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# Capacitors watch

1. Hi. I'm not sure how to do question b)i) , b)iii) and c)i)...... For Qb)iii) Do I draw the same graph as posted I posted (but with voltage)? Any help is appreciated, thanks

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2. (Original post by ohnanailikenanas)
Hi. I'm not sure how to do question b)i) , b)iii) and c)i)...... For Qb)iii) Do I draw the same graph as posted I posted (but with voltage)? Any help is appreciated, thanks

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The graph should be an exponential decrease
3. Thanks. Any idea on the other questions?

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4. (Original post by ohnanailikenanas)
Thanks. Any idea on the other questions?

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For ci)

Do you remember the rules for resistors in series and parallel? The rules for capacitor in series and parallel is the other way round. This gives you C. If you know R, then time constant = C x R.
5. I know the rules for series and parallel capacitors... Parallel you just add them, series is 1/C + 1/C + 1/C = 1/C But I don't know any values ... I think I need b)i before I calculate c)i)??

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6. (Original post by ohnanailikenanas)
I know the rules for series and parallel capacitors... Parallel you just add them, series is 1/C + 1/C + 1/C = 1/C But I don't know any values ... I think I need b)i before I calculate c)i)??

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It says calculate the new time constant in terms of the old one.

In this case I think it would be 2t???

You don't need the values You just need to do R(1(1/C + 1/C)) = 2RC = 2 x the old time constant ???
7. I'm confused.. I really don't know what I'm supposed to be doing. B)i) wants me to find the values of Vab at 2t, 3t and 4t. Is 2t half of 0.37? 3t half of 2t??

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8. (Original post by ohnanailikenanas)
I'm confused.. I really don't know what I'm supposed to be doing. B)i) wants me to find the values of Vab at 2t, 3t and 4t. Is 2t half of 0.37? 3t half of 2t??

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I don't know I have never seen one involving EMF before
9. Could you help with these questions? I was thinking for the second questjon I would draw this diagram..

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10. (Original post by ohnanailikenanas)
I'm confused.. I really don't know what I'm supposed to be doing. B)i) wants me to find the values of Vab at 2t, 3t and 4t. Is 2t half of 0.37? 3t half of 2t??

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(Original post by Kyx)
I don't know I have never seen one involving EMF before
The p.d. across the capacitor is given by the function when

i.e. at the time the switch changed over (t=0s), the p.d. across the combination was E volts.

After time t = CR, the p.d. has fallen to 0.37E.

If the timer was reset at that point to t=0s (when E = new p.d), then after a further t=CR time constant, the p.d. across the pair must fall by following the same function again. i.e. 0.37E.

So at t = 2CR, the new p.d. is

Do the same for t = 3CR, t=4CR....each time using the same 0.37E relationship.
11. (Original post by ohnanailikenanas)
I'm confused.. I really don't know what I'm supposed to be doing. B)i) wants me to find the values of Vab at 2t, 3t and 4t. Is 2t half of 0.37? 3t half of 2t??

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not quite

at the end of each period lasting 1 time constant it's 0.37 of the value it had been at the start of that period.

after 1RC it's 0.37 of V0
after 2RC it's 0.37 of the value it had at 1RC... which is 0.37 of 0.37 V0 i.e. 0.37 x 0.37... or 0.372
after 3RC it's 0.37 of the value it had at 2RC... 0.37 x 0.37 x 0.37 V0 or 0.373

and so on...

---
because V=V0e-t/RC

so when t=RC V=V0e-1
t=2RC V=V=V0e-2
etc.
12. OK, seems legit

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