Induced Potential, Transformers and the National Grid

Take a length of wire and push it down through the gap of a horseshoe magnet (between the N and S poles). A sensitive voltmeter connected across the ends of the wire will flick: while the wire is moving, a potential difference is induced across it.

This is electromagnetic induction — the generator effect.

Two equivalent set-ups.

1. Moving conductor in a stationary field. Push a straight wire through the field; the free electrons in the wire experience a force (motor effect on each charge) that drives them along the wire → charge separation → p.d. across the ends.
2. Stationary conductor in a changing field. Push a bar magnet into a coil (or pull it out). The magnetic field threading the coil changes; this induces an EMF around the coil → p.d. across the ends.

In both cases, what matters is that the magnetic field through the conductor is changing. Either move the conductor or move the field — the physics is the same.

Complete the circuit, get a current. If you connect the wire (or coil) into a closed circuit, the induced p.d. drives a current — the induced current. AQA's classic demo: pushing a magnet into a coil connected to a sensitive galvanometer makes the needle flick.

Stationary = nothing. A magnet held still inside a coil produces no induced current. The field threading the coil is constant; there is no change to induce a p.d. Movement (relative motion between magnet and coil) is essential.

Pushing the N pole INTO the coil induces a current → galvanometer deflects. Pull it OUT → deflection reverses. Magnet held still → no deflection.

The induced p.d. (and the induced current, if the circuit is closed) becomes larger if you increase any of the following:

1. Speed of relative motion — push the magnet in/out faster, or push the wire through the field faster. Faster motion → faster change in field through the coil → larger p.d.
2. Magnetic flux density (B) of the field — a stronger magnet gives a stronger field, so the same motion produces a bigger change in field and a bigger induced p.d.
3. Number of turns on the coil (N) — every turn of the coil is its own little 'circuit' contributing to the induced EMF. More turns → larger total induced p.d.
4. Area of the coil (or area swept by the conductor) — a bigger coil intercepts more field lines, so a given change in B gives a larger change in 'flux' through the coil.

These factors all reduce to the same idea (called Faraday's law in more advanced study): induced p.d. is proportional to how fast the magnetic flux through the coil is changing. AQA doesn't ask for Faraday's law by name at GCSE, but the four factors above are explicitly testable.

How to demonstrate each.

• Speed. Push a magnet slowly into a coil → small galvanometer deflection. Push it in quickly → larger deflection.
• Strength. Use a weak ceramic magnet → small deflection. Use a strong neodymium magnet → larger deflection.
• Turns. Replace a 50-turn coil with a 500-turn coil → ten times the deflection.
• Area. Use a coil with a larger diameter → larger deflection (for the same magnet and motion).

The size of the induced current (if the circuit is complete) depends on the induced p.d. AND on the resistance of the circuit ($V = IR$). A high-resistance circuit will show a small induced current even when the induced p.d. is reasonable.

When a magnet is pushed into a coil and an induced current flows, that current itself produces its own magnetic field around the coil. The direction of the induced current is always such that its magnetic field opposes the change that caused it. (This is the qualitative statement of Lenz's law.)

Example 1 — pushing N into coil. Push the N pole of a magnet into a coil. The induced current flows in a direction that makes the near end of the coil behave like a N pole — which repels the incoming N pole. You have to do work against this repulsion as you push the magnet in. That work becomes the electrical energy carried by the induced current. (Energy conservation!)

Example 2 — pulling N out of coil. Pull the N pole away. The induced current now reverses: the near end of the coil becomes a S pole, which attracts the receding N pole — opposing the change (you're trying to pull it away, the coil tries to hold on).

Example 3 — pushing S into coil. Same logic but with everything flipped. The near end of the coil becomes a S pole to repel the incoming S pole.

Why does this matter?

