Aurora 极光

Lodestone 磁石

Compass 指南针

Hans Christian Oersted (1777-1851) 21 April, 1820, during a university lecture: the discovery of electromagnetism.

Electric fields E are caused by charges. Magnetic fields B are caused by moving charges.

Current 电流 - - - - - - - - - - - - The electron current is the number of electrons per second that enter a section of a conductor.

Current 电流 - + + + + + + + + + + + + + + We usually pretend that positive charges are moving. (Sometimes this is true!)

Current 电流 + + + + + + + + + + + + + + Conventional current i is the amount of (positive) charge per second that enters a section of conductor. Its direction is opposite that of the electron current.

Current 电流 + + + + + + + + + + + + + + Units of current: Coulombs/second, or amperes

An electric current can deflect a compass needle.

An electric current can deflect a compass needle.

There is no deflection if the current is perpendicular to the compass.

The direction of the deflection depends on if the current is above or below the compass.

The direction of the deflection depends on the direction of the current.

How can we explain these experiments? By a field that curls around the current-carrying wire.

Magnetic field of a single moving charge: The Biot-Savart Law + Units: tesla (T) “Permeability of free space”

+ What direction is the magnetic field here?

- What direction is the magnetic field here?

- What direction is the magnetic field here?

+ The magnetic field here is pointing out of the wall. Which direction is the charge moving? +