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# Physics A2 - Unit 4 Multiple Choice, help required! Watch

1. I got stuck on a couple questions, and would appreciate any help!
Q11. http://prntscr.com/avta78

Not really fond of ratio type questions, but I'm assuming I'd use g = GM/r^2, I don't know how to go about this

Q15. http://prntscr.com/avtasq
Well from this, I know that there is a uniform electric field present, so therefore
E = V/d, and also F = EQ, so re-arranging to get E = F/Q and substituting this into E = V/d giving F = VQ/d, however the distance d for the charge Q to be placed in the middle if "d/2" and placing that into F = VQ/d gives me 2VQ/d. However that is incorrect. The correct answer is B.

Thank you very much!
2. Just going to give hints for each question.
11: change M into the mass equation with pV. Then further expand it to give radius. Also put g of the planet as yg of earth.

15: What is V in the centre?
I got stuck on a couple questions, and would appreciate any help!
Q11. http://prntscr.com/avta78

Not really fond of ratio type questions, but I'm assuming I'd use g = GM/r^2, I don't know how to go about this

Q15. http://prntscr.com/avtasq
Well from this, I know that there is a uniform electric field present, so therefore
E = V/d, and also F = EQ, so re-arranging to get E = F/Q and substituting this into E = V/d giving F = VQ/d, however the distance d for the charge Q to be placed in the middle if "d/2" and placing that into F = VQ/d gives me 2VQ/d. However that is incorrect. The correct answer is B.

Thank you very much!
For Q11. Try writing out the gravitational field strengths for each individually(using g=GM/r^2) then use the fact that the field strength from the planet is y times that of the earth (yxg of the earth= g of the planet) so you can equate the two terms you have and find the ratio.

For Q15. This is a very sneaky question. What do you know about electric fields between two parallel plates that are perpendicular? That should help you answer it.
4. (Original post by chrisidialu)
For Q11. Try writing out the gravitational field strengths for each individually(using g=GM/r^2) then use the fact that the field strength from the planet is y times that of the earth (yxg of the earth= g of the planet) so you can equate the two terms you have and find the ratio.

For Q15. This is a very sneaky question. What do you know about electric fields between two parallel plates that are perpendicular? That should help you answer it.
How do you have something both parallel and perpendicular?
5. (Original post by Vikingninja)
How do you have something both parallel and perpendicular?
wow im so sorry I meant to say what do you know about the electric field between two metal plates that are parallel to each other. I didnt mean to use the word perpendicular.
6. (Original post by chrisidialu)
wow im so sorry I meant to say what do you know about the electric field between two metal plates that are parallel to each other. I didnt mean to use the word perpendicular.
So the magnitude of electrostatic force, F is the same everywhere in a uniform field right?
So the magnitude of electrostatic force, F is the same everywhere in a uniform field right?
Correct. It's constant since E is constant.

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