Energy in Capacitor: Find Charge & U

In summary, the conversation discusses the scenario of a 20 pF capacitor charged to 3.0 kV being connected to an uncharged 50 pF capacitor. The conversation includes questions about the new charge on each capacitor, the initial and final energy stored in the capacitors, and whether electrostatic potential energy is conserved, lost, or gained in this scenario. Relevant equations and possible approaches are also mentioned.
  • #1
trans_pie
5
0
1. A 20 pF capacitor is charged to 3.0 kV and then removed from the charger and connected to an uncharged
50 pF capacitor.
(a) what is the new charge on each capacitor?
(b) Find the initial energy stored in the 20 pF capacitor and the final energy stored in the two capacitors. Is electrostatic potential energy conserved, lost or gained when the two capacitors are connected together?


2. Relevant equations
c=q/v
u=1/2 cv^2
 
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  • #2
Well what have you tried so far?
 
  • #3
hehe, google search found me this, I'm guessing a fellow UQ-er.

For a), I wasn't sure whether to reason:
1) that because both capacitors are conductors, the charge will spread evenly between them
or,
2) that is will spread proportionately between them based on their capacitance.

as for b), I've been unable to find a pertinent equation. But I would imagine that electrostatic potential should be conserved.
is u electrostatic potential...?
 
  • #4
Conradical said:
hehe, google search found me this, I'm guessing a fellow UQ-er.

For a), I wasn't sure whether to reason:
1) that because both capacitors are conductors, the charge will spread evenly between them
or,
2) that is will spread proportionately between them based on their capacitance.
The second one is right if you're thinking along the lines of conservation of charge and common potential difference. But you need to be more explicit as to how exactly are the charges distributed.

as for b), I've been unable to find a pertinent equation. But I would imagine that electrostatic potential should be conserved.
is u electrostatic potential...?
This requires the first part. Use the formula for energy in capacitors and sum both up, then you'll be able to see if it is conserved or not.
 
  • #5
wonderful.

thankyou sir.
 

1. How is energy stored in a capacitor?

Energy is stored in a capacitor through the separation of charges on its two plates. When a capacitor is connected to a power source, electrons from the negative plate are attracted to the positive plate, creating an electric field between the plates. This electric field stores the energy in the capacitor.

2. How can I calculate the energy stored in a capacitor?

The energy stored in a capacitor can be calculated using the formula E = (1/2)CV^2, where E is the energy in joules, C is the capacitance in farads, and V is the voltage across the capacitor in volts. This formula can also be written as E = (1/2)QV, where Q is the charge stored on the capacitor in coulombs.

3. How do I find the charge on a capacitor?

The charge on a capacitor can be found by multiplying the capacitance of the capacitor (in farads) by the voltage across it (in volts). This can be represented by the formula Q = CV. Alternatively, the charge can also be found by dividing the energy stored in the capacitor (in joules) by the voltage across it (in volts), using the formula Q = E/V.

4. Can the energy stored in a capacitor be negative?

No, the energy stored in a capacitor cannot be negative. This is because energy is a scalar quantity and cannot have a negative value. However, the voltage across a capacitor can be negative, which can result in a negative value for the energy formula. In this case, the negative value indicates that the capacitor is discharging energy.

5. How does the distance between the plates affect the energy stored in a capacitor?

The distance between the plates of a capacitor, also known as the plate separation, directly affects the capacitance of the capacitor. The greater the plate separation, the lower the capacitance and therefore, the lower the energy stored in the capacitor. This is because a larger distance between the plates results in a weaker electric field and less energy being stored.

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