Magnetic Fields and Forces

Magnetic fields are produced by moving charges — whether that is a current flowing through a wire, electrons orbiting a nucleus, or the aligned magnetic domains in a permanent magnet. This lesson covers the forces that magnetic fields exert on current-carrying conductors and moving charges, which form the basis of electric motors, loudspeakers, and particle accelerators.

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Magnetic Flux Density

Magnetic flux density (B) — often loosely called "magnetic field strength" — is defined by the force it exerts on a current-carrying conductor:

B = F / (IL sinθ)

or equivalently, the force on a current-carrying conductor in a magnetic field is:

F = BIL sinθ

where:

• F is the force on the conductor (N)
• B is the magnetic flux density (T — tesla)
• I is the current (A)
• L is the length of conductor in the field (m)
• θ is the angle between the conductor and the field direction

When the conductor is perpendicular to the field (θ = 90°), sinθ = 1 and the force is maximum:

F = BIL

When the conductor is parallel to the field (θ = 0°), sinθ = 0 and the force is zero.