Electric Fields question, can't figure it out!

It's part c that I'm stuck on (question is in the second picture, first is the diagram). Would it just be normal vector arrows, with the one for the -8nC charge twice as long as the other?

And I looked up the mark scheme and it says something about a parallelogram for the resultant, so now I'm super confused because we never covered anything like this in class. :s-smilie:

Help would be greatly appreciated. :smile:

Because you say you haven't covered it in class you may want to read up on bits covering the relevant principles on these links. But, I think you are already 'there', so may just readout of interest anyway. You should be able to spot the odd parallelogram-like figures that are similar to what is required of you.

('Superposition principle' section)

(All relevant, direct scenario covered in 'Electric field around two unlike charges' section.)

So, in answer to your question, you are right in saying that they do basically want normal vector arrows with relevantly scaled lengths.

Feel free to respond with any further query. :smile:

Reply 2

crozibear96

Original post by RVNmax

Thanks.

So, like this?

WP_001118.jpg245.5KB

Reply 3

RVNmax

Original post by crozibear96

Thanks.

So, like this?

Nearly. I think you missed where the point P was placed on the diagram in the question. You literally need to have it at the same place and take it from there, otherwise it's a bit too easy! (They are testing your vector directions.)

Also, what did you write underneath P, I cannot make it out.

Edit: Because you didn't realise where point P actually is, you hence didn't understand the parallelogram remark.

(edited 9 years ago)

Reply 4

crozibear96

Original post by RVNmax

Under the P it just says 80mm.

Here's a screenshot of the mark scheme (I drew the sketch);

Is that what it means?

phys.PNG52.9KB

Reply 5

RVNmax

Original post by crozibear96

Under the P it just says 80mm.

Here's a screenshot of the mark scheme (I drew the sketch);

Is that what it means?

The mark scheme matches what I said, that's good to know. It looks like you didn't get what I was saying in my previous post though :frown:

You see it tells you to copy the diagram in the question?
You LITERALLY have to copy it; P is not directly in between the two point charges.
It is slightly lower, but still equidistant. Once you have that sorted, then you can try and figure out the directions etc.

edit: in fact, for the purpose of understanding you can just answer the question on the diagram already provided to you if you want.

(edited 9 years ago)

Reply 6

crozibear96

Original post by RVNmax

The mark scheme matches what I said, that's good to know. It looks like you didn't get what I was saying in my previous post though :frown:

Oh, man, I didn't even see the P on the diagram! :eek:

Sorry.

I think I understand now though. Thank you for all your help!

Reply 7

RVNmax

Original post by crozibear96

Oh, man, I didn't even see the P on the diagram! :eek:

Sorry.

I think I understand now though. Thank you for all your help!

I didn't spot it right away either. Could very easily flummox you in the exam, which is why they tried to hint (not good enough imo) to copy the diagram.
It should say 'point P, as drawn on diagram' or something.

Reply 8

lerjj

Original post by crozibear96

Help would be greatly appreciated. :smile:

There are two ways of graphically representing vector addition- you can either form a triangle, OR you can draw the two vectors from a common origin, then reflect this to make a parallelogram which has the resultant as the diagonal of it. The two methods are completely equivalent.