Question

The "floating magnet trick" is shown in Figure \(10.50 .\) If the disk on the bottom were a permanent magnet, rather than a superconductor, the trick wouldn't work. The superconductor does produce un external field very similar to that of a permanent magnet. What other characteristic is necessary to explain the effect? (Hint: What happens when you hold two ordinary magnets so that they repel, and then you release one of them?)

Short answer

The effect observed in the floating magnet trick (where a superconductor floats above a magnet) requires a characteristic known as the 'Meissner effect' exclusive to superconductors. This effect allows superconductors to expel all magnetic field lines from within and create an opposing magnetic field to the incoming field, causing the repulsion or 'levitation' effect. A permanent magnet, however, cannot exhibit this effect - it can only attract or repel based on the alignment of its fixed poles.

Step-by-step solution

Understanding the Problem

The question asks why a superconducting disk can float above a magnetic disk and why a magnet cannot replace the superconductor. To answer this, it is necessary to understand how magnets and superconductors behave in a magnetic field.

Remembering Characteristics of Magnets and Superconductors

Magnets have fixed north and south poles that create a magnetic field. Similar poles tend to repel while opposite poles attract. Superconductors, on the other hand, can exhibit a behavior known as 'Meissner effect' where they expel all magnetic field within the material, creating an opposite magnetic field that repels the incoming field. Thus, no matter which pole of the magnet is pointed towards the superconductor, it will create an opposing magnetic field.

Relating The Behavior of Magnets and Superconductors

In the case of the floating magnet trick, the superconductor expels the magnetic field of the disk below, creating a repulsive force that makes it float. If the superconducting disk were replaced by a permanent magnet, that magnet would simply rotate until its opposite pole is aligned with the pole of the disk below (attraction), or it would be repelled and dispelled away (repulsion). Unlike superconductors, permanent magnets do not have the ability to create an opposing magnetic field to all magnetic directions.

Key concepts

Superconductivity

One of the most fascinating phenomena in physics is superconductivity. This is a property of certain materials that have zero electrical resistance when cooled below a critical temperature. But why is this significant? Imagine electrical current flowing through wires with no energy loss. This capability has revolutionary implications for power transmission and magnetic levitation trains.

At the heart of superconductivity lies the Meissner effect, where the material expels all internal magnetic fields. This is different from perfect diamagnetism, which merely repels an applied magnetic field. The process is quantum mechanical in nature and involves pairs of electrons, known as Cooper pairs, moving without resistance. The absence of electrical resistance allows superconductors to carry electrical current indefinitely once initiated. For students grappling with this concept, a focus on the distinction between ordinary conductors and superconductors – where the latter have zero resistance and exclude magnetic fields – can illuminate why the 'floating magnet trick' is possible only with a superconductor.

Magnetic Field Repulsion

Magnetic field repulsion is a characteristic often demonstrated by magnets when like poles are faced toward each other. However, in the context of the Meissner effect and superconductors, this repulsion behaves differently. A superconductor in the presence of an external magnetic field will create an induced magnetic field that pushes out the external field. This repulsion is not dependent on pole orientation, unlike ordinary magnets, making it unique and responsible for the levitation seen in the 'floating magnet trick'.

For students, understanding the Meissner effect can be made easier by comparing it with the behavior of two repelling ordinary magnets. If you release one of the repelling magnets, it will simply fly away. With superconductors, the induced magnetic field repels all orientations of the external magnetic field, resulting in a stable levitation. This concept highlights the non-conservative magnetic properties of superconductors and their potential to create stable levitating systems.

Magnets Behavior

Magnets exhibit behavior that is well-known and observable in everyday life. They have distinct north and south poles, and they interact with each other and magnetic fields in predictable ways: opposite poles attract and like poles repel. These interactions are at the core of many practical applications, from compasses to electric motors. However, when you bring similar poles close to each other, as soon as one magnet is let go, it will shoot away due to the repulsive force.

In the context of the 'floating magnet trick', it is essential for students to realize why a permanent magnet cannot replace a superconductor. Unlike superconductors, permanent magnets cannot adapt to change their magnetic orientation to continuously repel an external magnetic field. This distinction helps explain why permanent magnets can't maintain stable levitation above other magnets without additional stabilization mechanisms. By grasping these core behaviors of magnets, students can better appreciate the unique characteristics of superconductors and why they are critical for successful magnetic levitation.