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# e.m.f and other electricity questions!

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1. Hi,

Currently studying AQA Physics A, year 12. I have my Unit One exam coming up soon and am just reviewing the content. When given a question such as this:
'The p.d across the terminals of a cell was 1.1V when the current from the cell was 0.20A, and 1.3V when the current was 0.10A. Calculate a) the internal resistance of the cell and b) the cells' e.m.f'

I correctly answered the first part of the question, by essentially taking a gradient. Do I actually need to plot a graph for the second part, or is there another way to work it out? As far as I'm aware, on a graph of terminal p.d vs. current the emf is when you extrapolate back so the terminal p.d is zero?

Finally, has anyone got any good electricity resources that explain the content quite simply? Thank you :P
2. If you've drawn a graph then the EMF should be the y-incercept... if I'm thinking correctly! :P
3. (Original post by Xiomara)
Hi,

Currently studying AQA Physics A, year 12. I have my Unit One exam coming up soon and am just reviewing the content. When given a question such as this:
'The p.d across the terminals of a cell was 1.1V when the current from the cell was 0.20A, and 1.3V when the current was 0.10A. Calculate a) the internal resistance of the cell and b) the cells' e.m.f'

I correctly answered the first part of the question, by essentially taking a gradient. Do I actually need to plot a graph for the second part, or is there another way to work it out? As far as I'm aware, on a graph of terminal p.d vs. current the emf is when you extrapolate back so the terminal p.d is zero?

Finally, has anyone got any good electricity resources that explain the content quite simply? Thank you :P
y = mx + c

emf=c when y=0.
Spoiler:
Show

So m as you figured out, is the gradient of the line.
Delta y over delta x
so
(1.1-1.3) / (0.2-0.1) = - 0.2/0.1 = -2 ohm

Therefore, y = -2x + c

Take any point

So 1.1= -2 (0.2) + c
1.1 = - 0.4 + c
c = 1.1 + 0.4

c = 1.5 V

emf = 1.5V
4. (Original post by Xiomara)
Hi,

Currently studying AQA Physics A, year 12. I have my Unit One exam coming up soon and am just reviewing the content. When given a question such as this:
'The p.d across the terminals of a cell was 1.1V when the current from the cell was 0.20A, and 1.3V when the current was 0.10A. Calculate a) the internal resistance of the cell and b) the cells' e.m.f'

I correctly answered the first part of the question, by essentially taking a gradient. Do I actually need to plot a graph for the second part, or is there another way to work it out? As far as I'm aware, on a graph of terminal p.d vs. current the emf is when you extrapolate back so the terminal p.d is zero?

Finally, has anyone got any good electricity resources that explain the content quite simply? Thank you :P
The terminal pd, V, is the emf minus the volts dropped across the internal resistance.
The voltage dropped across the internal resistance r, is Ir, so if the emf is E you get

1.1 = E - 0.2r --- for the 1st case
and
1.3 = E - 0.1r --- for the second

Then it's just 2 (simultaneous) equations with 2 unknowns.
No need for graphs, gradients or intercepts.
5. (Original post by Stonebridge)
The terminal pd, V, is the emf minus the volts dropped across the internal resistance.
The voltage dropped across the internal resistance r, is Ir, so if the emf is E you get

1.1 = E - 0.2r --- for the 1st case
and
1.3 = E - 0.1r --- for the second

Then it's just 2 (simultaneous) equations with 2 unknowns.
No need for graphs, gradients or intercepts.
(Original post by TheGrinningSkull)
y = mx + c

emf=c when y=0.
Spoiler:
Show

So m as you figured out, is the gradient of the line.
Delta y over delta x
so
(1.1-1.3) / (0.2-0.1) = - 0.2/0.1 = -2 ohm

Therefore, y = -2x + c

Take any point

So 1.1= -2 (0.2) + c
1.1 = - 0.4 + c
c = 1.1 + 0.4

c = 1.5 V

emf = 1.5V

Thank you both so much! Positive rep for you both

I was wondering, could I please request your help for another question?

'Two 8 ohm resistors and a battery of emf 12.0V and internal reistance 8 ohms are connected in series with each other. Sketch the circuit diagram and calculate i. the power delivered to each external resistor, ii. the power wasted due to internal resistance'

This part of the question, I got correct answers of 2W and 2W, but the second part has me stumped.
'The two 8 ohm resistors in a) are connected in parallel with each other then connected to the same battery. Sketch the circuit diagram and calculate i. the power delivered to each external resistor, ii. the power wasted due to internal resistance'

I worked out the resistance of the resistors in parallel (4 ohms) new cell current = emf/total resistance, so 0.75A, then did I^2xR and I^2xr respectively to answer the questions, ending up with 2.25W and 4.5W. However, upon looking at the answers in the back of the book, the correct values are 2W and 8W. Where have I gone wrong?
6. (Original post by Xiomara)
Thank you both so much! Positive rep for you both

I was wondering, could I please request your help for another question?

'Two 8 ohm resistors and a battery of emf 12.0V and internal reistance 8 ohms are connected in series with each other. Sketch the circuit diagram and calculate i. the power delivered to each external resistor, ii. the power wasted due to internal resistance'

This part of the question, I got correct answers of 2W and 2W, but the second part has me stumped.
'The two 8 ohm resistors in a) are connected in parallel with each other then connected to the same battery. Sketch the circuit diagram and calculate i. the power delivered to each external resistor, ii. the power wasted due to internal resistance'

I worked out the resistance of the resistors in parallel (4 ohms) new cell current = emf/total resistance, so 0.75A, then did I^2xR and I^2xr respectively to answer the questions, ending up with 2.25W and 4.5W. However, upon looking at the answers in the back of the book, the correct values are 2W and 8W. Where have I gone wrong?
Check each section you go through in turn. So check to resistance of the parallel resistors.
Check the total resistance, check the current, check the power and go in that order.

Spoiler:
Show

Check the total resistance, seems like a basic error methinks.
7. (Original post by TheGrinningSkull)
Check each section you go through in turn. So check to resistance of the parallel resistors.
Check the total resistance, check the current, check the power and go in that order.

Spoiler:
Show

Check the total resistance, seems like a basic error methinks.
Thank you! But I still don't understand, how did I incorrectly calculate it?
8. (Original post by Xiomara)
Thank you! But I still don't understand, how did I incorrectly calculate it?

From there, what should the current be?

Maybe you didn't draw the diagram correctly so maybe that is confusing you?

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