Magnetic Field Formulas
(Direct from Maxwell)
| These three simple formulas can be derived directly from Maxwell's fourth equation, or Ampere's Law. | |
| Magnetic field due to an infinite, straight current filament | |
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 B is the magnetic field, in teslas. The direction of the field is tangent to a circle on a radius r (in meters) from the wire.
i is the current in the wire, in amperes. |
| Field inside a straight, infinite, air core solenoid | |
 |
 B is the magnetic field inside the solenoid, in teslas. The direction of the field is parallel to the axis of the solenoid. There is no field outside the solenoid.
i is the current in the wire, in amperes. n is the number of turns of wire per unit length of the solenoid, in 1/meters. |
| Field inside an air core toroid coil | |
 |
 B is the magnetic field, in teslas. The direction of the field is tangent to a circle on a radius r (in meters) from the center of the toroid.
i is the current in the wire, in amperes. N is the total number of turns of wire in the toroid. |
is the permeability constant
(1.26x10-6 H/m)