energy in capacitor

用戶37261

2013-04-10 10:09 pm

Considering a parallel-plate capacitor
Case1: capacitor is connected to a battery (V=constant)
Energy, E=CV²/2
--> E increase when C increase
--> E increase when adding dielectric(ε), increasing overlapping area(A), decreasing distance(d) or connecting another capacitor in parallel.
When we put a dielectric btw the plated, increase A or decrease d, we need to do work.
Q1: Does the work we do is totally equal to the difference btw initial energy and final energy?
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Case2: capacitor is isolated (Q=constant)
Energy, E=Q²/2C
--> E increase when C decrease
--> E increase when removing dielectric, decreasing A, increasing d. When we do these actions, we need to do work.
Q2: same with Q1
Q3: Does the energy inside the capacitor really increase when doing these actions?
Q4: What will happen to the energy when we continue increasing the distance btw the plates?

回答 (1)

天同

2013-04-12 11:14 pm

✔ 最佳答案

Case 1

In fact, because the capacitor is connected to battery, whcih supplies energy to the capacitor. Any increase of charge, and hence energy, comes from the battery.

Case 2
Since the capacitor is isolated, work done by an external agent is required to increase the energy stored in the capacitor.

For example, a force is needed to increase the separation between the plates, as work needs to be done against the attraction of the two plates that carry charges of opposite sign.

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