Question

A bar magnet is dropped through a vertical copper tube and is observed to fall very slowly, despite the fact that mechanical friction between the magnet and the tube is negligible (Figure 22.61). Explain carefully, including adequate diagrams.

Short answer

The magnetic field $\left(B\right)$is growing beneath the magnet, thus$-\frac{dB}{dt}$ is upward, and causes a counterclockwise current. A magnetic field is created at the magnet's position by the induced current, pointing upward. The magnet slows down as a result.

Step-by-step solution

A concept:

The induced current has a direction such that the magnetic field due to the induced current opposes the change in magnetic flux that induces the current.

A given data:

A bar magnet is dropped through a vertical copper tube and is observed to fall very slowly, despite the fact that mechanical friction between the magnet and the tube is negligible.

Induce current and magnetic field:

When a bar magnet is dropped vertically through a copper wire, above the magnet the magnetic field $\left(B\right)$ is decreasing, so $-\frac{dB}{dt}$is downward and induces current in clockwise direction. This induced current makes a magnetic field pointing down at the location of the magnet. So the induced current loop acts like a magnet with North Pole $\left(N\right)$on the bottom, which attracts the falling magnet. Thus the magnet slows down.

Below the magnet the magnetic field $\left(B\right)$is increasing, so $-\frac{dB}{dt}$is upward and induces a current in a counter-clockwise direction. This induced current makes a magnetic field point upward at the location of the magnet. Thus, the magnet slows down.

The schematic diagram for the given situation is shown in the below diagram.