3.7.5.6
A transformer uses changing flux to transfer electrical energy between two coils
Transformers — AQA A-Level Physics
Key Definition
Transformer — A device that changes the magnitude of an alternating voltage. It consists of a primary coil and a secondary coil wound on a shared soft iron core.
- A transformer has two coils of insulated wire, both wound around the same soft iron core. The primary coil is connected to the input AC supply. The secondary coil is connected to the output load.
- When an alternating 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). flows in the primary coil, it creates a changing magnetic field around that coil.
- The soft iron core channels this changing magnetic fluxThe product of magnetic flux densityMass per unit volume of a material. Measured in kg m⁻³. and the area perpendicular to the field. 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. (Wb). so that nearly all of it passes through the secondary coil as well. Iron is used because it is easily magnetised (high permeability) and has low hysteresis losses.
- The changing flux through the secondary coil induces an EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). across it, by Faraday's lawThe magnitude of the induced EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is proportional to the rate of change of magnetic 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)... This is the output voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). Informal term for potential difference..
- There is no electrical connection between the two coils. EnergyThe capacity to do work. Measured in joules (J). is transferred entirely through the changing magnetic fluxThe product of magnetic flux densityMass per unit volume of a material. Measured in kg m⁻³. and the area perpendicular to the field. 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. (Wb). in the core.
- A transformer only works with AC. If you connect DC, 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). is constant, the flux is constant, there is no change in flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weber-turns (Wb turns)., and no EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is induced in the secondary. After the initial switch-on transient, the output drops to zero.
- The iron core is laminated (built from thin insulated sheets) to reduce eddy currentThe rate of flow of charge. Measured in amperes (A). losses. More on this in the losses card.
Examiner Tips and Tricks
- When explaining how a transformer works, make sure you hit all four links in the chain: AC in primary -> changing flux in core -> flux links secondary coil -> EMF induced by Faraday's lawThe magnitude of the induced EMF is proportional to the rate of change of magnetic flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weber-turns (Wb turns)...
- Missing any link loses marks.
Common Mistake
MEDIUM
Students often: Don't say the transformer 'transfers current' or 'passes current' from primary to secondary.
Instead: The two coils are electrically isolated. EnergyThe capacity to do work. Measured in joules (J). is transferred via the changing magnetic fluxThe product of magnetic flux densityMass per unit volume of a material. Measured in kg m⁻³. and the area perpendicular to the field. Measured in weber (Wb). in the core, not by current flowing from one coil to the other.
Instead: The two coils are electrically isolated. EnergyThe capacity to do work. Measured in joules (J). is transferred via the changing magnetic fluxThe product of magnetic flux densityMass per unit volume of a material. Measured in kg m⁻³. and the area perpendicular to the field. Measured in weber (Wb). in the core, not by current flowing from one coil to the other.