Theme D — Fields
D.4.4 Lenz’s Law — IB Physics HL Revision Notes & Topical Questions
5 min read 9 parts Lesson 38 of 39 1 quiz
Lesson summary
D.4.4 Lenz's Law — IB PHYSICS HL notes: State Lenz's law, use it to find the direction of induced currents and explain self-induction, mutual induction and transformers.Statement — the induced e.m.f.
- Without the minus sign
- Push in a north pole
- Current direction
- Reverse the magnet
- Withdraw the magnet
- No free energy
Part 1 of 7Lenz's law
Statement
- Statement the induced e.m.f. is such that it will oppose the change causing it
Purpose
- Purpose it predicts the direction of an induced e.m.f. in a coil or wire
Basis
- Basis it is a consequence of conservation of energy
Equation
- ε = −N(ΔΦ)/Δt
- The minus sign shows the opposing induced field and the data-booklet form
Without the minus sign
- Without the minus sign the equation is Faraday's law alone
Part 2 of 7Experimental verification
Apparatus
- Apparatus a bar magnet, a coil and a sensitive ammeter, with no cell
Push in a north pole
- Push in a north pole the coil face becomes a north pole, repelling the magnet
Current direction
- Current direction anticlockwise at the entering face, found with the right-hand grip rule
Reverse the magnet
- Reverse the magnet the opposite deflection is seen
Withdraw the magnet
- Withdraw the magnet the coil end nearest the magnet becomes a pole that attracts the magnet and tries to keep it in
Poles
- Poles the poles of the coil swap as the magnet moves through
Part 3 of 7Energy conservation
No free energy
- No free energy electromagnetic effects do not create electrical energy from nothing
Work needed
- Work needed to induce and sustain an e.m.f., work must be done against the opposing force
Two loops
- Two loops if loop Y moves towards a loop X carrying a current, the induced current in Y opposes X's field,
- so work must be done to keep Y moving at constant speed
Part 4 of 7Self-induction
Definition
- Definition the effect in which a change in current tends to produce an induced e.m.f. that opposes the change in current in the same circuit
Back e.m.f.
- Back e.m.f. the induced e.m.f. opposing a change of current
Proportionality
- Proportionality back e.m.f. is proportional to minus the rate of change of current
Cause
- Cause it is an example of Lenz's law
Part 5 of 7Mutual induction
Definition
- Definition a change in current in one circuit induces an e.m.f. in a neighbouring circuit
Application
- Application transformers
Part 6 of 7Transformers
1Purpose
- Purpose to change a high alternating voltage at low current to a low voltage at high current, and the reverse
- It reduces energy loss in power lines
2Parts
- Parts a primary coil, a secondary coil and a soft iron core
3Operation
- Operation an alternating current in the primary produces an alternating magnetic field and flux linkage in the core
- The changing flux linkage in the secondary induces an alternating e.m.f. of the same frequency
4Step-up
- Step-up more turns on the secondary than the primary, giving a larger secondary voltage
5Step-down
- Step-down more turns on the primary, giving a smaller secondary voltage
6Core
- Core soft iron is used because it magnetises and demagnetises easily
Part 7 of 7More from the specification
More from the specification
Lenz's Law and Conservation of Energy
- Lenz's law states that an induced electromotive force (emf) and current in a conductor always oppose the change in magnetic flux that creates them, in accordance with energy conservation (ε = -N dΦ/dt).
Magnetic Flux and Flux Density
- Magnetic flux Φ through a coil of area A is defined by Φ = B A cos(θ),
- where B is magnetic flux density (in Tesla) and θ is the angle between the field and the normal to the coil plane;
- flux is maximum when the plane is perpendicular to field lines.
AC Generator Induced EMF
- Rotating a coil with constant angular speed ω in a uniform magnetic field produces a sinusoidal alternating emf across a connected voltmeter: ε = B A N ω sin(ωt), with maximum emf generated when the coil plane is parallel to the field.
Motional EMF in a Moving Conductor
- When a straight conductor of length L moves at speed v perpendicular to a magnetic flux density B, charge separation creates an induced motional emf equal to ε = B L v.
Lenz's Law and Conservation of Energy
- Lenz's law states that an induced electromotive force (emf) and current in a conductor always oppose the change in magnetic flux that creates them, in accordance with energy conservation (ε = -N dΦ/dt).
Magnetic Flux and Flux Density
- Magnetic flux Φ through a coil of area A is defined by Φ = B A cos(θ),
- where B is magnetic flux density (in Tesla) and θ is the angle between the field and the normal to the coil plane;
- flux is maximum when the plane is perpendicular to field lines.
AC Generator Induced EMF
- Rotating a coil with constant angular speed ω in a uniform magnetic field produces a sinusoidal alternating emf across a connected voltmeter: ε = B A N ω sin(ωt), with maximum emf generated when the coil plane is parallel to the field.
Motional EMF in a Moving Conductor
- When a straight conductor of length L moves at speed v perpendicular to a magnetic flux density B, charge separation creates an induced motional emf equal to ε = B L v.
Conductor
- Material that allows current to flow.
Density
- Mass per unit volume.
Perpendicular
- At right angles.
Voltmeter
- Instrument that measures p.d., connected in parallel.
Angular speed
- Magnitude of the angular velocity.
Potential difference
- Work done by a unit charge passing through a component.
Closed circuit
- Complete loop with a source in which current can flow
Key terms
| Term | Meaning |
|---|---|
| Lenz's law | The induced e.m.f. is such that it opposes the change causing it. |
| Back e.m.f. | An induced e.m.f. that opposes a change of current in the same circuit. |
| Self-induction | The effect in which a change in current induces an opposing e.m.f. in the same circuit. |
| Mutual induction | The effect in which a change of current in one circuit induces an e.m.f. in a neighbouring circuit. |
| Transformer | A device with primary and secondary coils on an iron core that changes alternating voltages. |
Avoid these mistakes
1
Saying the induced field always opposes the magnet's field — it opposes the change in flux.
2
Thinking the induced current always flows in the same sense — it reverses when the change reverses.
3
Ignoring the minus sign — it is the statement of Lenz's law.
4
Confusing self-induction with mutual induction.
5
Believing a transformer works with direct current — it needs a changing flux.
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