# Homework Help: Electric dipole moment problem

1. Sep 9, 2015

### gracy

1. The problem statement, all variables and given/known data
find the electric dipole moment of the system shown in figure

2. Relevant equations
In the simple case of two point charges, one with charge +q and the other one with charge −q, the electric dipole moment p is:

where d is the displacement vector pointing from the negative charge to the positive charge.
3. The attempt at a solution
I thought dipole moment would be zero because there is no dipole in the system.+2q and -q can not make a dipole because for a dipole we need two equal and opposite charges which is not the case here.+2q and +q can not make a dipole either.and +q and +q they don't fulfil the criteria so if no dipole "dipole moment "should be zero.other confusion that i am facing is whether this system is in equilibrium .or the charges can move due to repulsion/attraction.

2. Sep 9, 2015

### ehild

The charges are supposed to be fixed, they can not move.
You can consider the array as if there were two -q charges at the bottom left corner, that is there are two dipoles. The dipole of the array is the resultant.
A charge distribution can be characterized by its net charge and momenta. It can have a net dipole momentum, of quadrupole momentum and so on.

3. Sep 9, 2015

### gracy

Thanks.My answer came out to be qa√2

4. Sep 9, 2015

### ehild

That is the magnitude of the dipole moment. But it is a vector. Write it as a vector either with its components, or give the direction.

5. Sep 9, 2015

### gracy

it would be in between the two vectors at 45 degrees

6. Sep 9, 2015

### ehild

It is correct.

7. Sep 9, 2015