# Capacitance units help

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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

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

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#2

(Original post by

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=

iv. The pd after the extra charge was stored

V= Q/C =

Any help would be really apreciated

**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=7.4Any help would be really apreciated

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(Original post by

Remember it is 700 micro coulombs, so just needs to be converted to coulombs

**solC**)Remember it is 700 micro coulombs, so just needs to be converted to coulombs

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#4

(Original post by

Thanks for your help. Annoyingly it still doesn't work as it becomes 1000 times too small.

**VioletPhillippo**)Thanks for your help. Annoyingly it still doesn't work as it becomes 1000 times too small.

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

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#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)

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#6

(Original post by

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)

**.........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)

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#7

(Original post by

Drawing capacitance/voltage graphs for capacitors in higher physics last year was the good life.

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**RossB1702**)Drawing capacitance/voltage graphs for capacitors in higher physics last year was the good life.

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#8

(Original post by

Yeahhhhhhh. Can't even remember fun like that. Good times...

**.........Jeff458**)Yeahhhhhhh. Can't even remember fun like that. Good times...

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#9

**VioletPhillippo**)

Thanks for your help. Annoyingly it still doesn't work as it becomes 1000 times too small.

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#10

(Original post by

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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**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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#11

(Original post by

Nice. I did't take Physics this year (Bio, Chem, Maths). Pretty stupid idea considering Physics was my favourite subject last year!!!

**.........Jeff458**)Nice. I did't take Physics this year (Bio, Chem, Maths). Pretty stupid idea considering Physics was my favourite subject last year!!!

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(Original post by

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

**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

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#13

(Original post by

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.

**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.

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(Original post by

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

**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

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