(1) A capacitor is said to carry a charge Q. What is the net charge on the entir
ID: 1709450 • Letter: #
Question
(1) A capacitor is said to carry a charge Q. What is the net charge on the entire capacitor? (Explain your answer).(2) A medical defibrillator stores 1000 J of energy in a 100 microF capacitor. What is the voltage across the capacitor?
(3) You are given a thin copper sheet of 20 cm x 40 cm, a piece of paper (see table 23.1) of 20 cm x 25 cm x 0.2mm, plus wires as needed. How would you construct a capacitor? (You could cut the copper sheet). What will be the largest capacitance that you can achieve? What will be the breakdown voltage of your capacitor?
Explanation / Answer
1) net charge on the entire capacitor is also Q
2) Given
energy stored in the capacitor U = 1000 J
capacitance of the capcitor C = 100F
energy stored in the capacitor is
U = ( 1/2) C V 2
1000 = 0.5 * 100F * V 2
V2 = 20 *10 6
V = 4472.135 V
3) Given
Area of copper sheet A = 20 *40 cm 2
dielectric constant of paper is K = 3.5
distance between plates d = 0.2mm
largest capacitance of the capacitor
C = o A K / d
= ( 8.85*10 -12 ) ( 20*40*10 -4 ) ( 3.5 ) / 0.2*10 -3
= 1.239*10 -8 F
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