1. So far…

2. Moving electric charges make magnetic fields (磁场).
Biot-Savart Law
We can find the magnetic field caused by any current by using integration (积分).

3. Long current-carrying wire: Current-carrying loop:
magnetic dipole moment

4. Atoms can be magnetic dipoles.
So dipoles are the key to understanding the magnetism of matter.

5. Another example:
Solenoid 螺线管

6. The field in the middle is approximately uniform.
where n is the number of loops.

7. There are no magnetic “charges”.
Can you think of a way to make a magnetic field like this?
There are no magnetic “charges”.

8. Moving electric charges make magnetic fields… and magnetic fields make forces on moving electric charges.

9. -
The force is always perpendicular to the direction of motion: it cannot change the particle’s speed.

10. A charge moving in a uniform magnetic field will move in circles.
Radius of the circle:
(for v << c)

11. A charge moving in a uniform magnetic field will move in circles.
Accounting for relativity:
(for any speed)

12. The Shanghai synchrotron 同步加速器
Magnetic fields make the electrons move in a circle
Electric fields make them go faster – close to the speed of light!

15. Velocity selector: +

16. Velocity selector: +

17. Velocity selector: +