A light disc made of aluminium (a nonmagnetic material) is kept horizontally and is free to rotate a

Q: Why does the disc rotate if aluminium is non-magnetic?

Aluminium doesn't get attracted to magnets like iron does. However, it is a good conductor. The rotation is caused by electromagnetic induction (eddy currents), not by basic ferromagnetism.

Q: What are eddy currents?

They are loops of electrical current induced within conductors by a changing magnetic field, resembling swirling eddies in a river.

Q: Why doesn't the disc rotate in the opposite direction?

If it rotated in the opposite direction, the relative velocity between the magnet and the disc would increase, which would increase the change in flux. Lenz's law states it must oppose the change, so it moves in the same direction to reduce the relative velocity.

Q: Will the disc ever spin faster than the magnet?

No. If it spun faster, the relative motion would reverse, inducing currents that would slow it down. It will always spin slightly slower than the magnet.

Q: Does the disc heat up?

Yes! The eddy currents flowing through the electrical resistance of the aluminium dissipate energy as heat, causing the temperature to rise slightly.

Question

A light disc made of aluminium (a nonmagnetic material) is kept horizontally and is free to rotate about its axis as shown in the figure. A strong magnet is held vertically at a point above the disc away from its axis. On revolving the magnet about the axis of the disc, the disc will (figure is schematic and not drawn to scale)-

Options:

(A) rotate in the direction opposite to the direction of magnet's motion

(B) rotate in the same direction as the direction of magnet's motion

(C) not rotate and its temperature will remain unchanged

(D) not rotate but its temperature will slowly rise

Sigma Percentile · Accepted Answer

### The Mystery of the Non-Magnetic Disc

Imagine you are standing in a physics lab, and you see a light aluminium disc resting horizontally, completely free to rotate around its central axis. You take a strong bar magnet and start revolving it in a circle just above the disc.

Now, your intuition might tell you that nothing should happen. After all, aluminium is a **non-magnetic** material. It doesn't stick to your refrigerator magnets, so why should it care about this revolving magnet? But nature has a beautiful trick up its sleeve, and it all comes down to the magic of **Electromagnetic Induction**.

### The Birth of Eddy Currents

As the magnet revolves above the disc, the magnetic field lines piercing through the aluminium are constantly shifting and moving. According to **Faraday's Law of Induction**, whenever a conductor experiences a changing magnetic flux ($\Delta \Phi$), an electromotive force (EMF) is induced within it:

$$ \varepsilon = -\frac{d\Phi}{dt} $$

Because aluminium is an excellent electrical conductor, this induced EMF doesn't just vanish. Instead, it drives swirling, whirlpool-like loops of electrical current right inside the bulk of the metal. These are known as **Eddy Currents**.

### The Ultimate Rule of Opposition: Lenz's Law

Now that we have currents flowing in the disc, what do they do? To answer this, we must consult **Lenz's Law**. Lenz's Law is the universe's ultimate rule of stubbornness: it states that any induced current will flow in a direction that **opposes the cause** that produced it.

What is the "cause" in our setup? The cause is the **relative motion** between the moving magnet and the stationary disc. The eddy currents want to destroy this relative motion. They want the magnet and the disc to be at rest relative to each other.

### The Catch-Up Game

Since the magnet is being forced to revolve by an external agent (you), the eddy currents cannot stop the magnet. But they *can* move the disc! To minimize the relative motion, the disc decides to play a game of catch-up. It experiences a magnetic torque that drags it along, causing it to rotate in the **exact same direction** as the magnet.

This fascinating setup is historically known as **Arago's Disk**, discovered by François Arago in 1824, long before Faraday formally explained it! It forms the foundational principle behind modern induction motors and magnetic braking systems.

Therefore, the disc will rotate in the same direction as the direction of the magnet's motion.

Select Answer:

Visualized Solution

$Setup Analysis$

$Faraday’s Law of Induction$

$Eddy Currents$

$Lenz’s Law$

$Minimizing Relative Motion$

$Conclusion$

The Sigma Insight: Lenz's Law

The Mystery of the Non-Magnetic Disc

The Birth of Eddy Currents

The Ultimate Rule of Opposition

The Catch-Up Game

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