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20.5.3Magnetic fields and alternating currents (A Level)

Principles of Electromagnetic Induction

5 min read 11 parts Lesson 16 of 24 1 quiz

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Lesson summary

20.5.3 Principles of Electromagnetic Induction — AS PHYSICS 9702 notes: Describe electromagnetic induction experiments and the factors that control the induced e.m.f.Definition — the production of an e.m.f.

  • Steady flux
  • Magnet stationary
  • Magnet moving in
  • Magnet pulled out
  • Faster motion
  • Wire stationary

Section 1 of 11Electromagnetic induction

Definition
  • Definition the production of an e.m.f. in a conductor when there is relative movement between a conductor and a magnetic field,
    • or when the flux linking it changes
Current
  • Current if the conductor is part of a closed circuit the e.m.f. drives a current
Steady flux
  • Steady flux an unchanging flux, however large, induces nothing
Discovery
  • Discovery by Faraday in 1831
Uses
  • Uses generators, transformers, induction hobs, microphones and sensors

Section 2 of 11Magnet and coil experiment

Apparatus
  • Apparatus a bar magnet moved in and out of a coil connected to a sensitive voltmeter
Magnet stationary
  • Magnet stationary the reading is zero because the rate of change of flux is zero
Magnet moving in
  • Magnet moving in a momentary reading as the field lines cut the coil
Magnet pulled out
  • Magnet pulled out a reading in the opposite direction
Faster motion
  • Faster motion a larger e.m.f. because the rate of change of flux is larger

Section 3 of 11Wire in a field experiment

Wire in a field experiment
Apparatus
  • Apparatus a straight wire connected to a voltmeter and moved between two magnets, with no current to start with
Wire stationary
  • Wire stationary zero reading
Wire moving across the field
  • Wire moving across the field an e.m.f. is induced, with the wire, field and motion mutually perpendicular
Moving back
  • Moving back an e.m.f. in the opposite direction
Moving parallel to the field
  • Moving parallel to the field nothing is induced

Section 4 of 11Other demonstrations

Two coils
  • Two coils switching the current in a primary coil on or off gives a kick on a voltmeter across a nearby secondary, and zero when the current is steady
Rotating coil
  • Rotating coil a coil turned in a magnet's field gives an alternating e.m.f., the basis of the generator

Section 5 of 11What controls the size of the e.m.f.

Faster change of flux
  • Faster change of flux the magnet moves faster or the current changes more quickly
Stronger field
  • Stronger field a stronger magnet
More turns
  • More turns more turns on the coil
Larger area
  • Larger area a larger coil, or a longer wire in the field
Rule
  • Rule the induced e.m.f. is proportional to the rate of change of flux linkage, with direction given by Lenz's law

Section 6 of 11The moving-wire explanation

1Wire of length L moving at speed v across a field B
  • Wire of length L moving at speed v across a field B its free electrons are carried along and feel a magnetic force Bev along the wire
2Charge separation
  • Charge separation charge is pushed to one end until an electric field balances the force
3Induced e.m.f.
  • ε = BLv, the work per unit charge by the magnetic force over the length
4Flux cutting
  • Flux cutting the wire sweeps area Lv each second, so it cuts flux B × Lv per second, giving the same result

Section 7 of 11Eddy currents and everyday induction

Eddy currents
  • Eddy currents swirling currents induced in solid metal that heat it
  • Induction hobs heat a pan without the hob getting hot
Braking
  • Braking eddy currents oppose the motion that creates them, as in electromagnetic brakes on trains and roller coasters
Transformer cores
  • Transformer cores laminated with thin insulated layers to restrict eddy current paths
Microphone
  • Microphone sound moves a diaphragm and coil in a magnet's field to induce a voltage
Generators
  • Generators bicycle dynamos and wind turbines spin a magnet or coil to change flux linkage continuously

Section 8 of 11Conservation of energy

No energy is created
  • No energy is created the e.m.f. appears only if work is done against an opposing force
Magnet and coil
  • Magnet and coil the induced current makes a field that resists the motion, so the work done pushing in becomes electrical energy and then heat
If it helped the motion
  • If it helped the motion energy would be created from nothing, which is impossible
Lenz's law
  • Lenz's law this is its physical content

Section 9 of 11More from the specification

1Magnetic Flux and Flux Linkage
  • Magnetic flux through a surface of cross-sectional area A in a uniform field of magnetic flux density B is given by Phi = BA cos(theta),
  • where theta is the angle between the magnetic field lines and the normal to the plane of the area.
2Faraday's Law of Electromagnetic Induction
  • The magnitude of induced electromotive force in a circuit or solenoid is directly proportional to the rate of change of magnetic flux linkage: e.m.f. = -d(N * Phi) / dt.
3Maximum Induction Conditions
  • Induced e.m.f. reaches maximum magnitude when magnetic flux linkage changes most rapidly,
  • such as when a coil rotates perpendicular to magnetic field lines or when current in an adjacent circuit changes at an increasing rate.
4Magnetic Flux and Flux Linkage
  • Magnetic flux through a surface of cross-sectional area A in a uniform field of magnetic flux density B is given by Phi = BA cos(theta),
  • where theta is the angle between the magnetic field lines and the normal to the plane of the area.
5Faraday's Law of Electromagnetic Induction
  • The magnitude of induced electromotive force in a circuit or solenoid is directly proportional to the rate of change of magnetic flux linkage: e.m.f. = -d(N * Phi) / dt.
6Maximum Induction Conditions
  • Induced e.m.f. reaches maximum magnitude when magnetic flux linkage changes most rapidly,
  • such as when a coil rotates perpendicular to magnetic field lines or when current in an adjacent circuit changes at an increasing rate.
7Conventional current
  • Current drawn from the positive to the negative terminal.
8Density
  • Mass per unit volume.
9Normal
  • Line at 90° to a surface.
10Solenoid
  • Long coil of wire carrying a current

Section 10 of 11Key terms

TermMeaning
Electromagnetic inductionproduction of an e.m.f. by a changing magnetic flux linkage
Eddy currenta current induced in a solid piece of metal
Laminationa thin insulated layer in a transformer core
Induced e.m.f.ε = BLv for a moving wire
Generatora device converting kinetic energy into electrical energy by induction
Induction hoba hob heating cookware by eddy currents

Section 11 of 11Exam technique

1
Saying a stationary magnet in a coil induces an e.m.f. — the flux must change.
2
Saying a stronger steady field induces an e.m.f. — only a changing flux does.
3
Moving a wire parallel to the field and expecting an e.m.f. — it must cut field lines.
4
Forgetting that the direction reverses when the motion reverses.
Transformer Golden Equations Physics Standard

Transformer Turns Ratio & Efficiency

Ratio: Vp / Vs = Np / Ns = Is / Ip

  • Step-up reduces grid thermal loss (P = I2R).
  • Laminated core prevents eddy currents.