4. [ 15 points ] A parallel plate capacitor is made from two aluminum foil sheet

Question

4. [15 points] A parallel plate capacitor is made from two aluminum foil sheets, each 6.3 cm width and 5.4 m long. Between the sheets is a Teflon strip of the same width and length that is 0.035 mm thick.

a. What is the capacitance of this capacitor? (Dielectric constant of Teflon is 2.1).

b. If the capacitor is charged with a battery of 240 volts, what will be the energy stored in the capacitor?

c. If the changed capacitor is connected to a small electric motor, to what height can the motor lift a 4.0 g mass assuming 100% efficiency?

Explanation / Answer

Capacitance = k x 0 x area / separation

k teflon 2.1
0 = 8.854 10^-12 F/m
area = 0.063 * 5.4 = 0.3402 m2
separation= 3.5*10^-5m

C = 2.1 * 8.854 10^-12 * 0.3402 / 3.5 10^-5 = 0.18072 10^-6 F = 0.18072µF

E =1/2(CV^2) = 5.2047*10^-3 J

PE.cap = 1/2 C V^2
PE.mass = mgh

PE capacitaor = PE of mass

h = 1/2 C V^2 /gm

h= 5.2047*10^-3 J / (9.81m/s * .004kg)
h = 0.1326 m

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