Question

The potassium chloride molecule (KCl) has a dipole moment of 8.90*10^-30 C·m.

(a) Assuming that this dipole moment arises from two charges, each of magnitude 1.6*10^-19 C, separated by distance d, calculate d. Correct: Your answer is correct. 5.56e-11m (

(b) What is the maximum magnitude of the torque that a uniform electric field with magnitude 6.90*10^5 N/C can exert on a KCl molecule? Correct: Your answer is correct. 6.14e-24m

(c) Suppose that the electric field of part (b) points in the +y direction. What are the possible directions of the KCL dipole moment which would result in a torque having the magnitude calculated in part (b), and having a direction of +z? Choose all that apply. -y direction , -x direction, 45 degrees CCW from +x in the xy plane, +y direction, 45 degrees CW from +x in the xy plane, +x direction

(d) Continue to suppose that the electric field of part (b) points in the +y direction. If the dipole swings from an initial orientation pointing in a direction 20.0 degrees CCW away from the -x axis to a final orientation pointing in a direction 20.0 degrees CW away from the -x axis, what is the change in the dipole's electric potential energy? The motion is entirely in the xy plane. _J?

(I got a and b correct just need c and d and how to do them)

Answer 1

1)

Dipole moment = q*D =1.6*10^-19 C *D = 8.90*10^-30 Cm

Therfore distance, D = 8.90/1.6 * 10^-11 m = 5.56 * 10^-11 m

2)

The torque on a dipole is maximum when the angle between p and E is 90^o.

Torque = pEsin = p.E = 8.90*10^-30 Cm * 6.90*10^5 N/C = 61.41 * 10^-25 Nm

3)

electric field of part (b) points in the +y direction. the possible directions of the KCL dipole moment which would result in a torque having the magnitude calculated in part (b) will be perpendicular to y axis, ie if it lies in xz plane.

4)

If the dipole swings from an initial orientation pointing in a direction 20.0 degrees CCW away from the -x axis to a final orientation pointing in a direction 20.0 degrees CW away from the -x axis,

Potential energy is scalar product of p and E

initial potential energy, U_i=-pEcos = 0 as was 90^o.

Now, is 20^o. So, initial potential energy, U_f=-pEcos20 = 61.41 * 10^-25 *cos20 = 57.706 *10^-25 J

So, change in potential energy = U_f - U_{i =}57.706 *10^-25 J