Physics gcse question ( pls help, my exam is tomorrow )

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Thread starter 1 month ago
#1
"Why does a plastic ball covered in aluminum paint and placed between 2 metal plates ( One positively charged and the other negatively charged ) oscillate between the plates?"
The mark scheme's answer is the following:
- Ball moves towards +plate and becomes negatively charged
- it gets repelled and attracted to the -ve plate
- it becomes negatively charged and is then attracted back to the +plate

The answer to the concept of this question baffles me in so many ways because 1) If the ball is already neutral why would it have the urge to move towards any plate, 2) according to what did the ball move towards the positive plate and not the negative plate first and 3) assuming we followed through the mark scheme's answer, when the ball is repelled from the +ve plate and attracted to the negatively charged plate why didn't it become neutral after touching the -ve plate?
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1 month ago
#2
Yes, you are right in thinking the ball would not move at first, this is why you manually displace the ball to touch one of the plates. I'm not quite sure what the mark scheme is getting at. But if the ball has a neutral charge, it will not move to either of the plates, as the resultant force on it would be zero.

Regards your 3rd point, think about what happens when the -ve charged ball hits the +ve charged plate in terms of electrons. As the ball has an excess of electrons when -ve, when it hits the +ve plate, these electrons flow onto this plate, which leaves the ball with a lack of electrons, therefore it becomes positive. And as like charges repel, the ball is repelled to the negative plate, and this process continues
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Thread starter 1 month ago
#3
(Original post by adam.jmg04)
Yes, you are right in thinking the ball would not move at first, this is why you manually displace the ball to touch one of the plates. I'm not quite sure what the mark scheme is getting at. But if the ball has a neutral charge, it will not move to either of the plates, as the resultant force on it would be zero.

Regards your 3rd point, think about what happens when the -ve charged ball hits the +ve charged plate in terms of electrons. As the ball has an excess of electrons when -ve, when it hits the +ve plate, these electrons flow onto this plate, which leaves the ball with a lack of electrons, therefore it becomes positive. And as like charges repel, the ball is repelled to the negative plate, and this process continues
In the case of the 3rd point, I think you mean the ball will keep changing charges but wouldn't that cause the ball to keep on oscillating forever? Do we also manually stop it from moving if we want to end the process?
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1 month ago
#4
(Original post by Nasa1726)
In the case of the 3rd point, I think you mean the ball will keep changing charges but wouldn't that cause the ball to keep on oscillating forever? Do we also manually stop it from moving if we want to end the process?
to stop it from oscillating you would have to turn off the power supply to the circuit.

I think (correct me if I'm wrong) that if you were to manually stop it, and the circuit was still on, the ball would continue oscillating as it would still have a charge; unless the charge was neutralised by the contact with your hand.
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1 month ago
#5
(Original post by Nasa1726)
"Why does a plastic ball covered in aluminum paint and placed between 2 metal plates ( One positively charged and the other negatively charged ) oscillate between the plates?"
The mark scheme's answer is the following:
- Ball moves towards +plate and becomes negatively charged X
- it gets repelled and attracted to the -ve plate
- it becomes negatively charged and is then attracted back to the +plate
This mark scheme, is incorrect.
If the ball touches the positive plate it will become positively charged.

It is then repelled by the positive plate and attracted to the negative plate.
When it touches the negative plate, it becomes negatively charged and is repelled by that plate and attracted to the positive plate.
Last edited by Stonebridge; 1 month ago
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Thread starter 4 weeks ago
#6
(Original post by adam.jmg04)
to stop it from oscillating you would have to turn off the power supply to the circuit.

I think (correct me if I'm wrong) that if you were to manually stop it, and the circuit was still on, the ball would continue oscillating as it would still have a charge; unless the charge was neutralised by the contact with your hand.
ah yeah that makes sense
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Thread starter 4 weeks ago
#7
(Original post by Nasa1726)
ah yeah that makes sense
Thank you both
I've already taken my exam and I didn't see anything related to electricity, but I think I failed because it was the hardest exam I've ever taken, and I came out of the exam realizing that I don't understand physics and I need to work hard on it this summer.
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4 weeks ago
#8
What exam was it? I am guessing it was GCSE. I wouldn't worry about it too much as there is nothing you can do to change that result now (easier said than done). But, if you found it hard, no doubt that other people will so grade boundaries might be lower than usual.

