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# Capacitance watch

1. Hey. A quick question about capacitance , if I know the potential difference between two paralell plates whose area and distance is known before and after inserting a dielectric sheet, how do i calculate the initial capcitance ?

thanks guys
2. The standard formula for the capacitance of a parallel plate capacitor of plate area A and plate separation d is

and you say you know both A and d.
Epsilon 0 is the permittivity of free space.
If its air in there you can take the same value.

Maybe there is more to this question? You don't need to know the pd from the information given.
3. (Original post by Stonebridge)
The standard formula for the capacitance of a parallel plate capacitor of plate area A and plate separation d is

and you say you know both A and d.
Epsilon 0 is the permittivity of free space.
If its air in there you can take the same value.

Maybe there is more to this question? You don't need to know the pd from the information given.
cheers for the help, the pd drops from 2000V to 1000V when a dielectric material is placed between the parallel plates ( whose area is 2 squared metres and distance between is 1 cm) . how do i calculate the orginial capacitance ?
4. (Original post by Stonebridge)
The standard formula for the capacitance of a parallel plate capacitor of plate area A and plate separation d is

and you say you know both A and d.
Epsilon 0 is the permittivity of free space.
If its air in there you can take the same value.

Maybe there is more to this question? You don't need to know the pd from the information given.
the standard formula for capacitance has e0 x er where er is the relative permitivity. Air has a relative permitivity of just over 1, so the above equation gives the right answer .....for an air gap capacitor. Stick in a different dielectric and you really need to stick in the er term.
5. The change in voltage means that the capacitance has changed to twice the original value. C=Q/V so if V halves (while Q is assumed to stay the same) then C has doubled. This also tells me the dielectric constant (relative permittivity) was 2
But
The answer to your question is the same as I gave in my first post.
Use the formula I gave you for the capacitance in terms of e0, A and d
You have values given for A and d.
Do you have a value for epsilon 0? It's a standard constant equal to
8.85 x 10[sup]-12 Fm-1

And as the poster above has pointed out, the relative permittivity of air is very nearly equal to 1 (er=1.0005) so it's ok to use that formula.

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