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Edexcel PHYSICS Doubts

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astarmathsandphysics:
It is the correct answer according to my calculation. I made it 1.45*10^-3

dk bose:
Sorry sir, i made a mistake finding it ,but now i got it!!! here's my next question -
A 2.2*10^-6F capacitor is charged to a potential difference of 15V, and a 3.3*10^-6F capacitor is charged to a potential difference of 30V.
a)Calculate -  1)the charge on each capacitor,  2)the energy stored by each capacitor.
b)Show that, once Charge has re-distributed, the ratio of charge on each capacitor is equal to the ratio of the two capacitances. Hence calculate the charge on each capacitor and the final potential difference.
c)Calculate the total energy stored by the capacitors and suggest why this is less
than the total energy that would be stored by the capacitors individually.

astarmathsandphysics:
the capacitor q answers

dk bose:
A student is designing an alarm circuit. She wants a time interval between closing a switch and the circuit becoming active. She decides to charge a 2200*10^-6F capacitor through a resistor from a 5V supply. The potential difference across the capacitor must rise to 4.3V for the alarm circuit to become active.
Calculate the size of the resistor that she should connect in the circuit to achieve a 1 minute delay.

astarmathsandphysics:
Solve V=V_0 e^{-t/RC)
4.3=5e^-60/(R*2.2*10^-3))
4.3/5=0.86 =e^-60/(R*2.2*10^-3))
-60/(R*2.2*10^-3)) =ln 0.86=-0.151
R=180000 ohms

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