• Energy conservation. If the induced current's field helped the change, you'd get free energy — push the magnet a little and watch it accelerate itself into the coil while the coil also gave you free electricity. That would violate energy conservation. Lenz's law guarantees that the induced current always opposes the motion, so you have to do work to keep the change going.
• It explains why generators need driving. A generator (a coil rotating in a magnetic field) has to be turned by a steam turbine or a hand-crank. The induced current creates a magnetic field that opposes the rotation — the generator pushes back, and you have to keep doing work.
• It tells you the direction. AQA may give a setup and ask which way the induced current flows. Use the 'opposing the change' rule (or Fleming's right-hand rule, not required at GCSE).

The 'jumping ring' demo. A copper ring placed on the top of an electromagnet jumps off the magnet when the AC switches on. As the AC builds up the field through the ring, an induced current flows in the ring; the ring's field opposes the change → repulsion → ring jumps off.

Classic school experiments.

1. Magnet through a coil + galvanometer. Best for showing the four factors qualitatively. Vary speed, magnet strength, number of turns and coil area; record the relative galvanometer deflection each time.
2. Wire through a horseshoe magnet + sensitive voltmeter. Push a straight wire between the poles → voltmeter flicks. Useful for showing that a single conductor (not just a coil) can produce a p.d.
3. Search coil + AC source coil. Place a small 'search coil' (connected to an AC voltmeter or oscilloscope) near a coil carrying alternating current. The changing field of the AC coil induces an AC voltage in the search coil — the basis of transformers (4.7.3.4).
4. Drop a magnet through a copper tube. Watch it fall slowly! The induced currents in the tube produce a magnetic field that opposes the magnet's motion, slowing it down dramatically.

Everyday applications of the generator effect.

• Bicycle dynamo (4.7.3.2). A small wheel turned by the tyre rotates a magnet inside a coil → AC power for the bike's lamp.
• Mains generators (4.7.3.2). Steam from coal, gas, nuclear or biomass spins a turbine that turns a huge alternator. Wind, hydro and tidal turbines do the same job with no fuel.
• Electric guitar pickup. A small magnet magnetises the steel string. When the string vibrates, the changing field through a coil under the string induces a tiny AC voltage that drives the amplifier.
• Microphones (4.7.3.3, Physics only HT only). Sound vibrates a diaphragm, which moves a coil in a magnetic field → induced AC mirrors the sound.
• Induction hob. A coil under the cooktop carries a high-frequency AC. The changing field induces currents in the steel pan above it → the pan heats up.
• Contactless cards and Oyster cards. The card reader generates an AC field; this induces an AC voltage in a tiny coil in the card, powering its chip for the moment it is held over the reader.
• Regenerative braking on EVs and trains. When a vehicle brakes, the motor is operated as a generator: the wheels turn the coil in a magnetic field, inducing a current that recharges the battery while slowing the vehicle.

Recurring AQA exam contexts. Bike dynamo, generator vs motor, magnet through a coil with the four factors, induction hob (occasionally).

An alternator (AC generator) consists of:

• A coil of wire on an axle, free to rotate.
• A pair of permanent magnets (or an electromagnet) producing a steady magnetic field across the coil.
• Two slip rings — two complete metal rings on the axle, one connected to each end of the coil.
• Two carbon brushes that press against the slip rings, conducting the induced current to the external circuit.

As the coil rotates, the magnetic field threading the coil changes continuously. By the generator effect (4.7.3.1), an alternating p.d. is induced across the ends of the coil. Because each slip ring is connected to the SAME end of the coil throughout the rotation, the current in the external circuit reverses each time the coil rotates through 180° → the output is alternating current.

The output waveform. Plot induced p.d. against time and you get a sine wave:

• Peak p.d. when the coil's sides move at right angles to B (the wires 'cut' field lines fastest).
• Zero p.d. when the coil's plane is parallel to B (the wires move along the field, not across it).
• After half a turn, the same speed is reached but the wires now cut field lines in the opposite direction → induced p.d. reverses → negative peak.

The period of the sine wave equals the time for one full rotation of the coil. UK mains is generated at 50 Hz, so the coil at a power station rotates 50 times per second.

