# Can anyone help me solve these questions on capacitors?

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#### drew93

##### New Member
Can anyone help me with this please? Any help is greatly appreciated.

a. The plates of parallel-plate capacitor in vacuum are 0.5 cm apart and 1 m2 in area. Potential difference of 12 kV is applied across the capacitor. Calculate:
1. Capacitance
2. Charge on each plate
3. Magnitude of the electric field between the plates &
4. Energy density
5. Energy density (volume based) of a dry cell (1.5 V, 2500 mAh) and compare with that of the energy density calculated in 4.

b. Arrange the following in increasing order:
1. Power density value: Battery, Capacitor, Super capacitor and Fuel cell
2. Energy density value: Battery, Capacitor, Super capacitor and Fuel cell

#### birdman0_o

##### Active Member
Looks like homework, start by researching what equations you need. For part b its more common sense, or you can google energy density and power density.

#### drew93

##### New Member
Looks like homework, start by researching what equations you need. For part b its more common sense, or you can google energy density and power density.
Hello, I have the equations, I am just not sure where to plug in the numbers. I found the answer for capacitance, but for the other I do not know how to set the problem up. For energy density I have U = eoE2/2 . I know e not is 8.854 x 10 6 -12 but I dont know how to find E.

#### Pommie

##### Well-Known Member
Please note, it is called Homework Help, not Do my homework for me.

So with that in mind, question 1 should be easy. C=E0*A/d.

Where,
E0 = permattivity of a vacumn = 8.85 * 10^-12
A = area of plates
d = distance between plates.

You should get an answer around 200pF.

Mike.

#### drew93

##### New Member
I figured out the first 3. I just need help on the energy density problems. Could you help me set the equation up please?

Please note, it is called Homework Help, not Do my homework for me.

So with that in mind, question 1 should be easy. C=E0*A/d.

Where,
E0 = permattivity of a vacumn = 8.85 * 10^-12
A = area of plates
d = distance between plates.

You should get an answer around 200pF.

Mike.

#### birdman0_o

##### Active Member
For this question as I stated before, you do not know enough specifics to be able to use formulas, this is more of a research question.

#### MrAl

##### Well-Known Member
Can anyone help me with this please? Any help is greatly appreciated.

a. The plates of parallel-plate capacitor in vacuum are 0.5 cm apart and 1 m2 in area. Potential difference of 12 kV is applied across the capacitor. Calculate:
1. Capacitance
2. Charge on each plate
3. Magnitude of the electric field between the plates &
4. Energy density
5. Energy density (volume based) of a dry cell (1.5 V, 2500 mAh) and compare with that of the energy density calculated in 4.

b. Arrange the following in increasing order:
1. Power density value: Battery, Capacitor, Super capacitor and Fuel cell
2. Energy density value: Battery, Capacitor, Super capacitor and Fuel cell

Hello,

Energy density is the total energy per unit volume, so one way to do this assuming a uniform distribution of energy is to calculate the total energy and then divide that by the total volume.
For the two plate capacitor with large plate area over separation distance ratio we can probably assume negligible field fringing.

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