(d) Electromagnetic induction
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
6.15Induced voltageKnow that a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it and describe the factors that affect the size of the induced voltage6.16Generating electricityDescribe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field, and describe the factors that affect the size of the induced voltage617P TransformersDescribe the structure of a transformer and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides618P Step-up and step-down transformersExplain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy619P Transformer turns ratioKnow and use input voltage ÷ output voltage = primary turns ÷ secondary turns, Vₚ/Vₛ = Nₚ/Nₛ.620P Transformer power equationKnow and use input power = output power, VₚIₚ = VₛIₛ, for a 100% efficient transformer.
Explain how a changing magnetic field induces a voltage
A voltage is induced across a conductor when the magnetic field through or around it changes. This can happen when a conductor or coil moves through a magnetic field, when a magnet moves relative to a coil, or when the field itself changes.
Use the causal chain: relative motion or changing field → the conductor cuts magnetic field lines, or the field linkage through the coil changes → a voltage is induced. A current flows only if the conductor is part of a complete circuit; an induced voltage can still exist across an open circuit.
| Change | Effect on induced voltage | Why |
|---|---|---|
| move the conductor or magnet faster | larger magnitude | field lines are cut, or field linkage changes, more rapidly |
| use a stronger magnetic field | larger magnitude | the conductor encounters a greater field change |
| use more turns in the coil | larger magnitude | more turns experience the changing field |
| increase the length of conductor cutting the field | larger magnitude | more conductor takes part in the induction |
Reversing the direction of motion, or reversing the magnet's poles while keeping the motion the same, reverses the polarity of the induced voltage. If the motion repeatedly reverses, the induced voltage alternates. At an instant when the field linkage is not changing, no voltage is induced.
Generate an alternating voltage by rotation
A generator transfers mechanical energy to electrical energy by electromagnetic induction. Either rotate a coil in a magnetic field or rotate a magnet inside a coil: the relative rotation continually changes the magnetic field through the coil.
As the coil or magnet rotates, the field linkage changes in one direction and then the other. The induced voltage therefore reverses every half-turn, producing an alternating voltage. If an external circuit is complete, an alternating current flows.
| Generator change | Effect on output |
|---|---|
| faster rotation | larger peak voltage and more cycles each second |
| stronger magnetic field | larger peak voltage |
| more turns on the coil | larger peak voltage |
A generator needs relative motion between a coil and a magnetic field; neither part must be labelled as the only moving part. Do not confuse a generator with a motor: a generator uses motion to induce a voltage, whereas a motor uses current in a magnetic field to produce force and motion.
Explain how a transformer changes an alternating voltage
A transformer has a primary coil and a secondary coil of insulated wire wound around a shared soft iron core. The two coils are not electrically connected; the core links the changing magnetic field between them.
An alternating voltage drives an alternating current in the primary coil → the primary creates a changing magnetic field in the soft iron core → the changing field passes through the secondary coil → a voltage is induced across the secondary coil.
| Turns comparison | Voltage change | Transformer type |
|---|---|---|
| secondary has more turns than primary | secondary voltage is greater | step-up |
| secondary has fewer turns than primary | secondary voltage is smaller | step-down |
| both coils have the same number of turns | secondary voltage equals primary voltage in the ideal model | 1:1 transformer |
A transformer requires a changing magnetic field, so it works with alternating current and not with a steady direct current. Soft iron is used because it magnetises and demagnetises readily; the core transfers changing magnetic flux, not electric current, from one coil to the other.
Explain why the grid steps voltage up and down
In large-scale electricity supply, a step-up transformer raises the generator voltage before long-distance transmission. Step-down transformers then reduce the voltage near consumers to values suitable for distribution and use.
For approximately the same transmitted power, increasing voltage allows a smaller current. A smaller current causes less heating in transmission cables, so less electrical energy is wasted and more reaches consumers.
| Stage | Transformer action | Purpose |
|---|---|---|
| power station to transmission grid | step up voltage; current decreases | reduce heating and energy loss in long cables |
| transmission grid to local distribution | step down voltage | provide lower distribution voltages |
| local supply to users | step down as required | provide a safer, usable voltage for equipment |
The step-up transformer does not create extra energy: it trades higher voltage for lower current. The transmission voltage is reduced again before use because the very high grid voltage is unsuitable and dangerous for consumers.
Calculate with the transformer turns ratio
For an ideal transformer, the voltage ratio equals the turns ratio. Use primary quantities together and secondary quantities together.
VsVp=NsNp
Vp and Vs are the primary and secondary voltages; Np and Ns are the numbers of turns on the primary and secondary coils.
Method: 1. Write the ratio with primary values on top and secondary values below. 2. Substitute voltages in the same unit. 3. Rearrange before evaluating. 4. Check the direction: if Ns > Np, then Vs must be greater than Vp; if Ns < Np, then Vs must be smaller.
Example: a transformer has 160 primary turns, 45 secondary turns and a 12 V primary supply. Vs = 12 × 45 ÷ 160 = 3.375 V, so the output is about 3.4 V. This is a step-down transformer because the secondary has fewer turns and a lower voltage.
The symbols p and s identify coils, not larger and smaller values. Do not invert only one side of the equation, and do not confuse number of turns N with power P.
Calculate voltage and current in an ideal transformer
A 100% efficient transformer is an ideal model: its electrical input power equals its electrical output power.
VpIp=VsIs
VpIp is the primary input power and VsIs is the secondary output power. Voltage is in volts, current is in amperes and each product is in watts.
Because the two powers are equal, stepping voltage up makes current step down, and stepping voltage down makes current step up. This inverse change explains how a high transmission voltage can carry the same power with a smaller current.
Example: an ideal transformer takes 230 V and 4.5 A, and supplies 0.21 A. Vs = 230 × 4.5 ÷ 0.21 = 4928.6 V, so the output voltage is about 4.9 kV. The higher output voltage is consistent with the lower output current.
Use VpIp = VsIs only when the transformer is stated or assumed to be 100% efficient. A real transformer loses some energy, so its output power is less than its input power; equality is the ideal-model boundary, not a claim that every transformer is lossless.