1. +

2. +

3. +

4. +

5. +

6. -

7. -
A positive charge moving right makes the same field as a negative charge moving left.

8. 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

9. 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.

10. 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

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

12. 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

13. A long straight wire
Vectors:
Calculation…

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

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