What do the symbols in F = BIl mean?

F force (N), B flux density (T), I current (A), l length of conductor in the field (m).

State Fleming's left-hand rule.

First finger = field, second finger = conventional current, thumb = force (thrust). The three are mutually at right angles.

The flux density that produces a force of 1 N on a 1 m length of conductor carrying a current of 1 A at right angles to the field.

What happens to the force if the current direction is reversed?

The force reverses direction; its magnitude is unchanged.

What happens to the force if both the current and the field are reversed?

The force stays in the same direction.

In Required practical 10, why does the balance reading change?

The wire is pushed by the field, so (Newton's third law) the magnets are pushed with an equal and opposite force, which the balance registers.

In RP10, how is the force found from the balance?

Force = change in reading (kg) × g.

Why is the force on a conductor given by the direction of conventional current?

Conventional current direction is the agreed convention for the rule; the force on negative charge carriers moving one way equals that on positive carriers moving the other way.

Magnetic flux density and force on a conductorAQA A-Level Physics: Flashcards

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State the equation for force on a conductor perpendicular to a magnetic field.

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State the equation for force on a conductor perpendicular to a magnetic field.: F = BIl.
What do the symbols in F = BIl mean?: F force (N), B flux density (T), I current (A), l length of conductor in the field (m).
State Fleming's left-hand rule.: First finger = field, second finger = conventional current, thumb = force (thrust). The three are mutually at right angles.
Define the tesla.: The flux density that produces a force of 1 N on a 1 m length of conductor carrying a current of 1 A at right angles to the field.
Express 1 T in other units.: 1 T = 1 N A⁻¹ m⁻¹.
What is the force on a wire parallel to a magnetic field?: Zero.
What happens to the force if the current direction is reversed?: The force reverses direction; its magnitude is unchanged.
What happens to the force if both the current and the field are reversed?: The force stays in the same direction.
In Required practical 10, why does the balance reading change?: The wire is pushed by the field, so (Newton's third law) the magnets are pushed with an equal and opposite force, which the balance registers.
In RP10, how is the force found from the balance?: Force = change in reading (kg) × g.
In RP10, what is the gradient of a graph of balance reading (kg) against current?: Bl/g.
Why is the force on a conductor given by the direction of conventional current?: Conventional current direction is the agreed convention for the rule; the force on negative charge carriers moving one way equals that on positive carriers moving the other way.

Exam questions on Magnetic flux density and force on a conductor

A technician places a straight horizontal copper rod between the poles of a magnet. The rod carries a conventional current of 6.0 A towards the north. The magnet provides a uniform vertical magnetic field, directed downwards, with flux density 0.24 T over a 0.050 m length of the rod; the rod is at right angles to the field.
The current is halved and the length of rod in the field is doubled, with the flux density unchanged. Deduce the new force on the rod and its direction.2 marks
A student investigates the force on a wire using a top pan balance. A magnet assembly rests on the pan, with a rigid horizontal wire clamped so that 0.040 m of it lies at right angles to the uniform field between the poles, without touching the magnet. The balance is set to zero with no current. When a current of 2.5 A is switched on, the balance reading changes. The flux density between the poles is 0.095 T. Take g = 9.81 N kg⁻¹.
The student repeats the experiment for several currents with the length of wire in the field fixed, and plots the balance reading in kilograms against current. State the shape of the graph and explain how the flux density can be found from it.2 marks
A straight horizontal conductor of length 0.15 m and mass 8.0 g hangs from two very flexible leads in a uniform horizontal magnetic field of flux density 0.060 T, which is at right angles to the conductor. Current in the conductor is arranged so that the magnetic force on it acts vertically upwards. Take g = 9.81 N kg⁻¹.
Calculate the current required for the magnetic force on the conductor to balance its weight.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).