Scale of real generators. A coal, gas or nuclear power station uses a steam turbine to spin a giant alternator at 50 Hz. Output: tens of megawatts. Wind, hydro and tidal turbines do the same job mechanically. Car alternators are tiny in comparison — a few hundred watts — but use the same physics.

AC generator (alternator)

A dynamo (DC generator) is identical to an alternator except that the two slip rings are replaced by a split-ring commutator — exactly the same component as in a DC motor (4.7.2.3).

The induced p.d. across the ends of the coil is still alternating (the physics inside the coil is unchanged). But the split-ring commutator swaps the connection to the external circuit every half-turn — at exactly the moment when the induced p.d. inside the coil reverses. The result: in the external circuit, the current always flows in the same direction, although it varies in size (it's a 'pulsing' or 'humped' DC).

Output waveform. Plot output p.d. against time: it looks like the absolute value of a sine wave — a series of positive humps, never going negative.

• Peaks at the same instants as the AC version would peak.
• Drops to zero at the same instants as the AC version would cross zero.
• Never reverses sign.

Where dynamos are used. Bicycle dynamos and older car DC generators (largely replaced by AC alternators with rectifiers since the 1960s). Some hand-crank torches and emergency phone chargers.

Why are AC alternators preferred for mains? AC can be transformed up and down in voltage (4.7.3.4), which makes long-distance transmission far more efficient than DC. UK power stations therefore produce AC. Car batteries need DC, so a car alternator's AC output is rectified to DC by diodes before being sent to the battery.

Same components, different output. This is the headline of the spec point:

Component	Alternator	Dynamo
Coil	Yes	Yes
Magnetic field	Yes	Yes
Slip rings (2 complete)	Yes	No
Split-ring commutator	No	Yes
Brushes	Yes (2)	Yes (2)
Output	AC (sine wave)	DC (positive humps)

The choice of contacts is the only thing that determines whether you build an alternator or a dynamo.

AQA frequently asks you to sketch or interpret the oscilloscope trace of a generator's output.

Alternator (AC) trace.

• A sine wave centred on the time axis.
• Goes positive then negative each cycle.
• Period $T$ equals the time for one rotation of the coil.
• Frequency $f = 1/T$. UK mains: 50 Hz → period 20 ms.
• Peak voltage at $T/4$, $3T/4$, $5T/4$… (when coil sides move at 90° to B).
• Zero crossings at $0, T/2, T, 3T/2$… (when the coil plane is parallel to B).

Dynamo (DC) trace.

• A series of positive humps — same peaks as the AC version but never goes negative.
• Looks like the AC waveform 'folded' so all negative half-cycles are flipped to positive.
• Frequency of humps = 2 × frequency of rotation (because each rotation gives two humps, one for each half-turn).

How rotation speed changes the trace.

• Spin the coil faster → peaks get higher (faster motion → bigger induced p.d.) AND humps/cycles get closer together (higher frequency).
• Spin slower → peaks shrink AND cycles spread out.

How magnet strength changes the trace.

• Stronger magnet → bigger peaks. Frequency unchanged (rotation speed unchanged).

How turns change the trace.

• More turns → bigger peaks. Frequency unchanged.

Worked sketch interpretation. Given an oscilloscope screen showing a sine wave with peak = 12 V and period = 0.04 s:

• Peak voltage = 12 V.
• Frequency = 1/0.04 = 25 Hz.
• Coil rotates 25 times per second.
• This is half UK mains frequency — it's not a domestic generator.

Mains electricity generation. Almost all UK electricity comes from large alternators in power stations or renewable plants:

• Thermal stations (coal, gas, biomass, nuclear) burn fuel (or split nuclei) to make steam; the steam spins a turbine that turns the alternator.
• Hydroelectric and tidal stations use falling or moving water to spin turbines.
• Wind turbines use wind directly to turn the blades and rotor.
• Solar PV does NOT use the generator effect — it uses the photoelectric effect — but solar thermal plants do (heat → steam → turbine).

All generators are tied to the National Grid at 50 Hz. Synchronisation is critical: a generator out of phase with the grid would fight against it and burn out.

