Fleming's left-hand rule (HT only): length, l, in metres, m

The idea

In motor-effect calculations, the length l is the length of the conductor that lies inside the magnetic field and is perpendicular to the field lines. It is measured in metres, m, so always convert centimetres or millimetres before substituting into F = B I l. For example, a 5.0 cm wire inside a 0.20 T field carrying 3.0 A gives l = 0.050 m, so F = 0.20 T × 3.0 A × 0.050 m = 0.030 N. The equation F = B I l applies when the wire is at right angles to the magnetic field. The symbol l is italic because it is a quantity, while the unit symbol m is not italic. Recording l with its unit keeps the calculation consistent with B in tesla and I in amperes.

What earns the marks

• State that l is the length of conductor within the magnetic field, measured in metres (m).
• Convert lengths given in cm or mm into metres before using F = B I l.
• Recognise that the equation F = B I l is used when the conductor is perpendicular to the magnetic field.
• Keep the unit symbol m separate from the quantity symbol l, and give the final force in newtons.
• Substitute values correctly into F = B I l, for example 0.20 T × 3.0 A × 0.050 m = 0.030 N.

Common mistakes and how to put them right

• Substituting a length in centimetres directly into F = B I l.

  Put it right by dividing by 100 to convert to metres first.

• Using the whole wire length when only part of it lies in the magnetic field.

  Put it right by identifying only the length inside the field.

• Treating l as a unit rather than a quantity.

  Put it right by writing l = 0.050 m, where l is the quantity and m is its unit.

Exam tips

• Underline the length value and its unit in the question, then convert to metres before calculating.
• Write the equation, substitute values with units, and state the force unit in your answer.
• Ensure you only use the length of the wire that is actually within the magnetic field when calculating force.