1. The problem statement, all variables and given/known data Attached is the problem. This is the June 2010 AS Physics exam unit 4 question 16 (Edexcel) 2. Relevant equations See the attachments. 3. The attempt at a solution Q 16. (a) The voltage is always positive and so although the current must vary it is always positive and so the current is always traveling in the same direction. Therefore the graph shows a dc current. No problem here. (b) (i) "The capacitor stores charge" is the answer in the marking scheme, how does this answer the question? Does this mean that the voltage is then always constant and doesn't vary? (ii) E = 1/2 CV^2 this is easy! (c) (i) The capacitor charges and discharges. The graph appears to show the voltage increase exponentially as would be expected for a capacitor, but it also seems to discharge linearly. But should a capacitor not discharge exponentially? Also is it true that a capacitor in parallel with a power supply will away charge and then discharge continuously? This seems to be a different type of RC circuit where the capacitor and resistor are in parallel not in series. (ii) The marking scheme suggests using the V=V_0 exp(-t/RC) where t is substituted with 6x10^-3 seconds. How is this read from the graph? The graph seems to repeat itself every 10 ms, taking 4 ms to charge and 6 ms to discharge. I presume this is where the 6 ms comes from but why? The voltage seems to change from 3v to 5V so that R=6x10^-3/(10x10^-6)ln(3/5). C is known (C = 10x10^-6F) and so R can be found. (iii) Increase the capacitor, this is fine.