# Homework Help: Torque and work of dipole

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1. Sep 24, 2016

### ooohffff

1. The problem statement, all variables and given/known data
An electric dipole consists of +/- Q = 330nC separated by a distance of 1200nm. The dipole is initially oriented in the -i direction (along the negative x axis). The dipole is in a uniform external electric field of E=2500j N/C Determine the following:

a) The magnitude of the torque on the dipole when it is at +165° to the +x axis
b) The work done by the electric field to rotate the dipole to an angle of +110° to the +x-axis from its initial position

2. Relevant equations
|T| = |P| |E| sin θ
W = Uf-Ui

3. The attempt at a solution
I get what to plug in to solve the problems, but the only issue I have is that I'm not sure what θ should be in these equations because I can't really visualize between what two vectors the angle is supposed to be between? When it says it's initially oriented in the -i direction, does that mean it has already rotated 180°? I'm confused.

2. Sep 24, 2016

### drvrm

draw a diagram and then it will be/can be seen how your confusion stands..

3. Sep 24, 2016

### ooohffff

So for a) would theta be 165°-90°=75°?

And for b) would final theta be 110°-90°=20° and initial theta be 180°-90°=90°

4. Sep 25, 2016

### drvrm

what is your theta angle ?
As you write the equation for Torque the angle theta seems to be angle between the dipole moment and electric field...
Initially your dipole moment is along -i direction and E is in j direction so are you turning the dipole through 75 degree...check from diagram.
post the diagram for us to see.

5. Sep 25, 2016

### haruspex

No. Oriented at -i must mean at 180 degrees to the +x axis. It is then rotated to be at 110 degrees to the +x axis.
In part a) there is no actual rotation.

6. Sep 25, 2016

### haruspex

Yes.

7. Sep 25, 2016

### ooohffff

Okay, I did that and I got the right answer for a).

For b), plugging 20 degrees and 90 degrees didn't seem to work. Any ideas?

8. Sep 26, 2016

### haruspex

What are the starting and finishing angles in b)?

9. Sep 26, 2016

### ooohffff

Initial is 180, final is 110.

10. Sep 26, 2016

### ooohffff

11. Sep 26, 2016

### drvrm

the angle is between dipole moment and E and the work done in rotating a dipole through an elementary angular displacement should be torque times the angle say differential element of theta.

12. Sep 26, 2016

### ooohffff

Oh wait, the final angle is actually 20° (165°-90°) and the initial is 90°(180°-90°), like I said previously. I found something else in my problem that was wrong, and I got the right number. My work is negative, which makes sense because it is rotating clockwise to align with the electric field, right?