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Electric Fields Help please

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Let's say the ball was given +ve charge, the left plate connected to the +ve terminal of power supply and the right plate connected to the -ve terminal, as shown.

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image.jpg

I do understand that due to the uniform electric field the +ve charged ball accelerates uniformly to the right due to the constant field strength causing constant electrical force, so by F=ma constant acceleration. In other words I understand 1 to 2 in the graph. However I don't understand 2 to 3 in the graph. The only way 2 to 3 in the graph can occur is if the ball's charge becomes -ve. But I can't understand how this can occur. Alternatively the charges of the plates may reverse, so the left is -ve and the right is +ve. But it isn't specified in the question that the plates are connected to an alternating p.d. supply. So, what's the explanation for 2 to 3 in the graph and how?

THANK YOU SO MUCH:biggrin:
When the positively charged ball reaches the negative plate it picks up a negative charge from the negative plate and is then repelled by it.
It actually then goes on repeating this and goes backwards and forwards between the plates, picking up opposite charge at each bounce.
See it here. The black ping pong ball between the plates. (At high speed as the plates are close together.)
https://www.youtube.com/watch?v=8d7ohdDecQE
Reply 2
Avatar for Jaydude
Jaydude
11
The question says the ball is "covered in carbon coating." So when she "pulls the ball so that it touches one of the plates," the ball absorbs the charge that the plate was. Due to electric fields it will accelerate to opposite plate ( so on...), and it will then touch the other plate, same thing happens as before.

This is what my teacher said anyway. Surprisingly, the above answer isn't even in the markscheme, but that explains the graph imo.
(edited 10 years ago)
Reply 3
Avatar for krisshP
krisshP
OP
14
Original post by Stonebridge
When the positively charged ball reaches the negative plate it picks up a negative charge from the negative plate and is then repelled by it.
It actually then goes on repeating this and goes backwards and forwards between the plates, picking up opposite charge at each bounce.
See it here. The black ping pong ball between the plates. (At high speed as the plates are close together.)
https://www.youtube.com/watch?v=8d7ohdDecQE


Ah, okay. Thanks the help + video. Why doesn't weight have an effect? Shouldn't the ball slowly and eventually go lower due to its small weight?

PRSOM
(edited 10 years ago)
Original post by krisshP
Ah, okay. Thanks the help + video. Why doesn't weight have an effect? Shouldn't the ball slowly and eventually go lower due to its small weight?

PRSOM


I'm not sure what you mean "lower". The ball is on a string so it can't go lower.
The ping pong ball is light so it doesn't require much force (electrical) to move it.
A heavier ball would move more slowly between the plates as the acceleration would be less.
a=F/m
Reply 5
Avatar for krisshP
krisshP
OP
14
Thanks

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