A changing magnetic field through a coil or relative motion that cuts magnetic field lines can induce a potential difference. This is electromagnetic induction.
Moving a magnet into or out of a coil changes the field through the coil. A sensitive voltmeter connected across the coil can detect an induced voltage.The voltmeter measures potential difference, with polarity set by connections.
If the coil is part of a closed conducting circuit, the induced voltage drives a current. An open circuit can have induced voltage but no continuing current.
A stationary magnet held within a stationary coil does not produce a continuing induced voltage because the field through the coil is not changing.
Faster magnet movement, a stronger magnet or more coil turns can increase induced voltage when the other factors are fixed.
Reverse the motion or reverse the magnet's poles to reverse the induced voltage direction. Reversing both can keep the direction unchanged.
The field associated with the induced current opposes the original change. An approaching north pole induces a facing north pole that resists approach; a withdrawing north pole induces a facing south pole that resists separation.Both induced interactions resist the change that created them.
This opposition means mechanical work is needed to maintain motion when electrical energy is transferred. Induction does not produce free energy.
Investigating induction (Higher tier)
Connect a coil to a sensitive centre-zero voltmeter or data logger. Move a bar magnet in and out and record size and sign of voltage; compare with holding it still.
Change one factor at a time: magnet speed, magnet strength or number of turns. Keep coil size, movement path, starting positions and other variables consistent.
For fair comparisons, use repeatable movement and compare the same part of the movement or peak voltage. Repeat trials because hand movement gives varying speed.
Investigate direction by reversing magnet motion or poles individually. Label which end of the coil is connected to the positive voltmeter terminal.
A voltage pulse depends on the rate of field change, not simply how much magnetic material is sitting inside a coil.
Transformer operation (Higher tier)
A transformer has a primary coil and a secondary coil linked by a magnetic core. The coils are electrically separate in the usual ideal model. A changing magnetic field transfers energy between them.Coils are magnetically coupled, not joined by an electrical wire.
An alternating current in the primary creates a changing magnetic field in the core. The changing field through the secondary induces an alternating voltage.
A secondary current flows if a load completes its circuit. An unloaded secondary can have voltage with little output power because there is little load current.
A steady DC input does not provide the continuously changing field needed for normal transformer operation. Switching DC on or off can produce brief changes, not a sustained transformed output.
A step-up transformer increases AC voltage; a step-down transformer decreases it. More secondary than primary turns gives step-up behaviour; fewer gives step-down behaviour.
The core helps the magnetic field link both coils. Real transformers transfer some energy to the surroundings, including by heating, so their useful output power is less than their input power.
Ideal transformer power calculations
For a transformer assumed 100% efficient, input power = output power. Use VpIp = VsIs, with voltages in V and currents in A.
A primary of 230 V and 2 A supplies 460 W. If the ideal secondary is 23 V, its current is 460 ÷ 23 = 20 A.Lower output voltage permits greater current at the same power.
Increasing voltage reduces current for the same transferred power; reducing voltage increases current. A transformer cannot increase both output voltage and output current at unchanged input power.
Rearrange the ideal equation to find a missing voltage or current. For example, Is = VpIp/Vs; check whether the question explicitly assumes an ideal transformer.
If a real transformer has known efficiency, useful output power = efficiency fraction × input power. Account for losses rather than claiming exact equality.
The National Grid
The National Grid links generation and electricity transmission; local distribution networks deliver energy to consumers. Generation includes power stations and renewable sources.
Step-up transformers raise voltage before long-distance transmission. For the same power, higher voltage means smaller current.
Cable heating loss is P_loss = I²R. Reducing current greatly reduces heating losses for the same cable resistance, improving transmission efficiency.Halving current reduces I²R loss to a quarter.
Step-down transformers reduce voltage for distribution and again near domestic users. UK domestic supply is approximately 230 V AC; high transmission voltages would be unsuitable for direct household use.Illustrates the purpose of voltage changes rather than a universal list of grid voltages.
Some transmission lines operate at hundreds of kilovolts, for example 400 kV. Intermediate and generator voltages vary; the source's numbers are examples rather than one compulsory sequence.
High-voltage equipment requires insulation, separation and controlled access. Lower domestic voltage still presents a serious electrical hazard.
The grid transfers energy; it does not create it. Demand, generation and network operation must be managed, with some energy inevitably dissipated.
Alternators, dynamos and sound transducers (Higher tier)
(Higher tier) Moving a conductor relative to a magnetic field can produce (induce) a potential difference. Current flows if the circuit is complete. A stationary conductor in an unchanging magnetic field does not produce a continuing induced output.
(Higher tier) An alternator rotates a coil in a magnetic field and uses slip rings to maintain connections. As the coil moves through successive half-turns, the induced output changes direction, producing alternating current.
(Higher tier) A simple DC dynamo uses a split-ring commutator to reverse the coil’s connections every half-turn. This keeps the current in the external circuit flowing in one direction. Its size can still rise and fall (pulsate), rather than staying perfectly constant.
(Higher tier) In a moving-coil microphone, sound pressure variations move a diaphragm and attached coil in a magnetic field, inducing a varying voltage that represents the sound.A loudspeaker reverses the conversion, using current variations to produce motion and sound.
(Higher tier) A loudspeaker works by the reverse energy transfer: a varying current in a coil produces a varying force in a magnetic field, moving the cone to create sound. Headphones use the same principle on a smaller scale.
(Higher tier) Generators transfer kinetic energy to electrical energy; microphones transfer sound into electrical signals. Energy is conserved and friction, electrical resistance and other losses reduce useful output.
Transformer turns and transmission losses (Higher tier)
(Higher tier) For an ideal transformer, primary voltage ÷ secondary voltage = primary turns ÷ secondary turns: Vp/Vs = Np/Ns. Use alternating input; the changing core field induces the secondary voltage.
(Higher tier) A step-up transformer has more secondary turns and a higher secondary voltage; a step-down has fewer secondary turns and lower voltage. Turns ratio sets the voltage ratio, not the power gain.
(Higher tier) With ideal power balance, VpIp = VsIs. Raising voltage lowers current for the same transferred power; a transformer does not create extra energy.
(Higher tier) Power dissipated in a transmission cable is I²R. For fixed transmitted power and cable resistance, doubling transmission voltage halves current and reduces cable loss to one quarter.These relative values assume unchanged transmitted power and resistance.
(Higher tier) The National Grid uses step-up transformers before long-distance transmission and step-down transformers for consumers. High-voltage transmission reduces current-related heating, but requires suitable insulation and safety systems.
(Higher tier) Real transformers have energy losses, so output power is less than input. Use ideal assumptions only when stated, and distinguish useful output power from the heat lost in cables.
Revise electromagnetic induction with this narrated video. Use the player controls to pause, seek, adjust the volume or mute. Turn English captions on or off using the captions menu.
Mr Kidman's Classroom