal resistance, so you would need at least two equations. In fact, you can write down the equation for each set of values ((1.1V, 0.2A) and (1.3V, 0.1A)).

The pd across the terminals of a cell is $\text{(electromotive force)}-\text{(potential drop across internal resistance)}$, and potential drop across internal resistance is $\text{(current passing through internal resistance)} \times \text{(internal resistance)}$.





Are you sure that you wrote down this question exactly as it was given originally? Didn't you mean small "r" to be "R"?

If you did mean so, then you can use the Ohm's law which says that the current in the circuit is electromotive force over total resistance (i.e. internal resistance plus external resistance). Again, you have two unknown values and can write down two equations, one for each set of given values.

Thanks for the help, i meant "r", i am still a little bit confused, although your post is helpful