AQA A Level Physics

Revision Notes

7.8.1 Magnetic Force on a Current-Carrying Conductor

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Magnetic Force on a Current-Carrying Conductor

  • A current-carrying conductor produces its own magnetic field
    • When interacting with an external magnetic field, it will experience a force

  • A current-carrying conductor (eg. a wire) will experience the maximum magnetic force if the current through it is perpendicular to the direction of the magnetic field lines
    • It experiences no force if it is parallel to magnetic field lines

  • A simple situation would be a copper rod placed within a uniform magnetic field
  • When current is passed through the copper rod, it experiences a force which makes it accelerate

Copper rod experiment, downloadable AS & A Level Physics revision notes

A copper rod moves within a magnetic field when current is passed through it

  • The strength of a magnetic field is known as the magnetic flux density, B
    • This is also known as the magnetic field strength
    • It is measured in units of Tesla (T)

  • The force F on a conductor carrying current I at right angles to a magnetic field with flux density B is defined by the equation

F = BIL sinθ

  • Where:
    • F = force on a current carrying conductor in a B field (N)
    • B = magnetic flux density of external B field (T)
    • I = current in the conductor (A)
    • L = length of the conductor (m)
    • θ = angle between the conductor and external B field (degrees)

  • This equation shows that the greater the current or the magnetic field strength, the greater the force on the conductor
  • The length of the conductor, L in this equation is only the length that is within the field

Force on conductor (1), downloadable AS & A Level Physics revision notesForce on conductor (2), downloadable AS & A Level Physics revision notes

Magnitude of the force on a current carrying conductor depends on the angle of the conductor to the external B field

  • The maximum force occurs when sin θ = 1
    • This means θ = 90o and the conductor is perpendicular to the B field
    • This equation for the magnetic force now becomes:

F = BIL

  • The minimum force (0) is when sin θ = 0
    • This means θ = 0o and the conductor is parallel to the B field

  • It is important to note that a current-carrying conductor will experience no force if the current in the conductor is parallel to the field
    • This is because the F, B and must be perpendicular to each other

Worked example

A current of 0.87 A flows in a wire of length 1.4 m placed at 30o to a magnetic field of flux density 80 mT.Calculate the force on the wire.

Step 1: Write down the known quantities

    • Magnetic flux density, B = 80 mT = 80 × 10-3 T
    • Current, I = 0.87 A
    • Length of wire, L = 1.4 m
    • Angle between the wire and the magnetic field, θ = 30o

Step 2: Write down the equation for force on a current-carrying conductor

F = BIL sinθ

Step 3: Substitute in values and calculate

F = (80 × 10-3) × (0.87) × (1.4) × sin(30) = 0.04872 = 0.049 N (2 s.f)

Exam Tip

Remember that the direction of current flow is the flow of positive charge (positive to negative), and this is in the opposite direction to the flow of electrons

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.