# Homework Help: Magnitude and direction of the electric dipole

1. Sep 12, 2010

### tsang

1. The problem statement, all variables and given/known data
Three metal balls with unequal radii R1 < R2 < R3 are placed at the vertices of an equilateral triangle, whose sides have length a. The balls are connected by thin metal wires. A positively charged rod is brought into contact with one of the balls and transfers an amount of charge +Q to the system.
(a)Show that the charge Qi on the i-th sphere (i = 1,2,3) is given by
$$\frac{Qi}{Q}$$=$$\frac{Ri}{R1+R2+R3}$$
(b)Hence show that the potential V at the centre of the triangle is given by
$$V=\frac{Q\sqrt{3}}{4\pi\epsilon_0a}$$
(c)What is the magnitude and direction of the electric dipole moment of the system, if two of the balls have equal radii (e.g. R1 = R2)? Hint: electric dipole moments add like vectors.

2. Relevant equations
$$V=\frac{Q}{4\pi\epsilon_0R}$$
$$\vec{p}=q\vec{d}$$

3. The attempt at a solution
I kept trying this question,but still have no idea for part (a).
I think I have solved Part B,but I noticed the question uses "hence", does that mean part (b) should use part (a) as part of solution? But I simply subsititute everything to formula $$V=\frac{Q}{4\pi\epsilon_0R}$$,and I ended up part (b) without using Part (a),is there anything wrong with that?
Part (c) I tried to use formular $$\vec{p}=q\vec{d}$$,but I don't know what to do next.

2. Sep 12, 2010

### Mandeep Deka

Re: Electromagnetism

You are close to the answer of the first part

You wrote the expression of potential of a sphere, in terms of the charge it possesses and its radius. Give a thought, what do you think is the consequence of two conductors being joined by a metal wire and a charge be given on one of them??

3. Sep 12, 2010

### tsang

Re: Electromagnetism

Thank you,dear friend. But could you please give me more help on details? I truly have no idea about it. I actually don't know which formula to use for Part (a).
About Part (c), do you mean by I should think about torque? since one side would be positive and the other side would be negative?

4. Sep 13, 2010

### Mandeep Deka

Re: Electromagnetism

OK, lets get it clear

whenever two bodies are joined by a conducting wire or maybe placed in physical contact to each other, the essential consequence is that they are at EQUAL POTENTIAL!
i.e say for example you have a metal ball with charge Q, and radius 'R', and then you bring it in contact to an uncharged body, say another metal sphere of radius 'r', it will transfer a particular amount of charge to the uncharged sphere, after which you will find that the both the sphere are at same potential. That is what is the basic essence of the question.

In your question, once you give a total charge Q to the system, it will get distributed among the spheres, and at the end their potentials will be same.
Think about it a little more, you have the equation of potential of a sphere in terms of its radius and charge of it, you can do some simple math and get the answer

Hope its clear now!