What determines the direction of an induced current, and how does this follow from conservation of energy?
Use Lenz's law to determine the direction of an induced current and explain it in terms of conservation of energy.
How Lenz's law gives the direction of an induced current as the one that opposes the change producing it, and why this is required by conservation of energy.
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What this dot point is asking
You need to use Lenz's law to find the direction of an induced current and explain why it must oppose the change, linking this to conservation of energy.
The statement
This is the source of the minus sign in Faraday's law, : the induced EMF acts against the change in flux.
Applying it to a magnet and a coil
Picture a bar magnet being pushed, north pole first, toward a coil:
- The flux through the coil increases as the magnet approaches.
- By Lenz's law, the induced current opposes this increase, so it must create a magnetic field that pushes the magnet back - the coil's near face becomes a north pole.
- This means you must do work against a repulsion to push the magnet in.
Now pull the same magnet away:
- The flux decreases.
- The induced current now opposes the decrease, so the coil's near face becomes a south pole, attracting the magnet back to try to keep the flux up.
- Again you must do work, this time against an attraction.
Either way, the induced effect resists your motion.
Why: conservation of energy
The electrical energy generated comes from the mechanical work done against the opposing force - not from nowhere.
Eddy currents and braking
Lenz's law explains electromagnetic braking. A conductor moving through a magnetic field has circulating "eddy currents" induced in it. These currents oppose the motion, producing a retarding force that slows the conductor smoothly without contact - used in trains, roller-coasters and gym equipment.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2024 SACE Stage 23 marksA square conductive loop passes through a uniform magnetic field directed out of the plane of the page, perpendicular to the direction of movement. The loop is shown as it leaves the region of the magnetic field. Using Lenz's Law, determine the direction of the induced current in the loop as it leaves the region of the magnetic field.Show worked answer →
As the loop leaves the field, the outward (out of the page) flux through the loop is decreasing.
By Lenz's law, the induced current opposes this change, so it acts to maintain the outward flux through the loop.
To create a magnetic field out of the page inside the loop, the induced current must flow anticlockwise (by the right-hand rule, curling the fingers anticlockwise points the thumb out of the page).
Therefore the induced current is anticlockwise. 1 mark for identifying the flux is decreasing, 1 mark for applying Lenz's law (current opposes the decrease, maintaining outward flux), 1 mark for the anticlockwise direction. This opposition is required by conservation of energy, as the induced current resists the motion removing the loop.