Edexecel Physics Unit 4 January 2013 Revision Thread

i really dont get it could u like explain how i select the appropriate scale. for example should i select 1m = 1cm or something like that.

A student places a capacitor across the terminals of this power supply. Suggest how this produces a constant voltage.

And you given the diagram of a power supply. You still saying it has nothing to do with the question?

The whole thing is so stupid.

i really dont get it could u like explain how i select the appropriate scale. for example should i select 1m = 1cm or something like that.

the diagram is for output of the power supply so because the capacitor has now been put the output will no longer be the same because the capacitor is storing this charge

You realise that capacitor doesn't affect the power supply at all?

you are not understanding what im saying. Just take a break relax and look at the question and make sense of what we just said to you. It's really not that difficult your trying to make things wayy too complicated.

the diagram is for output of the power supply so because the capacitor has now been put the output will no longer be the same because the capacitor is storing this charge

May be, but everything you say just doesn't make any logical sense to me.

We given a power supply with varying voltage. A capacitor is attached to it. Question asks to suggest how it produces constant voltage, but it just doesn't, right?

You said:

1) The diagram is not relevant. But the question clearly mentions that exactly this power supply with voltage according to the diagram is used.

2)


And it's just not true, because the power supply will keep supplying energy the same way, and it's not affected by the capacitor (at least, in this case).

I appreciate your help, but it's just seems so wrong to me.

are u talking about the circuit in the diagram in part c?

you just answered your own question. "The power supply will keep supplying energy the same" so now answer the question. If you still want to argue ask your teacher.

No, the link you gave.

lol for that we are the one switching the point. when we have to charge we connect it to the top one when we have to discharge we connect it to the circuit which will include only resistor and capacitor.

Yeah, of course. Just in this question we have no switches and no resistor (before part C).

oh yeah. infact it did not mention anything about those components

Someone help me with these questions i cant seem to get the right answer

1) A capacitor charges through a 0.22M ohm resistor. Calculate the capacitance if the time taken for it to reach full charge is 5 minutes.


2) A 47 micro Farad capacitor is charged to a potential difference of 15V and then discharged through a 100k ohm resistor. How long does it take for half of the energy stored in the capacitor to have dissipated in the circuit.


This is what i did for part 1)

Tau = RC

5 minutes - 300 seconds

300= 0.22 x 10 ^6 x C

C = 300/ 0.22 x 10^ 6 = 1.36 x 10 ^ -3 Farad. (but the answer is 2.7 x 10^-4 F )

300 sec =5RC, RC=60.

ahhhhhhh how do you do part 2!

W = 1/2 CV^2

soo W = 0.5 x 47 x 10^-6 x 15 ^2

W = 5.3 x 10^ -3 J (total energy)

Then we need to work out the time constant (tau) :

Tau = 100 x 10^3 x 47 x 10^-6 = 4.7 seconds

Sooo half the energy stored ( 5.3 x 10 ^-3) / 2 = 2.65 x 10^-3 J

It's correct, but after that I can't follow where your conclusion came from.

Half energy stored = 1/2CV^2. Do you know how to proceed from here?

i was going to do that but then i thought it was wrong caz it doesnt make sense we just worked out the energy? so why would it equal energy again? haha

Well, you need to find out the energy first to calculate half-energy, to get the values for voltage. So, there's nothing weird about it.