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- A positive charge moving right makes the same field as a negative charge moving left.

Calculating current = drift speed of electrons n = density of electrons (number per unit volume) In a time Δt, the electrons move a distance Number of electrons in the yellow volume Electron current

The direction is opposite that of the electron current. Calculating current Electron current = drift speed of electrons n = density of electrons (number per unit volume) Conventional current Conventional current The direction is opposite that of the electron current.

Magnetic field of a current Sum over all charges in the short length of wire dl A short piece of wire, carrying a current i

Magnetic field of a current A short piece of wire, carrying a current i

Magnetic field of a current Biot-Savart Law for a current-carrying wire: The vector points in the direction of the conventional current. A short piece of wire, carrying a current i

A long straight wire Vectors: Calculation…

A long straight wire where R is the perpendicular distance from the wire.

For a very long wire (L >> r):