6.3FieldsCore

Magnetic Flux Density and Force on a Current

A current-carrying wire in a magnetic field feels a genuine mechanical push — the motor effect, and the basic principle behind every electric motor and loudspeaker ever built.

35 min Video by Malmesbury Education Module 6.3 Magnetic fields

Motor Effect F=BIL — Physics A-level PracticalWatch the full walkthrough before the notes below.

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What you'll be able to do

Define magnetic flux density and the tesla
Use $F = B I L sin θ$ for the force on a current-carrying conductor
Use Fleming’s left-hand rule to find the direction of the force
Explain why force is zero when current is parallel to the field

Magnetic flux density

Magnetic flux density, $B$ , measures field strength in tesla (T), defined via the force it produces on a current-carrying conductor.

Force on a current-carrying conductor

The force on a current-carrying wire depends on flux density, current, length in the field, and the angle between current and field — maximum when perpendicular, zero when parallel.

F = B I L sin θ

Force on a current-carrying conductor.

1Perpendicular:

sin θ = 1

, so

F = B I L

F = 0.60 \times 4.0 \times 0.25

Answer

F = 0.60

Fleming’s left-hand rule

The force direction is perpendicular to both current and field, found using Fleming’s left-hand rule: First finger = Field, seCond finger = Current, thUmb = force (thrust).

Tip — "FBI": First finger Field, seCond finger Current, thUmb thrust — always the LEFT hand for the motor effect.

Equation recap

F = B I L sin θ

Force on a current-carrying conductor.

Common mistakes to avoid

Using F=BIL even when the wire isn’t perpendicular to the field.

F=BIL only applies at θ=90° — use the full F=BILsinθ otherwise.

Using the right hand for the motor effect.

Fleming’s LEFT hand is for force from a current; the right hand is for generators/induced current.

Key takeaways

Magnetic flux density B is measured in tesla.
F = BILsinθ; maximum when perpendicular, zero when parallel.
Fleming’s left-hand rule: First=Field, seCond=Current, thUmb=thrust.

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