1. Lecture 21: FRI 06 MAR Magnetic fields Ch.28.8-10 Physics 2102 Jonathan Dowling 28.8 Force on Current in Wire 28.9 Torque on Current Loop 28.10 Magnetic Dipole Moment

2. Magnetic Force on a Wire. L

3. . i (28-12)

4. Torque on a Current Loop: Principle behind electric motors. Net force on current loop = 0 For a coil with N turns,  = N I A B sin , where A is the area of coil Rectangular coil: A=ab, current = i But: Net torque is NOT zero!

5. Magnetic Dipole Moment N = number of turns in coil A=area of coil. We just showed:  = NiABsin  Right hand rule: curl fingers in direction of current; thumb points along  Define: magnetic dipole moment  As in the case of electric dipoles, magnetic dipoles tend to align with the magnetic field.

6. Electric vs. Magnetic Dipoles -Q  QE +Q p=Qa

7. DC Electric Motor