# PHY5 Jun 05 ThreadWatch

This discussion is closed.
13 years ago
#101
(Original post by Steve8052)
Thanx ur a LIFESAVER!!! i was just trying to calculate how many marks i lost in the paper. But however i still dnt understand how that would be the charge across that one capacitor? Is it something to do with the fact that the other two were in parallel?
And btw, what did you get for the displacements of that standing wave question in PHY4?
Your method was entirely correct, just longer than it needed to be.

Basically, you take the two 2µF capacitors in parallel to act as a single 4µC capacitor. Which means you now have two 4µF capacitors in series. As you worked out, the equivalent capacitance of two 4µF capacitors is 2µF, and therefore charge in circuit = 48µC.

Remember, when capacitors are in series, charge on both capacitors are the same, but potential difference may be different (not in this case since both were 4µF)

When capacitors are in parallel, the charge stored across each capacitor is not (necessarily) the same.

My displacements were 2cm, 2.5cm, 1.6cm (in no particular order)
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13 years ago
#102
(Original post by Steve8052)
Thanx ur a LIFESAVER!!! i was just trying to calculate how many marks i lost in the paper. But however i still dnt understand how that would be the charge across that one capacitor? Is it something to do with the fact that the other two were in parallel?
And btw, what did you get for the displacements of that standing wave question in PHY4?
If you treat the two capacitors that were in parallel as one capacitor in series with the equivalent capacitance calculated in the first part of the question, then you have effectively got a circuit with two 4microF capacitors in series. The charge stored in capacitors in series (regardless of their capacitancies) is the same for each capacitor and the same as the total charge stored.

EDIT: Hash got there before me!
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13 years ago
#103
(Original post by Steve8052)
Oh damn, theres a mark gone.
Oh and in the standing wave question was the displacement -2,-2.5 and -1.6?
yes , thats wat i did , some of my friends got 2 , 2.6 and 1.6 , but i thought the displacement should be negative or else a standing wave wont form
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13 years ago
#104
(Original post by Hash)
My displacements were 2cm, 2.5cm, 1.6cm (in no particular order)
so if i got -2, -2.5, -1.6 would it be wrong because i thought it must be negative for the standing wave to form.
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13 years ago
#105
(Original post by nas7232)
I started from R as R was the minimum value. [1 mark]

Then i plotted g/9, g/16 and drew the graph. [2 marks]

Same, I agree with you.
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13 years ago
#106
(Original post by Steve8052)
so if i got -2, -2.5, -1.6 would it be wrong because i thought it must be negative for the standing wave to form.

umm possibly wwrong. But it was only one mark for each value. You either won't be penalised at all, or you may get 2/3 of the marks.
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13 years ago
#107
i kno dats wat im not sure about did they have to be positive for the wave to be 'standing'. N i just remembered, the equipotential for the Earth Q in PHY5, are they supposed to get further apart? Sorry if i'm being a pain in the neck but just want to clarify these things. really bugging me @ the moment.
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13 years ago
#108
(Original post by Steve8052)
i kno dats wat im not sure about did they have to be positive for the wave to be 'standing'. N i just remembered, the equipotential for the Earth Q in PHY5, are they supposed to get further apart? Sorry if i'm being a pain in the neck but just want to clarify these things. really bugging me @ the moment.
Yes they are
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13 years ago
#109
i got 2,1.6,2.5 [no specific order as ican't remember]
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13 years ago
#110
(Original post by MBR50)
I'm quite worried for you, because I got nearly exactly the same! I got a C in AS which was a B after resits, but I didn't think the paper went that well for me... but I suppose all that is a huge boost in confidence! *confused*
Hey I got the same answers as well mark so don't worry! We've both failed! lol
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13 years ago
#111
(Original post by kevla)
Hey I got the same answers as well mark so don't worry! We've both failed! lol
OR

We have all aced it
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13 years ago
#112
i kno its big headed an all but apart from possibly the GPE thingy only worth 2 marks im pretty confident all of my answers weere absolutly correct there on page 3
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13 years ago
#113
(Original post by tktaylor6)
i kno its big headed an all but apart from possibly the GPE thingy only worth 2 marks im pretty confident all of my answers weere absolutly correct there on page 3
i hope so
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Thread starter 13 years ago
#114
o da big 2000 views mark...im so proud of ma thred
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13 years ago
#115
(Original post by lgs98jonee)
E=V/d=5kN/C

F=EQ=5kN/C * 1.60x 10^-19 C
=8x10^-16N
got that too!! for the next 1 or the one after u had to use formula eV=1/2mv^2 to calculate v right??
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13 years ago
#116
(Original post by DmAsTeR)
got that too!! for the next 1 or the one after u had to use formula eV=(1/2)mv^2 to calculate v right??
dont forget the brackets I used that too
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13 years ago
#117
(Original post by Phil23)
dont forget the brackets I used that too
good... i replied to the wrong post earlier on...have not got the same offers for uni as u... mine r: firm - imperial - biomedical engineering AAB
insurance - UCL - biochemical engineering ABB (my mate got BBB for same course )
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13 years ago
#118
(Original post by DmAsTeR)
good... i replied to the wrong post earlier on...have not got the same offers for uni as u... mine r: firm - imperial - biomedical engineering AAB
insurance - UCL - biochemical engineering ABB (my mate got BBB for same course )
woh - they must hate you
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13 years ago
#119
(Original post by Phil23)
woh - they must hate you
they should have given me BBB but i confidently (also stupidly) said im definitely gonna get A in math... my other friend never told them any of his grades and got the BBB offer
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13 years ago
#120
(Original post by nas7232)
i got 2,1.6,2.5 [no specific order as ican't remember]
I got the same answers, but I'm not quite sure about my sketch.
0
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