Magnetic Flux Linkage (AQA A Level Physics): Revision Note

Magnetic Flux Linkage

• More coils in a wire mean a larger e.m.f is induced

• The magnetic flux linkage is a quantity commonly used for solenoids which are made of N turns of wire

• The flux linkage is defined as:

The product of the magnetic flux and the number of turns of the coil

• It is calculated using the equation:

Magnetic flux linkage = ΦN = BAN

• Where:

  • Φ = magnetic flux (Wb)

  • N = number of turns of the coil

  • B = magnetic flux density (T)

  • A = cross-sectional area (m²)

• The flux linkage ΦN has the units of Weber turns (Wb turns)

Flux Linkage in a Rotating Rectangular Coil

• When the magnet field lines are not completely perpendicular to the area A, then the perpendicular component of magnetic flux density B is taken

• The equation then becomes:

Φ = BA cos(θ)

• Where:

  • Φ = magnetic flux (Wb)

  • B = magnetic flux density (T)

  • A = cross-sectional area (m²)

  • θ = angle between magnetic field lines and the line perpendicular to the plane of the area (often called the normal line) (degrees)

• This means the magnetic flux is:

  • Maximum = BA

    • when cos(θ) =1 therefore θ = 0^o

    • The magnetic field lines are perpendicular to the plane of the area

  • Minimum = 0

    • when cos(θ) = 0 therefore θ = 90^o

    • The magnetic fields lines are parallel to the plane of the area

• An e.m.f is induced in a circuit when the magnetic flux linkage changes with respect to time

• This means an e.m.f is induced when there is:

  • A changing magnetic flux density B

  • A changing cross-sectional area A

  • A change in angle θ

The magnetic flux through a rectangular coil decreases as the angle between the field lines and plane decrease

• Magnetic field lines may not be completely perpendicular to the plane of the area that they pass through

• Therefore, the component of the flux density which is perpendicular is equal to:

ΦN = BAN cos(θ)

• Where:

  • N = number of turns of the coil