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# Unit 4 Physics Edexcel A2 and Edexcel IAL Watch

1. (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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2. (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?
3. (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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4. (Original post by physicsmaths)
The conventional current... Which way does it go .

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Positive to negative.
5. (Original post by target21859)
Positive to negative.
Yes so.. To the left.

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6. 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.
7. (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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8. (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
9. (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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10. (Original post by physicsmaths)
So what is the time to discharge?

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Approximately 3 times the time constant
11. (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?
12. (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?
Oh yeah actually it is 5 times. I'm really not sure about the relation. Could you give me a hint?
13. (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?
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.
14. (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.
15. Hi All

I have put together some videos and Unit 4 Paper Walkthroughs

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
16. 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!
17. (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
18. (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!
19. (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
20. 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
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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