Potential Difference across capacitors in parallel.

[OUmecheng]

Homework Statement

Two capacitors, C1 = 20 micro Farads and C2 = 5 Micro Farads are connected in parallel and the resulting combination is connected to a 12-V battery. Find (a) the Equivalent capacitance of the combination, (b) the electric charge on each capacitor, (c) the potential difference across each capacitor, and (d) the energy stored in each capacitor.

Homework Equations

Ceq = C1 + C2

Q = CV

Ec = QV/2 = (C(V^2))/2 = (Q^2)/2C

The Attempt at a Solution

(a) 2.0x10^-6 F + 5x10^-6 F = 2.5x10^-5 F(b) Q = CV,

Q1 = (20x10^-6 F)(12V) = 2.4x10^-4 C

Q2 = (5x10^-6 F)(12V) = 6x10^-5 C(c) Not quite sure what this is asking for?(d) Ec = CV^2/2

Ec1 = c1V^2 / 2 = 0.00144 F*V^2

Ec2 = c2V^2 / 2 = 3.6x10^-4 F*V^2

As you can see I've worked most of this out, seems simple enough, but I don't know what to do for (c)?

 

[kuruman]

It's simpler than you think. Draw yourself the circuit and look at it.

 

[OUmecheng]

It's simpler than you think. Draw yourself the circuit and look at it.

errrr charge over capacitance?

 

[kuruman]

How did you do part (b)? What voltage did you assume across each capacitor?

 

[kuruman]

errrr charge over capacitance?

Please don't edit previous posts. Just post a new response.

Your "attempt at a solution for (b) says


(b) Q = CV,

Q1 = (20x10^-6 F)(12V) = 2.4x10^-4 C

Q2 = (5x10^-6 F)(12V) = 6x10^-5 C

Tell me in English what do the numbers highlighted in red represent?

 

[OUmecheng]

Please don't edit previous posts. Just post a new response.

Your "attempt at a solution for (b) says


(b) Q = CV,

Q1 = (20x10^-6 F)(12V) = 2.4x10^-4 C

Q2 = (5x10^-6 F)(12V) = 6x10^-5 C

Tell me in English what do the numbers highlighted in red represent?

Battery Voltage/Potential Difference?

 

[kuruman]

Battery Voltage/Potential Difference?

It is not a ratio. In plain English the equation Q = CV means

The charge on a capacitor is the same as the capacitance multiplied by the potential difference (or voltage) across the terminals of the capacitor.

Can you answer (b) now?

 

[OUmecheng]

It is not a ratio. In plain English the equation Q = CV means

The charge on a capacitor is the same as the capacitance multiplied by the potential difference (or voltage) across the terminals of the capacitor.

Can you answer (b) now?

So the potential difference is the same as charge on a capacitor divided by the capacitance.

V1= Q/C1
V2= Q/C2

Thanks!

 

[kuruman]

Actually, you are doing it backwards. Memorize this definition

"Two circuit elements in parallel share the same potential difference (or voltage) across their terminals"

and use it constructively.

Cheers.