If you have other exams just focus on revising that content and doing as best as you can on them. And use this whole experience to improve your revision methods to suit you best in A levels!

That was a very cliché thing to say but it does have truth in it, and if you let it get to you now it will affect your other exams. Only a few more weeks until it is over. I am going to try to keep that mentality for tomorrow as I have my first A level physics paper

(Original post by Nasa1726)
Thank you both
I've already taken my exam and I didn't see anything related to electricity, but I think I failed because it was the hardest exam I've ever taken, and I came out of the exam realizing that I don't understand physics and I need to work hard on it this summer.
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Thread starter 4 weeks ago
#9
(Original post by adam.jmg04)
What exam was it? I am guessing it was GCSE. I wouldn't worry about it too much as there is nothing you can do to change that result now (easier said than done). But, if you found it hard, no doubt that other people will so grade boundaries might be lower than usual.

If you have other exams just focus on revising that content and doing as best as you can on them. And use this whole experience to improve your revision methods to suit you best in A levels!

That was a very cliché thing to say but it does have truth in it, and if you let it get to you now it will affect your other exams. Only a few more weeks until it is over. I am going to try to keep that mentality for tomorrow as I have my first A level physics paper
Yes it was gcse and my entire class found it hard so thank god I'm not the main issue but still I know I should give physics my entire attention once I'm done since I'm planning to take it in A-level and yeah you're def right about making the most of my experience rn so that I can learn from my mistakes. And btw I wish you all the luck tmrw, I'm so sure you're going to ace that exam!
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4 weeks ago
#10
(Original post by Nasa1726)
Yes it was gcse and my entire class found it hard so thank god I'm not the main issue but still I know I should give physics my entire attention once I'm done since I'm planning to take it in A-level and yeah you're def right about making the most of my experience rn so that I can learn from my mistakes. And btw I wish you all the luck tmrw, I'm so sure you're going to ace that exam!
Don't overdo it in the summer holidays, take the time to enjoy it as you don't really have to worry about work because it will basically be starting from fresh. But when you start A levels I would recommend going over everything you learn and try not to forget it over time, so be constantly looking over previous topics keeping them in your head. This will make it much easier when it comes to revising for the AS and A level exams as you will not have to re-learn content which is a big-time waster and what I had to do (just produces more stress). If you can balance out work and pleasure then you will do great in the end. How I tried to manage my time was to do school work up to around 5-6pm, then after that I would stop doing work no matter what and just do something else not related to school. Also, I would recommend doing all the homework the day it is set and DONT WASTE FREE PERIODS. In yr 12 it might seem very relaxed but it will all catch up to you in yr13 if you are too relaxed. I'm not trying to scare you this is just advice from my experience, it might be different for you.

regarding my first physics paper, it was hard but I managed to answer every question without guessing anything, but I'm never too confident after exams as I don't want to raise my hopes too much. But in all, I think it was ok. If you ever need any help during a levels or any advice, just @ me or DM me!
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Thread starter 4 weeks ago
#11
(Original post by adam.jmg04)
Don't overdo it in the summer holidays, take the time to enjoy it as you don't really have to worry about work because it will basically be starting from fresh. But when you start A levels I would recommend going over everything you learn and try not to forget it over time, so be constantly looking over previous topics keeping them in your head. This will make it much easier when it comes to revising for the AS and A level exams as you will not have to re-learn content which is a big-time waster and what I had to do (just produces more stress). If you can balance out work and pleasure then you will do great in the end. How I tried to manage my time was to do school work up to around 5-6pm, then after that I would stop doing work no matter what and just do something else not related to school. Also, I would recommend doing all the homework the day it is set and DONT WASTE FREE PERIODS. In yr 12 it might seem very relaxed but it will all catch up to you in yr13 if you are too relaxed. I'm not trying to scare you this is just advice from my experience, it might be different for you.

regarding my first physics paper, it was hard but I managed to answer every question without guessing anything, but I'm never too confident after exams as I don't want to raise my hopes too much. But in all, I think it was ok. If you ever need any help during a levels or any advice, just @ me or DM me!
Mannnn thank you so much for your advice and help, I'll definitely keep it in my head
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