# Will this dipole rotate or change position?

• I
• annamal
In summary, the external electric field is in black and two charges with their electric fields are drawn in orange. The net force on the dipole is zero, but the net torque is positive, due to the attraction of the charges towards the field. Correct, but not the net torque. You were asking about how the dipole would move in this situation. In summary, the external electric field is in black and two charges with their electric fields are drawn in orange. The net force on the dipole is zero, but the net torque is positive, due to the attraction of the charges towards the field.

#### annamal

Will this dipole rotate or change position? The external electric field is in black. Two charges with their electric fields are drawn in orange.

Calculate the net force on the dipole and the net torque (with respect to the center of the dipole) , you will find them both zero.

Another way to solve this, is via the concept of dipole moment. If the dipole moment is parallel to the external Electric Field then the dipole is in stable position.

So, the e-field arrows go from + charges (potential, really) towards minus. This will exert a coulomb force to push/pull the charges. But, it's stable like a pencil standing on it's point (really, an unstable equilibrium) any perturbation in alignment, which will always happen somehow, will create a rotational force to make them switch positions. Then it really will be stable (a stable equilibrium, once things settle down), but with the charges separated a bit more than if there was no field. The concept is simple, the + charge wants to move with the arrows, the minus charge wants to move the other way.

berkeman
This confuses me a bit because the external electric field in black is as though we placed a positive charge to the very left of the image, right next to the blue +q, which means, the blue +q should be repelled and move away with the red -q moving forward.

annamal said:
which means, the blue +q should be repelled and move away with the red -q moving forward.
That's what @DaveE just said. And if the two charges are held rigidly at their current separation (hence, a diople), they will be rotated by those forces until they align with the field. But since the E field is constant in magnitude in the area that you show, what is the "Net" force on the two charges overall?

berkeman said:
That's what @DaveE just said. And if the two charges are held rigidly at their current separation (hence, a diople), they will be rotated by those forces until they align with the field. But since the E field is constant in magnitude in the area that you show, what is the "Net" force on the two charges overall?
Shouldn't it be 0 in this unstable equilibrium position?

Shouldn't what be zero? The net force and torque? I suppose so, but it's a very unstable equilibrium as drawn, and will experience a torque fairly soon that flips it into the stable equilibrium position (after a period of oscillations that will depend on any damping that is present).

DaveE
berkeman said:
Shouldn't what be zero? The net force and torque? I suppose so, but it's a very unstable equilibrium as drawn, and will experience a torque fairly soon that flips it into the stable equilibrium position (after a period of oscillations that will depend on any damping that is present).
You asked about the net force. The net force should be 0.

annamal said:
You asked about the net force. The net force should be 0.
Correct, but not the net torque. You were asking about how the dipole would move in this situation.

## 1. What factors can cause a dipole to rotate or change position?

There are several factors that can affect the rotation or position of a dipole, including external electric or magnetic fields, thermal fluctuations, and interactions with neighboring dipoles.

## 2. How does the orientation of a dipole affect its rotational behavior?

The orientation of a dipole, or the direction of its moment, plays a crucial role in determining its rotational behavior. Dipoles tend to align themselves with external electric or magnetic fields, causing them to rotate or change position accordingly.

## 3. Can a dipole rotate or change position spontaneously?

Yes, a dipole can undergo spontaneous rotation or positional changes due to thermal fluctuations. This is known as Brownian motion and is a random, continuous process.

## 4. Is there a way to control the rotation or position of a dipole?

Yes, external electric or magnetic fields can be used to manipulate the rotation and position of dipoles. Additionally, the design and arrangement of dipoles in a material can also affect their behavior.

## 5. How is the rotational behavior of dipoles relevant in real-world applications?

The rotational behavior of dipoles is crucial in many scientific and technological fields, such as molecular biology, materials science, and electronics. Understanding and controlling dipole rotations can lead to advances in drug delivery, energy storage, and data storage technologies.