IGCSE Physics 0625

4.1 Simple phenomena of magnetism

Objective: Explain that magnetic forces are due to interactions between magnetic fields

Magnetic forces are a fundamental aspect of the physical world, responsible for phenomena ranging from the operation of electric motors to the Earth's magnetic field. This section explores the basic principles of magnetism, focusing on the idea that magnetic forces arise from the interaction between magnetic fields.

Key Concepts

• Magnetic Fields: A region around a magnet or a current-carrying conductor where a magnetic force can be detected.
• Magnetic Poles: The points on a magnet where the magnetic field is strongest. Opposite poles attract, and like poles repel.
• Magnetic Force: The force exerted by a magnet on another magnetic material or on a moving electric charge.

Evidence for Magnetic Fields

We can observe the existence of magnetic fields through several phenomena:

• Compass Needle Alignment: A compass needle aligns itself with the Earth's magnetic field. This demonstrates the presence of a magnetic field around the Earth.
• Magnetic Attraction: Magnets attract ferromagnetic materials like iron, nickel, and cobalt. This attraction is due to the interaction between the magnetic field of the magnet and the magnetic domains within the ferromagnetic material.
• Magnetic Force on Moving Charges: A magnetic field exerts a force on a moving electric charge. This is the principle behind electric motors.

How Magnetic Forces Work

Magnetic forces are not simply a direct contact force. Instead, they arise from the interaction between magnetic fields. A magnetic field is a vector field, meaning it has both magnitude and direction. When a magnet or a current-carrying conductor is placed in another magnetic field, the magnetic field of the first object exerts a force on it. The direction of this force depends on the relative orientation of the fields and the polarity of the magnets involved.

The force experienced by a moving charge in a magnetic field is described by the Lorentz force law:

$$F = q(v \times B)$$

Where:

• F is the magnetic force (a vector).
• q is the charge of the moving particle.
• v is the velocity of the charge.
• B is the magnetic field vector.
• × represents the cross product.

Experiments Demonstrating Magnetic Forces

Several simple experiments can demonstrate the interaction of magnetic fields:

Experiment 1: Attraction of Iron Filings

A bar magnet is brought near a small pile of iron filings. The iron filings align themselves along the magnetic field lines, demonstrating the presence of a magnetic field around the magnet.

Experiment 2: Repulsion of Like Poles

Two bar magnets are brought close together with like poles facing each other. They will repel each other, demonstrating that like magnetic poles exert repulsive forces.

Experiment 3: Magnetic Force on a Current-Carrying Conductor

A wire carrying an electric current is placed in a magnetic field. The wire will experience a force, demonstrating the interaction between magnetic fields and moving charges (currents).

Summary

Magnetic forces are a consequence of the interaction between magnetic fields. Magnets produce magnetic fields, and these fields exert forces on other magnets and on moving electric charges. Understanding this interaction is crucial for understanding a wide range of magnetic phenomena.

Concept	Description
Magnetic Field	Region around a magnet or current where magnetic force is exerted.
Magnetic Poles	Points on a magnet with the strongest field; North and South poles.
Magnetic Force	Force exerted by a magnet or current on another magnetic material or moving charge.