- #1

oxon88

- 176

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## Homework Statement

A multi-plate variable capacitor has 4 pairs of plates. The plates, when closed, are separated in air by 0.01mm. A capacitance range of 10 - 400 pF is required

a) estimate the required radius, R of each plate.

b) The capacitor is set to the maximum of 400 pF and is charged to 10v throught a 50KΩ resistor. determine: i) the initial value of current flowing. ii) the time constant for the circuit.

## Homework Equations

capacitance = ε

_{r}ε

_{0}A(n-1) / d

Initial current = V/R

Time constant = R x C

## The Attempt at a Solution

a) 400x10

^{-12}= ε

_{r}ε

_{0}A(8-1) / (1x10

^{-5})

can anyone explain what these 2 values are? (ε

_{r}& ε

_{0}) i think one is dialectric strength. which for air is 3MV m

^{-1}?

b)

i) 10V/50Kohm = 0.2mA

ii) (50x10

^{3}) x (400x10

^{-12}) = 20 μs