Coulomb's law

is maith liom chocolate

(i) State the Coulomb Law and explain how the electrical field is defined through the
Coulomb force. State the unit of the electrical field. [3 marks]
Four charges q = –5 µC are fixed at the corners of a
square with a length of a=1.5 cm
(ii) Make a sketch of the four charges and include
vector arrows representing all individual
Coulomb forces exerted on charge 4 as well as
the resulting net force on charge 4. [2 marks]
(iii) Calculate the magnitude of the resulting net force
on charge 4.

Can anyone help with this, my exam is tomorrow and I'm freaking out .. I can do the question if it were charges in a triangle, how different is it when they're in a square??

Fergus_Nash

Here comes the Moderator any second now with his/her BOLD CAPITALS.

is maith liom chocolate

huh???

raah!

He's making scathing social commentary... Or something.

With all these coulomb law charge arrangement questions you can normally use some symmetry in the problem. There doesn't seem to be any useful cancellations or anything here though :P. There is one where you can use the symmetry to avoid another direction calculation, and where you only need to do one rather than two magnitude calculations.

For example, two of the charges at the corners adjacent to where the charge you want to calculate the force on will exert the same magnitude of force on the charge. You use Coulombs law, and you will get the magnitude in each case (it's the same for each of them), and for each of them the direction is along the side of the square joining it to the charge of interest.

So you could say that the force on the corner charge is :

Fi - Fj. This is a force to the right from the charge to the left and a force downward from the charge above.

Then you use pythagoras' theorem to find how far a way the opposite corner charge is. It will be sqrt ( 2* 1.5^2 ) . Put this distance into Coulomb's law to then find the magnitude of force exerted on the corner charge from this opposite charge.

You would then look at the diagram again, and would see from the trigonometry of the situation that the force vector of this opposite corner charge would be:

( cos 45 ) * sqrt ( 2* 1.5^2 ) i - ( cos 45 ) * sqrt ( 2* 1.5^2 ) j

= (1 / sqrt(2) ) * sqrt ( 2* 1.5^2 ) i - ( 1 / sqrt(2) ) * sqrt ( 2* 1.5^2 ) j

Where you know it's cos45 because the line bisects the angle of 90 degrees. I.e, it goes straight to the corner. You then just add this force to the other two forces you calculated and find the magnitude.

Of course, you could use the symmetry of the situation to conclude that these forces will have the same direction, and find the magnitudes of the two side charges, and the corner charge before, and then simply add them. The resultant force is then
mag(cos45) i + mag (cos 45) j

So the question, jsut like the triangle one, is about using coulomb's law to calculate the magnitude of the forces, and then using the diagram and vectors to find out the direction of each of the forces.

On looking back at that I realise it is rather messy unlatex'd and unhelpful, but since I've already written it I may as well post it. I understand how stressful it can be to not know something before a repeat exam, especially when you may be away from college or what have you. I have one coming up myself, I'm taking a rest at the moment

Edit: I assumed that the charge you were interested in was the bottom corner one. There's a word for the point where you want to evaluate the electric field. It's... the field point or something. It's a useful piece of terminology anyway, though I don't know how appropriate it is for when you are finding the force on a charge.

is maith liom chocolate

oh wow, thank you so much, crap I forgot to say where the charge was, it's the one on the top right but I think I can manipulate what you said to fit that anyways, I shall give it a go now, thank you again

What makes things even worse is this is my first year doing physics and I will never have to do it again