# Isaac physics question help

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#1

I have been trying to answer this question on Isaac physics (GCSE) for a while but cannot figure out how to do the 3rd part on 29.7
The answer to part a is 91000A
Part b is 5000A
Part d is 225MW
I have checked all of these answers on the website and they are correct however for some reason when I use the method that I used for d on part c I don’t get the correct answer. Does anyone have any ideas on how to answer this?
Last edited by Emily~3695; 6 months ago
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6 months ago
#2
Did you use I2R in part (d)

It would be the same formula in part (c) but with a different current.

Otherwise can you show us your working to see where the error is.
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#3
(Original post by Stonebridge)
Did you use I2R in part (d)

It would be the same formula in part (c) but with a different current.
That’s what I did which is why I don’t understand why it is the wrong answer
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#4
For part d I worked it out by doing
(5000)^2 x9 which got 225000000W so 225MW
When I tried to do it for part c by doing (91000)^2 x9 the answer is much larger than the original power ( 74529MW) and this answer is definitely wrong so I’m not sure what I did wrong
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6 months ago
#5
(Original post by Emily~3695)
For part d I worked it out by doing
(5000)^2 x9 which got 225000000W so 225MW
When I tried to do it for part c by doing (91000)^2 x9 the answer is much larger than the original power ( 74529MW) and this answer is definitely wrong so I’m not sure what I did wrong
Try using the non-round up value of current from (a).
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#6
(Original post by Eimmanuel)
Try using the non-round up value of current from (a).
This got me the right answer so thank you
I think I just expected it to use the rounded answer because it didn’t accept the actual answer for a, I guess I was just overcomplicating it a bit so thanks
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6 months ago
#7
I think the advice from all examining boards is just this.
Always use your exact (unrounded) intermediate result when you need it to calculate another value. Don't use the rounded value.
Only use the rounded value as a final answer, and always to the number of sig figs as advised.
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