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# AQA A2 Unit 4 electric fields question + unit 4 tips?

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1. Cant remember how to work out these kind of questions , if anyone could help It'd be much appreciated

Say if two charges , +6nC and +4nC are on the same line , 48mm apart , how do you find the point of where there would be no force on a charge , sorry if this is a really stupid question.

Also anyone else got tips for the aqa unit 4 paper ? I do all of them backwards so as to have the multiple choice part last but was wondering if people had any others
2. (Original post by Ragghorn)
Cant remember how to work out these kind of questions , if anyone could help It'd be much appreciated

Say if two charges , +6nC and +4nC are on the same line , 48mm apart , how do you find the point of where there would be no force on a charge , sorry if this is a really stupid question.

Also anyone else got tips for the aqa unit 4 paper ? I do all of them backwards so as to have the multiple choice part last but was wondering if people had any others
The field strength from one charge will be equal to the field from the other charge, so the field strength at that point overall will be zero, hence no force.

So if it's a multiple choice question, I would say your best option is to plug in the possible answers to Q/(4piEo x distance to the point^2). Where Q is the 6nC charge or 4nC. The correct answer will be when you get the same field strength from each charge.

If its not multiple choice, you would have to make a simultaneous equation which is a bit more complicated. As far as I know, you would then have to say that...

Q1/(4piEo x distance from charge^2) = Q2/(4piEo x distance from other charge^2)
You need to sub in the distance from the 4nC charge as (48mm - the distance from the 6nC) in order to make it work.
The constants will cancel and you will have to do some rearranging to solve it.

Let me know if there's anything you don't understand.
3. (Original post by ixguard)
The field strength from one charge will be equal to the field from the other charge, so the field strength at that point overall will be zero, hence no force.

So if it's a multiple choice question, I would say your best option is to plug in the possible answers to Q/(4piEo x distance to the point^2). Where Q is the 6nC charge or 4nC. The correct answer will be when you get the same field strength from each charge.

If its not multiple choice, you would have to make a simultaneous equation which is a bit more complicated. As far as I know, you would then have to say that...

Q1/(4piEo x distance from charge^2) = Q2/(4piEo x distance from other charge^2)
You need to sub in the distance from the 4nC charge as (48mm - the distance from the 6nC) in order to make it work.
The constants will cancel and you will have to do some rearranging to solve it.

Let me know if there's anything you don't understand.
ah okay perfect , for some reason i was making both field strengths use the same distances from the point of no force . Thank you , I worked it out and got 26.42mm from the 6nC charge so that seems correct as it would be more than 24mm from the 6nC charge. Thanks again

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