A transformer consists of two coils (primary and secondary) wound on a shared soft iron core
Electromagnetism - OCR A-Level Physics
- A transformerA device that changes the voltage of an a.c. supply using electromagnetic induction between two coils on an iron core. consists of two coils (primary and secondary) wound on a shared soft iron core.
- 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). in the primary creates a 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, which links to the secondary and induces an e.m.f.
- Transformers only work with a.c.Alternating current — current that reverses direction periodically, typically as a sine wave. because a steady 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). produces a constant flux with no change - hence no induced e.m.f.
$$\frac{V_s}{V_p} = \frac{N_s}{N_p}$$
- For an ideal (100% efficient) transformer: powerThe rate of energy transfer. Measured in watts (W). $in = powerThe rate of energy transfer. Measured in watts (W). out$, so V_p $I_{p} = V_{s} I_{s}$.
- A step-upA transformer where the secondary voltage is greater than the primary (more turns on the secondary coil). transformer has N_s > N_p (increases 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., decreases currentThe rate of flow of charge. Measured in amperes (A).).
- A step-downA transformer where the secondary voltage is less than the primary (fewer turns on the secondary coil). transformer has N_s < N_p (decreases voltageThe energyThe capacity to do work. Measured in joules (J). transferred per unit charge between two points. Measured in volts (V). Informal term for potential difference., increases current).
- EnergyThe capacity to do work. Measured in joules (J). losses in real transformers include: resistive heatingEnergy lost as heat due to the resistance of the coil windings in a transformer (P = I²R). (I²R in coils), eddy currentsLoops of induced current in a conductor exposed to a changing magnetic field; they cause energy loss as heat. in the core (reduced by lamination), flux leakageLoss of magnetic flux between the primary and secondary coils of a transformer, reducing efficiency. (not all flux links both coils), and hysteresisThe energy loss in a magnetic core caused by the repeated magnetisation and demagnetisation of the material during each AC cycle. (energy used magnetising and demagnetising the core each cycle).
Common Mistake
MEDIUM
Wrong: A transformer can work with d.c.
Right: A transformer requires a changing flux to induce an e.m.f. D.C. produces a constant flux, so $\varepsilon = -d\Phi/dt = 0. A brief e.m.f$. is induced only when the d.c. is switched on or off.
Right: A transformer requires a changing flux to induce an e.m.f. D.C. produces a constant flux, so $\varepsilon = -d\Phi/dt = 0. A brief e.m.f$. is induced only when the d.c. is switched on or off.