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Capcitors

How do i work out capacitance? I've got the time and voltage. I used a data logger in my circuit and a smoothing unit and i didn't use a resistor of any sort because my teacher said the data logger has enough resistance in it. So hw do i workout capacitance? I plotted a graph with time along the x axis and the log of V on the y axis, does the gradient give me the capactitance?
Reply 1
Avatar for moojoo
moojoo
8
C=Q/V

use Q= It using time you got an current to work out charge

and you have voltage

and there you go!
Reply 2
Avatar for microwave69
microwave69
OP
1
moojoo
C=Q/V

use Q= It using time you got an current to work out charge

and you have voltage

and there you go!


i dont have cthe current though....... i used in my circuit:
power pack 12v
electrical leads
my capacitor
smoothing unit
dataloger - deltronics extra sense
voltage thing (connected to datalogger on 20mv)

thats it!
Reply 3
Avatar for Jonathan
Jonathan
12
hang on... was the data logger in series or parallel with the capacitor? Did you start with the capacitor shorted and then start recording data when you disconnected, or did you start with an open circuit or something else shorted?

I think I know which you'll need to use.
Reply 4
Avatar for microwave69
microwave69
OP
1
um im not actually sure :s-smilie: to get the the computer to start recording the results i just pulled one of the leads out of the smoothing unit thus disconnecting the power supply and measuring the capcitance of my capcitor
Reply 5
Avatar for microwave69
microwave69
OP
1
okay ive kind of sorted it out, i just need R (resistance) now, thing is,how do i measure the resistance of the data logger OR does any one know the resistance of the Deltronics Extra Sense data logger??
Reply 6
Avatar for Jonathan
Jonathan
12
It's ridiculous that your teacher told you to use that instead of a resistor. Presumably it was connected across the capacitor, and when this RC system was disconnected from the supply, the cap discharged through this resistor (the voltmeter/logger). It should have near-infinite resistance anyway!

The thing is, if you don't have resistance, you won't be able to work out capacitance, because you have two unknowns in the equation: R and C.

The equation you'd use would be

V at any time = Vo e-t/RC

where Vo is the initial voltage across the resistor and capacitor before the discharging, t is the amount of time it's been discharging for, and RC is the resistance times capacitance, or 'time constant', of the system.

You could, however, work out the 'time constant' RC by rearranging the equation:

V = Vo e-t/RC
ln V = ln Vo + -t/RC
ln Vo - ln V = t/RC
ln (Vo / V) = t/RC

RC = t / ln (Vo / V)

and put in any result taken at any point in time during the experiment, because it should work.

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