What Are the Effects of Five Charges in an Electrical Field?

[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

 

[Doc Al]

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

 

[Doc Al]

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