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Unit 4 Physics Edexcel A2 and Edexcel IAL

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Original post by target21859
Hi, I have two questions. For the first picture how do you know which way the current is flowing? And for the second picture why is the answer B? Thanks in advance.


First one current is direction of conventional flow so positive charge since it is antiproton you know the directoon.
For B think of Nphi increasibg steadilyy now its gradient is increasing so A or B, you can see gradient at t=0 is not 0 hence it is B.


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Reply 21
Original post by physicsmaths
First one current is direction of conventional flow so positive charge since it is antiproton you know the directoon.
For B think of Nphi increasibg steadilyy now its gradient is increasing so A or B, you can see gradient at t=0 is not 0 hence it is B.


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So the current flows to the left since antiprotons have a negative charge and because the current flows in the opposite direction to the flow of negative charges?
Original post by target21859
So the current flows to the left since antiprotons have a negative charge and because the current flows in the opposite direction to the flow of negative charges?


The conventional current... Which way does it go .


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Reply 23
Original post by physicsmaths
The conventional current... Which way does it go .


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Positive to negative.
Original post by target21859
Positive to negative.


Yes so.. To the left.


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Reply 25
Thanks for the help. I just have 1 more question. For the question in the picture, I worked out the value for the time constant but I'm not sure what it means. I looked at the mark scheme but I still don't understand the significance.
Original post by target21859
Thanks for the help. I just have 1 more question. For the question in the picture, I worked out the value for the time constant but I'm not sure what it means. I looked at the mark scheme but I still don't understand the significance.


what does the time constant represent?


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Reply 27
Original post by physicsmaths
what does the time constant represent?


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The time taken for the capacitor to lose 1/e of its charge
Original post by target21859
The time taken for the capacitor to lose 1/e of its charge


So what is the time to discharge?



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Reply 29
Original post by physicsmaths
So what is the time to discharge?



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Approximately 3 times the time constant
Original post by target21859
Approximately 3 times the time constant

I thought it was 5 time the time constant?
Anyway so what is this relation to the flicks between the switch?
This shoudl give the answer.
Reply 31
Original post by physicsmaths
I thought it was 5 time the time constant?
Anyway so what is this relation to the flicks between the switch?
This shoudl give the answer.

Oh yeah actually it is 5 times. I'm really not sure about the relation. Could you give me a hint?
Reply 32
Original post by physicsmaths
I thought it was 5 time the time constant?
Anyway so what is this relation to the flicks between the switch?
This shoudl give the answer.

1/400=2.5*10^-3 and tau=2.2*10^-4 and in the mark scheme it said 400hz is suitable because 1/400<tau. I don't understand this relation.
Original post by target21859
1/400=2.5*10^-3 and tau=2.2*10^-4 and in the mark scheme it said 400hz is suitable because 1/400<tau. I don't understand this relation.


1/400 is the time. So you want something not equal to but less.
Hi All

I have put together some videos and Unit 4 Paper Walkthroughs
on my youtube channel

These include several full (non IAL) papers and ALL 100 multiple choice questions asked since 2010.

The non IAL thread is here

Good luck with your exams everybody!

EJS
Hi, I'm really struggling with converting between Joules, kg and MeV/c^2. I never know when to use c^2 in the equations.

I've worked out that to convert from MeV/c^2 to energy:
mass (MeV/c^2) x eV = energy (J)

Can anyone please tell me the similar equations for converting kg to energy and MeV/c^2 to kg?

Thanks - any help would be massively appreciated!
Original post by candycake
Hi, I'm really struggling with converting between Joules, kg and MeV/c^2. I never know when to use c^2 in the equations.

I've worked out that to convert from MeV/c^2 to energy:
mass (MeV/c^2) x eV = energy (J)

Can anyone please tell me the similar equations for converting kg to energy and MeV/c^2 to kg?

Thanks - any help would be massively appreciated!

E=mc^2
When converting from kg into ev/c^2, use E=mc^2 then divide answer by eV
When converting from ev/c^2 to kg, first convert into joules and then use m=E/c^2
(edited 7 years ago)
Original post by Mowerharvey
E=mc^2
When converting from kg into ev/c^2, use E=mc^2 then divide answer by eV
When converting from ev/c^2 to kg, first convert into joules and then use m=E/c^2


So would it be right to convert kg to eV/c^2 using: (kg x c^2)/eV
And kg to joules using: kg x c^2 ?

Thank you!
Original post by candycake
So would it be right to convert kg to eV/c^2 using: (kg x c^2)/eV
And kg to joules using: kg x c^2 ?

Thank you!


Yes that would be correct
I've gone totally wrong on this question but I can't see where my method is different to that on the mark scheme - can anyone point out where I have gone wrong please?

Show that the GPS satellite takes about 40 000s to complete one orbit of earth
mass of earth = 6 x 10^24 kg
radius of earth = 6400m
Height of satellite above Earth's orbit = 20200 m

My method:
F=Gm1m2/r^2
F = mrw^2
rw^2 = Gm/r^2
w = sqrt(Gm/r^3)
w = sqrt ((6.67 x 10^-11 x 6 x 10^24)/(6400+20200)^3)
w = 4.611
T = 2pi/w
T = 2pi/4.611
T = 1.33s

I have obviously gone astray somewhere but I can't see where! Thanks

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