# Electric Field Vectors

1. Jan 11, 2014

### MathewsMD

Is the electric force on a charged particle always in the same direction of the field? What if it is an uncharged particle?

If you have an electron, with the field vectors pointing radially inward, then place a proton in the field, then yes since the proton is attracted towards the electron as well. But if you place an electron in the field, the electric force will not be in the same direction as the force, right?

The same concept applies with an electric field about a stationary proton, if you have another proton, then yes, the directions of the electric force and field are the same. But if you have an electron, this is not true.

Could anyone please clarify or confirm anything I have said? Thank you!

2. Jan 11, 2014

### Staff: Mentor

This is true only if the particle is positively charged. As you note later, if the particle is negatively charged, the force is in the opposite direction to the field.

Then there is no force.

3. Jan 11, 2014

### MathewsMD

Also, based on this:

I understand why C is correct. But isn't option A and technically C right too?
Since the field vectors would be pointing downward and right with the greatest magnitude in situation 1.

4. Jan 11, 2014

### MathewsMD

Sorry for the extra questions. They're just related and I rather not open a new thread.

For:

Why is the force exerted by a uniform electric field not parallel to it? The force and electric field vectors are parallel, right? Why does the field have to not be parallel to the dipole moment in order for there to be torque? Why can't it be just perpendicular as well? Dipole moments are a little new and any explanation would be very helpful!

5. Jan 14, 2014

### MathewsMD

Any help? I am trying to think about it logically but am missing something.

6. Jan 14, 2014

### Staff: Mentor

I would tend to agree that A is true too. I'll look more at this... It would help if you could write down the relative total vector fields for each configuration...

7. Jan 14, 2014

### Staff: Mentor

The key is the uniformity of the electric field. The net force of a uniform electric field on an electric dipole is what?

There may be a temporary force on the ends of the dipole if it is not yet aligned with the E-field, but eventually what happens?

8. Jan 14, 2014

### MathewsMD

Hmmm..don't dipoles become aligned to become parallel in the electric field? Therefore there would be no torque, but isn't there still a parallel force from the electric field? You stress the uniformity though...are the field vectors supposed to cancel, regardless of the object in it?

9. Jan 15, 2014

### Staff: Mentor

You have the correct intuition. If the dipole is not yet aligned, there is a net torque. If there is no damping, the dipole will oscillate.

Once the dipole is aligned with the E-field, there is no net force or torque. I have a feeling that you can write the equations that show that...