Magnetic Forces and Applications in Physics

1. Today’s agenda: Review and some interesting consequences of F=qvxB. You must understand the similarities and differences between electric forces and magnetic forces on charged particles. Magnetic forces on currents and current-carrying wires. You must be able to calculate the magnetic force on currents. Magnetic forces and torques on current loops. You must be able to calculate the torque and magnetic moment for a current-carrying wire in a uniform magnetic field. Applications: galvanometers, electric motors, rail guns. You must be able to use your understanding of magnetic forces and magnetic fields to describe how electromagnetic devices operate.

2. Reminder: signs Include the sign on q, properly account for the directions of any two of the vectors, and the direction of the third vector is calculated “automatically.” If you determine the direction “by hand,” use the magnitude of the charge. Everything in this equation is a magnitude. The sign of r had better be +!

3. y  z x z  y x Reminder: left- and right-hand axes This is a right-handed coordinate system: This is not: For the magnetism part of physics 2135, you MUST use right-hand axes. And you’d better use your right hand when applying the right-hand rule!

4. y  z x z  y x Handy way to “see” if you have drawn right-hand axes: Z ? y x ? I personally find the three-fingered axis system to often (but not always) be the most useful way to apply the right-hand rule.

5. “In and does it matter which finger I use for what?”             You’ll learn about F = IL x B later in today’s lecture.             No, as long as you keep the right order. No, as long as you keep the right order. All three of these will work:

6.       This works: Switching only two is wrong! This doesn’t: “The right-hand rule is unfair! Physics is discriminating against left-handers!” No, you can get the same results with left-hand axes and left-hand rules. See thisweb page.

7. But Physics 24 does discriminate against left-handers! This is Captain Jack Crossproduct. He visits our classes occasionally (see the physics on the blackboard behind him). You don’t want to see what he does with his scimitar when he sees a left hand used for the right hand rule! The right hand rule is just a way of determining vector directions in a cross product without having to do math.

8. Magnetic and Electric Forces The electric force acts in the direction of the electric field. The electric force is nonzero even if v=0. The magnetic force acts perpendicular to the magnetic field. The magnetic force is zero if v=0.

9. + + Magnetic and Electric Forces The electric force does work in displacing a charged particle. E D F The magnetic force does no work in displacing a charged particle! v  B ds Amazing! not really F