Please do all these questions. Name: PHY 227 F16 Pre-REC 7 Section: Consider an
ID: 1785624 • Letter: P
Question
Please do all these questions.
Name: PHY 227 F16 Pre-REC 7 Section: Consider an electric dipole consisting of charges +q and -q held at a separation d and kept parallel to the Y-axis. There is a constant external electric field E in the space that is independent of the dipole charges. This independent electric field is uniform and points along the negative X direction. Consult the homework problem "Torque on a Dipole in a Uniform Field" in Homework #1 (problem #9, tutorial). 1. Write down the electric dipole vector in terms ofq, d, unit vector along X-axis and the unit vector along Y-axis. 2. Write down the forces on tq and -q due to the external electric field in terms of q, E, unit vector along X-axis and the unit vector along Y-axis. 3. What is the net force on the dipole? 4. Imagine a rotation axis that intersects the dipole perpendicularly at its midpoint. How will the dipole rotate about this axis, clockwise or anticlockwise? 5. What is the magnitude of the torque acting on the +q charge about this rotation axis? What is the magnitude of the torque acting on the -q charge about this rotation axis? 6. Are the torques on tq and -q acting in the same direction or are they opposing each other? What is the magnitude of the net torque acting on the dipole due to the electric field? Write the net torque in terms of the electric dipole moment and the electric field. 7. Write down the most general vector equation relating the net torque on the electric dipole to the electric dipole moment and the external independent electric field. [Hint: refer to your textbook ifyou have trouble.] In recitation this week we will consider forces and torques on magnetic dipoles.Explanation / Answer
1. Electric dipole moment vector is, p=q*d*j, where j is the unitvector in positive y direction
2. force on positive charge = -qE i, where i is the unit vector in positive x direction.
Force on negative charge = qE i, where i is the unit vector in positive x direction
3. Net force on dipole is zero as both the forces cancel each other out.
4. Anticlockwise as the force on positive charge(upper) is in negative x direction and force on negative charge(lower) is in positive x direction.