The induced EMF opposes the change that produces it — Lenz's law
Electromagnetic Induction — AQA A-Level Physics
- This is the reason for the minus sign in Faraday's lawThe magnitude of the induced EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). is proportional to the rate of change of magnetic flux linkageThe product of magnetic flux and the number of turns of a coil. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area.-turns (Wb turns)... The EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). acts in the direction that would resist the change.
- Think about it from energyThe capacity to do work. Measured in joules (J). conservation. If the induced currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). aided the change instead of opposing it, you'd get a runaway effect: the currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). would increase the flux change, which would increase the currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A)., which would increase the flux change — energyThe capacity to do work. Measured in joules (J). from nothing. That violates conservation of energyThe capacity to do work. Measured in joules (J).Energy cannot be created or destroyed, only transferred from one form to another. The total energy of a closed system remains constant..
- When you push a magnet towards a coil, the induced current creates its own magnetic field that repels the magnet. You have to do work to push the magnet in. That work is what provides the electrical energy.
- When you pull the magnet away, the induced current flips direction to attract the magnet. Again, you have to do work against the attractive force.
- The opposition is always to the change, not to the flux itself. If flux is increasing, the induced current opposes the increase (tries to reduce flux). If flux is decreasing, the induced current opposes the decrease (tries to maintain flux).
Show Solution
The north pole approaches the solenoid, so the magnetic fluxThe product of magnetic flux densityMass per unit volume of a material. Measured in kg m⁻³. and the area perpendicular to the field. Measured in weberThe SI unit of magnetic flux. One weber is the flux through an area of 1 m² when the magnetic flux density is 1 T perpendicular to the area. (Wb). through the solenoid is increasing (more field lines threading through the coil).
The induced current must oppose the increase in flux. To do this, the solenoid must generate its own magnetic field that opposes the approaching north pole. The end of the solenoid facing the magnet must become a north pole (to repel the incoming north pole).
For the end facing the magnet to be a north pole, the field lines must come out of that end towards you. By the right-hand grip rule (curl fingers in the direction of current, thumb points in field direction), the current flows anticlockwise as seen from the end facing the magnet.
The current flows anticlockwise as viewed from the end of the solenoid facing the approaching magnet.
Instead: The current opposes the change. If flux is increasing, the induced field acts to reduce it. If flux is decreasing, the induced field acts to maintain it. The current doesn't just blindly oppose whatever flux is there.