University:
Kentucky State University
Course:
PHY 212 | General Physics II
Academic year:

2023
Views:

175

Pages:

5
Author:

Athena Harrison

Energy Storage in Capacitors (defibrillator shock!!) work to charge a capacitor: • capacitor already has charge q, voltage (difference) V • move extra charge element dq from one plate to the other • external work required: dW = dq V. V + - q dW  V dq  dq C from q=CV • start with zero charge, end up with Q: Q Q 0 0 W   dW   dq + 2 Q q q Q2 dq   . C 2C 0 2C +q -q • work required to charge the capacitor = change in potential energy U f  Ui  Wext • when starting from empty capacitor: U i  0 potential energy stored in capacitor: Using Q=CV, three equivalent expressions: Q 2 CV 2 QV U   . 2C 2 2 All three equations are valid; use the one most convenient for the problem at hand. Q2 U . 2C Quiz: the slope is: 1/C Example: a camera flash unit stores energy in a 150 F capacitor at 200 V. How much electric energy can be stored? CV 2 U 2 U 6 2 150  10 200    2 J U3 J If you keep everything in SI (mks) units, the result is “automatically” in SI units. Where does the stored energy reside? Energy is stored in the capacitor: A C 0 and V  Ed d 1 2 U  C  V  2 V + E 1  0 A  2 U   Ed   2 d  1 U    0 Ad  E 2 2 The “interior volume of the capacitor” is Ad - +Q d -Q area A energy density u (energy per unit volume): 1 0 Ad  E 2  U 2 1 u   0 E 2 Ad Ad 2 energy resides in the electric field between the plates 1 u  0 E 2 2 The amount of energy density transferred by the electric waves is proportional to the square of the magnitude of the electric field. V + - E +Q d -Q area A

Energy Storage in Capacitors

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