# Internal resistance & emf

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#1
The question I have been set is:

The pd across the terminals of a cell was 1.1V when the current from the cell was 0.2A and 1.3V when the current was 0.1A. Calculate
a) The internal resistance of the cell
b) The cell's emf

I've come back to this a few times over the last few days & I'm just stuck. Looking at the formulas we've learned I need emf to calculate the internal resistance & I need the internal resistance to calculate emf. I'm sure it's something silly I just haven't got my head around! Can anyone help?
0
6 years ago
#2
(Original post by lovali)
The question I have been set is:

The pd across the terminals of a cell was 1.1V when the current from the cell was 0.2A and 1.3V when the current was 0.1A. Calculate
a) The internal resistance of the cell
b) The cell's emf

I've come back to this a few times over the last few days & I'm just stuck. Looking at the formulas we've learned I need emf to calculate the internal resistance & I need the internal resistance to calculate emf. I'm sure it's something silly I just haven't got my head around! Can anyone help?
You need to write the formula twice, once for each of the sets of values you have, and solve as simultaneous equations.

The formula should be
V = E - Ir
where V is terminal pd (given each time), E is emf, I is current (given each time) and r is internal resistance.
Plug the numbers in and solve simultaneously for two unknowns.

Do you know how to solve simultaneous equations?
0
#3
(Original post by Stonebridge)
You need to write the formula twice, once for each of the sets of values you have, and solve as simultaneous equations.

The formula should be
V = E - Ir
where V is terminal pd (given each time), E is emf, I is current (given each time) and r is internal resistance.
Plug the numbers in and solve simultaneously for two unknowns.

Do you know how to solve simultaneous equations?
That would explain things! Yet again we have been set a question that involves things we haven't touched on in class.

Last time I did simultaneous equations was in high school nearly 20 years ago This is the first year the college has offered Physics at access so we've run into these issues a fair amount.

I've had a look at a site that solves it & takes you through the steps used & it really doesn't make sense.
0
6 years ago
#4
(Original post by lovali)
That would explain things! Yet again we have been set a question that involves things we haven't touched on in class.

Last time I did simultaneous equations was in high school nearly 20 years ago This is the first year the college has offered Physics at access so we've run into these issues a fair amount.

I've had a look at a site that solves it & takes you through the steps used & it really doesn't make sense.
It may help with the simultaneous equations to ask on the maths forum.
I hope you'll find that the high school maths soon comes back to you.
We'll help here with the physics if you have any problems. It will help us to help you when you post if you can say how far you've got with a problem and what it is you don't understand.
Good luck.
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