Car alternators. The car engine drives a small alternator via a belt. The AC output is rectified to DC by a diode bridge and used to charge the battery and run the electrical systems. Before the 1960s, cars used dynamos (with commutators) directly — alternators with rectifiers are more reliable and lighter.

Bicycle dynamos. A small wheel pressed against the bike tyre rotates a magnet inside a fixed coil. This is technically an alternator (slip rings, AC output), but the term 'dynamo' is used colloquially. The AC output runs the bike's lights directly — flicker is too fast to notice when cycling.

Regenerative braking. In an electric car or train, the drive motor is reversed in operation when braking: the moving wheels turn the rotor in the magnetic field, generating an induced current that recharges the battery. The motor is now acting as a generator. This recovers a significant fraction of the vehicle's kinetic energy (typically 20–60 %).

Microgrids and emergency generators. A diesel generator on a backup system is a small alternator with a diesel engine driving it. Sized in kW for homes, MW for hospitals.

Why is the National Grid AC? Because AC voltages can be transformed up and down with transformers (4.7.3.4) — and transmission at very high voltage (400 kV) means low current and small I²R losses in the cables. DC transmission is possible but needs expensive converters at each end.

A standard moving-coil microphone has three essential parts:

1. Diaphragm. A thin, light, circular sheet of plastic or metal foil at the front of the microphone. Sound waves cause it to vibrate.
2. Voice coil. A small cylindrical coil of fine wire fixed to the back of the diaphragm. The coil sits inside a cylindrical gap in a permanent magnet.
3. Permanent magnet. A strong magnet (often neodymium) shaped to create a radial magnetic field in the gap where the coil sits — exactly the same arrangement as in a loudspeaker.

When sound waves hit the diaphragm, they push it inward and outward as compressions and rarefactions arrive. The coil, glued to the diaphragm, oscillates with it — the coil's wires move back and forth through the radial field.

Because the coil's wires are cutting magnetic field lines as they move, an alternating p.d. is induced across the coil (generator effect, 4.7.3.1). If the microphone is connected to an amplifier or recorder, an alternating current flows. That AC carries an electrical 'copy' of the original sound wave.

permanent magnet

V induced AC (matches sound waveform)

Moving-coil microphone — generator effect

The chain of events when sound hits a microphone:

1. Sound wave arrives. Compressions and rarefactions of air push the diaphragm inward (compression) and pull it outward (rarefaction).
2. Diaphragm vibrates. Because the diaphragm is light and flexible, it follows the air pressure variations very closely. A 1 kHz sound makes the diaphragm vibrate 1000 times per second.
3. Coil oscillates in the magnetic field. The voice coil is glued to the back of the diaphragm, so it oscillates with it — moving along its axis, in and out of the magnet's gap.
4. Induced p.d. Because the coil's wires move at right angles to the radial magnetic field, they cut field lines (generator effect). A potential difference is induced across the ends of the coil. As the coil's motion alternates direction, the induced p.d. alternates too — producing an alternating current if the circuit is closed.

Frequency. Every time the diaphragm completes one oscillation cycle, the coil completes one cycle, and the induced AC completes one cycle. Frequency of induced AC = frequency of the sound.

Amplitude. Louder sounds cause larger pressure variations → larger diaphragm displacement → larger coil velocity → larger induced p.d. The amplitude of the AC matches the loudness of the sound.

Complex signals. Real speech and music contain many frequencies simultaneously. The diaphragm responds to all of them at once, and the induced AC contains the same mix of frequencies and amplitudes. This is why a high-quality microphone can faithfully capture a full orchestra.

Energy chain. Sound energy → kinetic energy of the diaphragm and coil → electrical energy in the induced current (some is lost to friction and air resistance). The microphone is a transducer — sound to electrical.

