Complete each stage to unlock the next one.
On the syllabus: A-Level Physics
Drop a strong magnet through a copper tube. It falls in slow motion — far slower than free fall — even though copper is not magnetic. Drop the same magnet through a plastic tube: normal free fall. What invisible force is slowing the magnet, and where does it come from?
Drop a strong magnet through a copper tube. It falls in slow motion — far slower than free fall — even though copper is not magnetic. Drop the same magnet through a plastic tube: normal free fall. What invisible force is slowing the magnet, and where does it come from?
When a magnetic field changes through a conductor, it induces a current. That current creates its own magnetic field — and by Lenz's Law, this field always opposes the change that caused it. Nature resists change.
Lenz's Law states that induced currents always oppose the change in magnetic flux that caused them. This is a consequence of energy conservation — if induced currents aided the change, we could create energy from nothing. The induced EMF is given by Faraday's Law.
Magnetic flux Φ = BA cos θ, where B is field strength, A is coil area, and θ is the angle between field and normal to the coil. EMF is induced whenever flux changes — by changing B, A, or θ. Lenz's Law gives the direction of the induced current. Think of it as electromagnetic inertia: the coil resists changes in flux just as a mass resists changes in velocity. When flux increases, induced current opposes the increase. When flux decreases, induced current tries to maintain it. Applications: eddy current brakes in trains and roller coasters (smoother, no wear), induction cooktops (alternating magnetic field induces currents in the pot which heats it directly), metal detectors (distorted field detected), and all electric generators. The slow fall of a magnet through a copper tube is a beautiful classroom demonstration of Lenz's Law.