Question

A thin diamagnetic rod is placed vertically between the poles of an electromagnet. When the current in the electromagnet is switched on, then the diamagnetic rod is pushed up, out of the horizontal magnetic field. Hence the rod gains gravitational potential energy. The work required to do this comes from

• A. the current source
• B. the lattice structure of the material of the rod
• C. the magnetic field
• D. the induced electric field due to the changing magnetic field

Answer 1

The Correct Option is C

To understand why the diamagnetic rod is pushed up and where the work comes from, let's break down the concepts involved:

1. Diamagnetism: Diamagnetic materials are characterized by their tendency to create an opposing magnetic field when exposed to an external magnetic field. This effect causes them to be pushed out of strong magnetic fields, contrary to paramagnetic or ferromagnetic materials that are attracted to magnetic fields.
2. Electromagnet and Magnetic Field: When the electromagnet is turned on, it creates a magnetic field. A diamagnetic rod placed in this magnetic field experiences a repulsive force that pushes it out of the magnetic field region.
3. Movement and Potential Energy: As the diamagnetic rod moves upwards, it gains gravitational potential energy. According to the work-energy principle, this requires work to be done on the rod.
4. Source of Work: The work required to move the rod and overcome gravitational and any resistive forces comes specifically from the magnetic field itself. The energy supplied by the magnetic field is transferred to the rod, allowing it to gain potential energy.

Having analyzed the effects involved, let's evaluate each option:

• The current source: While it supplies the current that creates the magnetic field, the actual work done on the rod is not directly from the current source.
• The lattice structure of the material of the rod: The crystalline lattice does not provide energy for movement; it defines the material's properties.
• The magnetic field: The magnetic field does the work required to push the diamagnetic material out of the magnetic field, hence providing the energy needed for the rod to gain gravitational potential energy. This is the correct answer.
• The induced electric field due to the changing magnetic field: Although a changing magnetic field can induce an electric field, this is not relevant to the work done on the diamagnetic rod in this context.

Therefore, the work required to move the diamagnetic rod comes from the magnetic field.