A moving-coil microphone and a loudspeaker contain the same three components (coil, magnet, diaphragm/cone). They differ only in how energy flows:

Step	Loudspeaker (motor effect)	Microphone (generator effect)
Input	Alternating current (from amplifier)	Sound waves (from air)
Mechanism	$F = BIL$ on the coil	Coil cuts field lines → induced p.d.
Output	Sound waves	Alternating current
Physics	Motor effect (4.7.2.4)	Generator effect (4.7.3.3)

The same hardware can do both. Press your ear to an unplugged speaker and you'll hear something — the cone is acting as a microphone diaphragm, and the coil generates a tiny AC. Wire that signal into an amplifier and you've made a microphone (poor quality, but it works).

Why are dedicated microphones better at being microphones? Designers optimise the diaphragm to be very light (so it responds to even faint sounds) and the coil to have low mass and good electrical properties. A loudspeaker is optimised the other way — for moving lots of air, not for sensitivity.

This is the headline AQA exam question. 'Compare the operation of a microphone and a loudspeaker.' Mark scheme:

• Both contain a coil in a magnetic field with a diaphragm/cone.
• Loudspeaker uses the motor effect: an AC current produces a force on the coil; the coil moves; the cone moves; sound waves are produced.
• Microphone uses the generator effect: sound waves move the diaphragm; the diaphragm moves the coil; the moving coil induces an alternating p.d.
• They are 'energy-reversed' — loudspeaker turns electrical into sound; microphone turns sound into electrical.

Moving-coil microphones are the workhorse of stage performance, broadcast and karaoke. They're robust, handle loud sounds well, and don't need a power supply. The Shure SM58 (used by almost every band you've ever seen live) is a moving-coil microphone — pure 4.7.3.3 physics inside.

Other microphone types (not on AQA spec, but useful context).

• Condenser microphones use a thin metal-coated diaphragm as one plate of a capacitor. Sound changes the diaphragm's position → capacitance changes → AC output. Need a power supply (often 48 V 'phantom power'). Found in studios.
• Ribbon microphones use a thin metal ribbon (no coil!) suspended in a magnetic field. Sound moves the ribbon → induced p.d. directly across its ends. Same generator effect, simpler geometry. Used for warm vocal recording.
• MEMS microphones (in mobile phones and headphones) are tiny silicon chips. They use capacitance changes like condenser mics but at micro-scale. Same principle.
• Carbon microphones (the old telephone earpiece) used carbon granules whose resistance changed with pressure. Obsolete now.

Why study only moving-coil at GCSE? Because it directly mirrors the loudspeaker — students see the symmetry. The other types use different physics that's out of scope for GCSE.

Modern UK applications of microphones.

• Live music and theatre PA systems.
• Phone calls — every UK mobile phone has at least two microphones (one for voice, one for noise cancellation).
• Voice recognition — Alexa, Siri and Google Assistant rely on microphones to digitise speech.
• Broadcasting — TV and radio studios use specialised condenser or ribbon microphones for high fidelity.
• Hearing aids — a tiny microphone captures sound, processes it, then plays it through a miniature loudspeaker into the ear.
• Sonar and ultrasound — specialised 'microphones' (hydrophones, ultrasound transducers) work at frequencies above and below human hearing.

Recording chain. Microphone → preamplifier → analogue-to-digital converter → computer → audio file. The microphone is the first transducer in this chain — without it, there's nothing to record.

A simple transformer has three parts:

1. Primary coil — a coil of insulated wire connected to the input AC supply. The number of turns is $n_p$.
2. Secondary coil — a separate coil of insulated wire connected to the output (the load circuit). The number of turns is $n_s$.
3. Soft iron core — a continuous loop of iron (often shaped like a rectangle or 'O') that passes through the middle of both coils.

The two coils are not electrically connected to each other. They are connected only by the magnetic field that runs through the shared iron core. This is critical for safety (e.g. isolation transformers in medical equipment).

Why soft iron? Soft iron is easy to magnetise and easy to demagnetise (4.7.1.4). When the primary coil's current alternates, the iron's magnetisation alternates with it — efficiently transferring the changing field to the secondary coil. A 'hard' magnetic material (like steel) would retain its magnetisation and waste energy as heat.

