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# Capacitance units help

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1. Hi,

I'm just doing some summary questions from the textbook. For the last question I got the right values for the answers but the wrong order of magnitude and I'm not sure what I'm doing wrong. I feel like it's to do with units but I've got the other questions right using the same method.

A capacitor is charged by using a constant current of 24microamps to a pd of 4.2 in 38s. It is then charged from 4.2V by using a current of 14microamps in 50s.

i. Charged stored at a pd of 4.2
Q=It = 24×38= 912micro coulombs (answer given in the back as 0.91 micro coulombs)

ii. Capacitance = Q/V= 0.91/4.2 = 0.22 micro Farads. (Correct)

iii. The extra charge stored at a current of 14 micro amps.
Q=It= 14×50= 700 micro coulombs (correct)

iv. The pd after the extra charge was stored
V= Q/C = 700/0.22 = 3200 which should be 3.2 as the final answer is 3.2+4.2=7.4

Any help would be really apreciated
2. (Original post by VioletPhillippo)
Hi,

I'm just doing some summary questions from the textbook. For the last question I got the right values for the answers but the wrong order of magnitude and I'm not sure what I'm doing wrong. I feel like it's to do with units but I've got the other questions right using the same method.

A capacitor is charged by using a constant current of 24microamps to a pd of 4.2 in 38s. It is then charged from 4.2V by using a current of 14microamps in 50s.

i. Charged stored at a pd of 4.2
Q=It = 24×38= 912micro coulombs (answer given in the back as 0.91 micro coulombs)

ii. Capacitance = Q/V= 0.91/4.2 = 0.22 micro Farads. (Correct)

iii. The extra charge stored at a current of 14 micro amps.
Q=It= 14×50= 700 micro coulombs (correct)

iv. The pd after the extra charge was stored
V= Q/C = 700/0.22 = 3200 which should be 3.2 as the final answer is 3.2+4.2=7.4

Any help would be really apreciated
Remember it is 700 micro coulombs, so just needs to be converted to coulombs
3. (Original post by solC)
Remember it is 700 micro coulombs, so just needs to be converted to coulombs
Thanks for your help. Annoyingly it still doesn't work as it becomes 1000 times too small.
4. (Original post by VioletPhillippo)
Thanks for your help. Annoyingly it still doesn't work as it becomes 1000 times too small.
I think you're having unit problems...

The best way to work is to *always* convert everything to SI units at the start, then do all your calculations, then convert back (if required).

The problem with working with a mixture of μF μA and whole volts is that sometimes it will appear to work correctly and sometimes you'll get an answer that is orders of magnitude out - which is exactly what's happened to you here
5. From what I am hearing you need help converting. I always convert units for ease at the start. Just incase you don't know then to convert micro to normal it is times ten to the negative 6 (x10^-6)
6. (Original post by .........Jeff458)
From what I am hearing you need help converting. I always convert units for ease at the start. Just incase you don't know then to convert micro to normal it is times ten to the negative 6 (x10^-6)
Drawing capacitance/voltage graphs for capacitors in higher physics last year was the good life.

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7. (Original post by RossB1702)
Drawing capacitance/voltage graphs for capacitors in higher physics last year was the good life.

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Yeahhhhhhh. Can't even remember fun like that. Good times...
8. (Original post by .........Jeff458)
Yeahhhhhhh. Can't even remember fun like that. Good times...
How's physics going for you so far ? I'm actually quite enjoying it partly because my teachers is sound as. About to finish unit 1 and probably have our nab soon after October break.

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9. (Original post by VioletPhillippo)
Thanks for your help. Annoyingly it still doesn't work as it becomes 1000 times too small.
Ah sorry that was the only one I instantly noticed, but it looks like from the start you've forgotten to convert units from micro
10. (Original post by RossB1702)
How's physics going for you so far ? I'm actually quite enjoying it partly because my teachers is sound as. About to finish unit 1 and probably have our nab soon after October break.

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Nice. I did't take Physics this year (Bio, Chem, Maths). Pretty stupid idea considering Physics was my favourite subject last year!!!
11. (Original post by .........Jeff458)
Nice. I did't take Physics this year (Bio, Chem, Maths). Pretty stupid idea considering Physics was my favourite subject last year!!!
Those subjects are probably more relevant for medicine except maths. Doubt you need to study advanced higher maths or physics for medicine. Honestly you would have liked physics this years it's so interesting for example currently we are learning about the power output giving off by large stars in space, Einsteins Theory of general relativity which expands on higher physics special relativity, black holes, rotational motion when objects travel around circular shapes, gravitation for objects that are affected by gravity in our atmosphere and other planets.

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12. (Original post by Joinedup)
I think you're having unit problems...

The best way to work is to *always* convert everything to SI units at the start, then do all your calculations, then convert back (if required).

The problem with working with a mixture of μF μA and whole volts is that sometimes it will appear to work correctly and sometimes you'll get an answer that is orders of magnitude out - which is exactly what's happened to you here
Thanks. I'm just confused because for example in part a. I've worked the answer out in microamps as this is the unit given in the question. The answer I got was 912 microamps but the answer is given as 0.91microamps.
13. (Original post by VioletPhillippo)
Thanks. I'm just confused because for example in part a. I've worked the answer out in microamps as this is the unit given in the question. The answer I got was 912 microamps but the answer is given as 0.91microamps.
Oh sorry - it's an error in your book... even had another thread about it a while ago... http://www.thestudentroom.co.uk/show....php?t=2021236
14. (Original post by Joinedup)
Oh sorry - it's an error in your book... even had another thread about it a while ago... http://www.thestudentroom.co.uk/show....php?t=2021236
Thanks, I didn't spot that thread

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