So far…
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).
Long current-carrying wire: Current-carrying loop: magnetic dipole moment
Atoms can be magnetic dipoles. So dipoles are the key to understanding the magnetism of matter.
Another example: Solenoid 螺线管
The field in the middle is approximately uniform. where n is the number of loops.
There are no magnetic “charges”. Can you think of a way to make a magnetic field like this? There are no magnetic “charges”.
Moving electric charges make magnetic fields… and magnetic fields make forces on moving electric charges.
- The force is always perpendicular to the direction of motion: it cannot change the particle’s speed.
A charge moving in a uniform magnetic field will move in circles. Radius of the circle: (for v << c)
A charge moving in a uniform magnetic field will move in circles. Accounting for relativity: (for any speed)
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!
Velocity selector: +
Velocity selector: +
Velocity selector: +