Apply: force on a conductor (at right angles to a magnetic field) carrying…

The idea

This Higher tier equation gives the force on a straight conductor carrying a current when the conductor sits at right angles to a uniform magnetic field. The force in newtons equals magnetic flux density in tesla multiplied by current in amperes multiplied by the length of conductor in metres within the field: F = B × I × L. For example, a 0.5 T field acting on a 2 A current in 0.3 m of wire gives F = 0.5 × 2 × 0.3 = 0.3 N. Learners apply the equation, keep units consistent and recognise that the force is greatest at right angles and zero when the conductor is parallel to the field. Rearranging to find B, I or L is also expected.

What earns the marks

• Selects the relationship force = magnetic flux density × current × length and substitutes the given values correctly.
• Calculates the force in newtons using consistent units of tesla, ampere and metre.
• Rearranges the equation to determine magnetic flux density, current or length when the force is known.
• Recognises that the force is greatest when the conductor is at right angles to the field and zero when parallel.

Common mistakes and how to put them right

• Multiplies the values without checking units, giving an answer in the wrong unit.

  Put it right: Convert lengths to metres and keep tesla and ampere before calculating.

• Assumes the force is largest when the conductor is parallel to the field.

  Put it right: State that the force is greatest at right angles and zero when parallel.

• Forgets to rearrange before substituting when finding current or flux density.

  Put it right: Isolate the required quantity first, then substitute.

Exam tips

• Write the equation, substitute values with units, then state the unit of the answer.
• Convert any length given in centimetres to metres before multiplying.
• Check whether the question asks for force, flux density, current or length, and rearrange before calculating.