3.7.5.3
Four ways to change the flux linkage through a coil
Magnetic Flux & Flux Linkage — AQA A-Level Physics
- Look at the equation $N\Phi = BAN \cos \theta. There are four quantities you could change$, and each one would change the flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area.-turns (Wb turns).:
1. Change B — the magnetic flux densityMass per unit volume of a material. Measured in kg m⁻³.The strength of a magnetic field. The force per unit length per unit currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). on a currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).-carrying conductor perpendicular to the field. Measured in teslaThe SI unit of magnetic flux density. One tesla is the flux density when a force of 1 N acts on a 1 m conductor carrying 1 A perpendicular to the field. (T).
- Strengthen or weaken the external field. Move a magnet closer or further away, or change the currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). in an electromagnet.
- This is what happens when you push a bar magnet into a coil. The field through the coil increases, the flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area.-turns (Wb turns). increases, and an EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is induced.
2. Change A — the area of the coil
- Stretch or compress the coil to change its cross-sectional area.
- In practice this is hard to do with a rigid coil, but a flexible conducting loop being pulled open or squeezed shut would change A and therefore Φ.
3. Change θ — the angle between the field and the normal
- Rotate the coil in the field. This is the most important case — it's how generators and alternators work.
- As the coil rotates, θ changes continuously, so $N\Phi = BAN \cos \theta changes continuously$, and a continuously changing EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is induced.
4. Change N — the number of turns
- Add or remove turns from the coil. More $turns = more flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area.-turns (Wb turns).$.
- In practice, N is usually fixed. But the principle matters: transformers work because the flux links different numbers of turns on the primary and secondary coils.
Examiner Tips and Tricks
- A common 2-mark question: 'State two ways the flux linkage through a coil can be increased.' Pick any two: increase B, increase A, increase N, or rotate the coil so θ is closer to 0°.
- Give a specific physical action, not just 'change B'.