Capacitors: Using U=1/2QV

MY QUESTION IS PART (ii) BY THE WAY! :smile:

Q: A capacitor of capacitance 0.10F, initially charged to a P.D of 3V is connected to a resistor. For the period while the P.D drops from 3V to 1V,

i) Calculate the charge which flows from one capacitor plate to another.

I've found this to be 0.2C (from using Q=VC) which is correct

ii) Calculate the energy transferred from the Capacitor

Using U=1/2QV , i've found U = 0.5 x 0.2 x 2 = 0.2J

However the markscheme says to use U=1/2CV^2 instead, which ends up with U = 0.5 x 0.1 x 2^2 = 0.4J

Shouldn't using either U =1/2QV or U=1/2CV^2 come to the same answer? Or have i used the wrong value for something?

Reply 1

Zenarthra

3

Original post by minnigayuen

MY QUESTION IS PART (ii) BY THE WAY! :smile:

Q: A capacitor of capacitance 0.10F, initially charged to a P.D of 3V is connected to a resistor. For the period while the P.D drops from 3V to 1V,

i) Calculate the charge which flows from one capacitor plate to another.

I've found this to be 0.2C (from using Q=VC) which is correct

ii) Calculate the energy transferred from the Capacitor

Using U=1/2QV , i've found U = 0.5 x 0.2 x 2 = 0.2J

However the markscheme says to use U=1/2CV^2 instead, which ends up with U = 0.5 x 0.1 x 2^2 = 0.4J

Shouldn't using either U =1/2QV or U=1/2CV^2 come to the same answer? Or have i used the wrong value for something?