Why a continuous loop? The iron core guides the magnetic field from the primary to the secondary with minimal loss. A straight bar would let field lines spread out into the air, weakening the link.

Step-up transformer (n_s > n_p)

Here is the step-by-step physics of an operating transformer:

1. AC supplied to primary. An alternating current flows in the primary coil. Each turn of the primary creates its own magnetic field (4.7.2.1, solenoid).
2. Alternating flux in the core. The primary's alternating current produces an alternating magnetic field. The soft iron core guides this field around the loop so that the same alternating flux passes through the secondary coil.
3. Induced p.d. in the secondary. The alternating magnetic flux through the secondary coil constantly changes. By the generator effect (4.7.3.1), an alternating p.d. is induced across the secondary coil — at the same frequency as the primary's AC.
4. AC in the secondary circuit. If the secondary is connected to a load (e.g. a lamp or a National Grid cable), the induced p.d. drives an alternating current through that load.

The key insight. Each turn of either coil 'sees' the same amount of magnetic flux change per second. So:

• Voltage induced per turn is the same on both coils.
• More turns → bigger total voltage.

That gives:

$\frac{V_p}{V_s} = \frac{n_p}{n_s}$

where $V_p$ and $V_s$ are the primary and secondary voltages, and $n_p$ and $n_s$ are the corresponding numbers of turns.

Why doesn't DC work? A direct current produces a steady magnetic field in the core. A steady field does NOT change, so it cannot induce a p.d. in the secondary. The transformer would produce no output (except for a brief pulse when the DC is first switched on or off).

This is why the grid is AC. Without AC, you can't use transformers; without transformers, you can't easily change voltages; without voltage changes, you can't transmit electricity efficiently over long distances.

The transformer equation:

$\frac{V_p}{V_s} = \frac{n_p}{n_s}$

• $V_p$ — potential difference across the primary (V).
• $V_s$ — potential difference across the secondary (V).
• $n_p$ — number of turns on the primary.
• $n_s$ — number of turns on the secondary.

Step-up transformer. $n_s > n_p$ → $V_s > V_p$. The secondary voltage is larger than the primary voltage. Used to step grid voltage from a power station (e.g. 25 kV) up to the National Grid transmission voltage (e.g. 400 kV).

Step-down transformer. $n_s < n_p$ → $V_s < V_p$. The secondary voltage is smaller than the primary voltage. Used to step grid voltage down from 400 kV (transmission) to 230 V (UK domestic mains), or further down to a few volts for chargers and gadgets.

Worked example — step-up. A power-station transformer has 500 turns on its primary and 50 000 turns on its secondary. The primary p.d. is 25 kV. Calculate the secondary p.d.

$\frac{V_p}{V_s} = \frac{n_p}{n_s}$ $\frac{25\,000}{V_s} = \frac{500}{50\,000}$ $V_s = 25\,000 \times \frac{50\,000}{500} = 2\,500\,000\,\text{V} = 2.5\,\text{MV}$

(In practice the National Grid uses 275 kV or 400 kV — but the maths is the same.)

Worked example — step-down. A phone charger transformer has 2300 turns on its primary and 50 turns on its secondary. Mains voltage is 230 V. Calculate the secondary p.d.

$V_s = V_p \times \frac{n_s}{n_p} = 230 \times \frac{50}{2300} = 5.0\,\text{V}$

— exactly the voltage a USB charger delivers.

Sanity check. Always check the direction of stepping. If $n_s > n_p$, expect $V_s > V_p$ (step up). If $n_s < n_p$, expect $V_s < V_p$ (step down). If your answer goes the wrong way, you've flipped the ratio.

An ideal transformer is 100 % efficient — none of the electrical energy is lost as heat. Power in equals power out:

$V_p I_p = V_s I_s$

• $V_p I_p$ — input power (W), delivered by the AC supply to the primary.
• $V_s I_s$ — output power (W), delivered by the secondary to the load.

