# C-R time constant question

Discussion in 'Homework Help' started by fan_boy17, May 9, 2012.

1. ### fan_boy17 Thread Starter New Member

Apr 17, 2012
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The capacitor is initially uncharged. the switch is moved to position 1 first for 1.5s and then after 1.5s the switch is moved to position 3 and remains there for 3.5s.
What is the maximum instantaneous energy stored in the capacitor during the period from t = 0 to t = 5 s?

how do i go about solving this problem. Does the capacitor reach its max voltage at 1.5s since in position 3 the capacitor is being discharged.

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2. ### StayatHomeElectronics AAC Fanatic!

Sep 25, 2008
1,020
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Given the circuit setup when the switch is in position 1, you should be able to determine the length of time it takes for the capacitor to charge. It should be a function of the resistor and capacitor in the circuit.

3. ### fan_boy17 Thread Starter New Member

Apr 17, 2012
12
0
it doesnt reach 5 time constant. the switch is moved to position 3 after 1.5s so what i get for the voltage across the capacitor after 1.5s is 1.2 x 10^-5 V. since the capacitor will be discharged when in position 3 the maximum voltage will hence be 1.2x10^-5 V.
which gives Energy= (0.5)(5)(1.2x10^-5)^2.

is this correct?

4. ### StayatHomeElectronics AAC Fanatic!

Sep 25, 2008
1,020
71
That does appear to be correct, although I did not verify the actual voltage number. The components give a rather large time constant, which results in a rather small voltage charge in 1.5s. The instantaneous is given by Energy = 0.5*C*V^2, which you have calculated.

5. ### WBahn Moderator

Mar 31, 2012
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7,691
I strongly suspect that the capacitance is supposed to be 5µF and not 5F. Look at the space between the '5' and the 'F'; I think they intended to go back and insert a 'µ' manually. But, you pretty much have to go with what you've got.

The only issue I have with your solution is that you won't track units:

Energy= (0.5)(5F)(1.2x10^-5V)^2 = 30pJ.

6. ### #12 Expert

Nov 30, 2010
18,076
9,691
I just blogged the formulas for RC time constants. My blog #3