6: Fields and Forces
6.3 Magnetic Force and Field
Magnetic Fields
Magnetic field is sometimes referred to as ‘magnetic flux’.
Magnetic fields are caused by electric currents. These may be large (as in a solenoid) or tiny (such as that associated with electrons moving in orbits).
While electric and gravitational forces cause charged bodies or those containing mass to accelerate, magnetic fields are different; they cause a magnet placed in the field to rotate. The direction of the field lines is the direction of the force experienced by a north pole placed in the field.
Magnetic Flux Density
We cannot talk about a ‘magnetic field strength’ as force per unit pole. This is impossible as a single pole cannot exist on its own.
Instead we refer to ‘magnetic flux density’, B.
Magnetic flux can be thought of as the number of field lines passing through a certain area. Measured in Webers (Wb).
Magnetic flux density is the amount of magnetic flux per unit cross sectional area. Measured in Tesla.
Hence magnetic fields are sometimes referred to as ‘B fields’.
Field patterns due to electric currents
Wire:
Solenoid:
E.g.1
E.g.2
Applications of Fleming’s LH Rule
1. A free charged particle in a magnetic field will always experience a force perpendicular to its motion. This will provide a centripetal force, resulting in circular motion:
2. An electric motor
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F = BIL
F = BqV
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