Torque on an Electric Dipole
For a rigid dipole in a uniform field, tau = p x E and |tau| = pE sin theta, where theta is the angle from p to E.
Why this shows up in the exam
Charged rods in sheet fields · Finding dipole orientation · Comparing torques in different fields
Learn the idea
A uniform field exerts a turning effect p cross E but no net force on an ideal dipole. The opposite forces on the two charges form a couple that tries to align p with E. Torque vanishes when parallel or antiparallel.
🧠 Memory hook: Torque is p cross E: alignment is the goal.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- tau = p x E — dipole torque vector
- |tau| = p E sin theta — torque magnitude in a uniform field
How to approach it
- 1Construct p from negative to positive
- 2Find the angle to E
- 3Use the right-hand rule and pE sin theta
Common slip-ups that cost marks
- •Using cos theta in torque
- •Assigning a net force in a uniform field
- •Measuring theta from the wrong vector
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
Two point charges 1 microC and 2 microC are 1 m apart in vacuum. Take k = 9 x 10^9 SI. Find the force magnitude.
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