Unstable equilibrium in case of dipoles

1. Sep 11, 2015

gracy

I know in case of unstable equilibrium of dipoles when dipole makes zero angle with electric field intensity ,But how we decide that this is unstable equilibrium I know in case of unstable equilibrium dipole returns to it's place after the small displacement ,I want to know why or how?why/how does it return to it's original position after the small displacement ?

2. Sep 11, 2015

blue_leaf77

Shouldn't the dipole be in equilibrium when it makes zero angle with the E field?

3. Sep 11, 2015

gracy

I meant 180 degrees

4. Sep 11, 2015

blue_leaf77

When the angle is 180 degrees, the dipole will also be in equilibrium position. Free body diagram can explain all this.

5. Sep 11, 2015

gracy

main question is

6. Sep 11, 2015

blue_leaf77

See these pictures, red is positive and blue is negative. After all, the system of a dipole in a uniform monodirection electric field is the same as a pendulum under gravitational field.

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7. Sep 11, 2015

nasu

In the first case (stable), if you rotate it a little bit from equilibrium the torque of the electric forces will rotate the dipole back to the zero angle.
In the second case (unstable) the torque will rotate it away from equilibrium.
The torque is zero for both cases (zero and 180) but but is not zero if the angle is changed a little bit.

8. Sep 11, 2015

gracy

Why?That's my main concern!

9. Sep 11, 2015

nasu

Why what? Why there are electric forces or why they rotate this way?
If it's the second part. just look at the force diagrams in the two cases.

10. Sep 11, 2015

gracy

I am not able to grasp the diagrams.

11. Sep 11, 2015

gracy

I want to know why Dipole tend to restore it's original position when displaced a little from 0 degree (with Electric field)but the dipole does not do the same(does not restore it's position)when displaced a little from 180 degree (with Electric field)
Want to understand with the help of force diagram but I am not getting those in post #6 because I want to take dipole as a whole not positive and negative charges

12. Sep 11, 2015

blue_leaf77

Actually I have given you a very straightforward hint: pendulum under gravitation. Are you familiar with such mechanical system? In term of energy, it's a matter of which energy state is more favorable for this system to be in. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/diptor.html
Easy. Just move those force arrow lines to the complete dipole picture.

13. Sep 12, 2015

nasu

I don't mean the diagrams posted here.
Just draw the forces on each charge, from the external field. The forces between the charges are not relevant. In order to have a stable dipole these forces are already balanced by some sort of repulsive forces.
Imagine a rigid stick with forces applied to the ends and see which way will rotate.
The forces are along the vertical direction, either up or down, depending of the sign of the charges.