Combined with the turns-ratio equation. If a transformer steps voltage UP by a factor of 10 ($V_s = 10 V_p$), the current is stepped DOWN by the same factor ($I_s = I_p / 10$). Conversely, a step-down transformer (e.g. $V_s = V_p / 10$) gives a step-up in current ($I_s = 10 I_p$).

This is the key to efficient transmission on the National Grid. Power losses in cables are $P = I^2 R$ (4.2.4 in spec 4.2). High voltage → low current → tiny $I^2 R$ losses. That's why long-distance grid lines run at 275 kV or 400 kV, not at 230 V — at low voltage, the same power would require thousands of times more current and the cables would melt from $I^2 R$ heating.

Worked example — step-up with current. A step-up transformer takes 25 kV at 800 A and outputs 400 kV. What is the output current (assume 100 % efficient)?

$V_p I_p = V_s I_s$ $I_s = \frac{V_p I_p}{V_s} = \frac{25\,000 \times 800}{400\,000} = 50\,\text{A}$

So 25 kV × 800 A = 20 MW going in, and 400 kV × 50 A = 20 MW coming out. Voltage up by ×16, current down by ×16, power unchanged.

Worked example — step-down with current. A phone charger transformer steps 230 V down to 5.0 V. The phone draws 2.0 A from the secondary. Calculate the current in the primary (assuming 100 % efficiency).

$I_p = \frac{V_s I_s}{V_p} = \frac{5.0 \times 2.0}{230} = 0.043\,\text{A} \approx 43\,\text{mA}$

A heavy phone draw (2 A on the secondary) is only 43 mA on the mains — efficient at the wall socket.

Real transformers are not ideal. Up to 5 % of energy is lost to:

• Heating of the coils (I²R losses — use thick wire).
• Eddy currents in the core (use a laminated iron core to block them).
• Hysteresis (energy lost in magnetising and demagnetising the iron — soft iron minimises this).
• Magnetic flux leakage (some field escapes the core into the air).

Large grid transformers can be 98–99 % efficient. AQA only requires the ideal $V_p I_p = V_s I_s$ at GCSE.

The UK National Grid relies on transformers to move electricity from generators to homes efficiently. The full chain:

1. Power station alternator. Generates AC at around 25 kV (depending on the station).
2. Step-up transformer (at the power station). Increases the voltage from 25 kV to 275 kV or 400 kV.
3. Long-distance transmission lines. Carry electricity at high voltage, low current — minimising $I^2 R$ losses over hundreds of km.
4. Step-down transformer (at a 'grid' substation). Decreases voltage from 400 kV to 132 kV for regional distribution.
5. Further step-down transformers. From 132 kV to 33 kV to 11 kV at smaller substations.
6. Local step-down transformer (street-level). From 11 kV down to 230 V (single-phase) or 400 V (three-phase) for delivery to homes and businesses.

The chain involves at least four transformers between the generator and the lamp on your bedside table. Each step balances voltage (lower = safer to handle, harder to transmit) against current (higher = more cable losses, but easier to insulate the cable).

Why isn't the whole grid at 230 V? Because at 230 V, transmitting 20 MW would require $I = P/V = 20\,000\,000 / 230 \approx 87\,000$ A. Even a cable with very low resistance would melt instantly under that current, and the $I^2 R$ losses would dissipate most of the energy as heat in the cable before it reached anyone.

Why isn't the whole grid at 400 kV? Because 400 kV is dangerous — pylons must be tall to keep the cables away from the ground, transformers must be huge, and switching is expensive. Domestic 230 V is a compromise between safety, equipment size and human convenience.

Stepping down for chargers and appliances. Inside your phone charger, USB-C laptop charger, doorbell, or model railway transformer, a small step-down transformer reduces 230 V to a few volts. (Modern chargers also use switching electronics, but a transformer is usually still at the core.)

Linking to 4.2.6 The National Grid. Spec 4.2.6 (Electricity topic) covers the National Grid as a whole; spec 4.7.3.4 (this section) focuses on the transformer as the device that makes voltage changes possible. AQA may test either context.