# Field of five charges

xma123
http://imgur.com/a3vxU

Really haven't a clue how to go about this, it's the last question on the paper and nothing before it is anything near as complex as this.

Any help much appreciated

Mentor
What's the field at the center from a single charge? Then find the vector sum of all the field contributions. Take advantage of symmetry.

voko
Imagine the top charge is also absent. What would the field be due to the four charges?

xma123
So due to each charge the field is E=kq/r2

The 4 in the middle cancel, giving a field of either positive or negative of the single field charge above. So would the answer be option 3?

voko
What is the direction of j? Of the field?

xma123
What is the direction of j? Of the field?

The picture is all the info given on this

Mentor
So due to each charge the field is E=kq/r2
Good. That's the magnitude of the field, so be sure to count the direction.
The 4 in the middle cancel, giving a field of either positive or negative of the single field charge above.
Yes, the 4 in the middle cancel. But I don't know what you mean by "either positive or negative". Which is it? What's the direction of the field from the top charge?

voko
You have enough information to deduce the direction of the field by the top charge. As for the direction of j, I am sure you have heard of the standard basis vectors i, j and k. What are they?

xma123
Apologies, it's late. The j vector acts in the y direction, so the top charge would act in the upwards of y, +j vector and therefore option 3.

By positive or negative I was referring to the difference betweent options 3 and 4.

voko
the top charge would act in the upwards of y

Why is that?

xma123
Why is that?

Oh damn, they're positive charges, so the field acts downwards away from the top charge.

This would give a "negative" field in the y direction and so option 4 is correct?

voko
they're positive charges, so the field acts downwards away from the top charge

Correct!

xma123
Correct!

Thanks very much for your patience, much appreciated