Magnetic Fields and Forces: The Basics (VCE SSCE Physics): Revision Notes

Magnetic Fields and Forces: The Basics

Introduction to magnetism

A permanent magnet is an object that attracts certain metals like iron, nickel, and cobalt. You have probably noticed how a magnet can pick up paper clips or stick to a refrigerator door. This attractive force is an example of magnetism in action.

All magnets share several important properties. Regardless of their shape (whether they are bar-shaped, horseshoe-shaped, or cylindrical), every magnet has two regions where the magnetic effect is strongest. These regions are called poles. By convention, we label these poles as the north pole and the south pole.

Magnetic dipoles

The combination of a north pole and a south pole is known as a magnetic dipole. This is a fundamental characteristic of magnetism. If you break a magnet in half, you do not get a separate north pole and south pole. Instead, each piece becomes a smaller magnet with its own north and south poles. This process continues no matter how many times you break the magnet - even down to the atomic level.

Electrons and protons are subatomic particles that each possess their own magnetic dipoles. This demonstrates that magnetic dipoles exist at the smallest scales in nature.

What is a magnetic field?

The space around a magnet where magnetic forces can be detected is called a magnetic field. We cannot see magnetic fields directly, but we can visualise them using iron filings. When iron filings are sprinkled around a magnet, they align themselves along invisible lines, revealing the pattern of the magnetic field.

The magnetic field is represented by the symbol $B$ and is a vector quantity - it has both magnitude (strength) and direction. The SI unit for measuring magnetic field strength is the tesla, symbol $\text{T}$.

Direction of magnetic field lines

The direction of the magnetic field at any point is defined as the direction of the force that would act on a north pole placed at that point. We represent this using arrows on field line diagrams. The arrows point away from north poles and toward south poles.

Measuring magnetic fields

The strength of a magnetic field is measured in tesla ($\text{T}$). Different sources of magnetism produce fields of varying strengths:

Faraday's rules for magnetic field lines

In 1846, Michael Faraday introduced the concept of magnetic field lines to explain how magnets interact. He established three fundamental rules that apply to all magnetic field lines:

Uniform magnetic fields

In some configurations, magnetic fields can be uniform - meaning the field strength remains constant across a region. A U-shaped magnet creates a region of approximately uniform magnetic field between its poles. In this region, the field lines run parallel to each other with equal spacing.

When a magnetic field is not changing with time, we call it a static field.

Magnetic dipoles versus monopoles

An important distinction exists between different types of fields in physics. We can classify field sources as either monopoles (single poles) or dipoles (pairs of opposite poles).

Field type	Only monopoles	Only dipoles	Monopoles and dipoles
Gravitation	✔
Magnetism		✔
Electricity			✔

Gravitation involves only monopoles because mass comes in only one variety. All masses attract each other.

Magnetism involves only dipoles. North and south magnetic poles always exist together in pairs. Despite extensive searching, physicists have never found an isolated magnetic pole (a magnetic monopole).

Electricity involves both monopoles and dipoles. Electric charge comes in two varieties (positive and negative) that can exist separately as monopoles or together in pairs as dipoles.

Representing magnetic fields in three dimensions

Magnetic fields exist in three-dimensional space. To represent three-dimensional field patterns on a flat page, we use a convention called the arrow convention:

Electromagnetism

In 1820, Danish physicist Hans Christian Ørsted made a crucial discovery while demonstrating electrical circuits to students. He noticed that when he placed a compass near a wire carrying electric current, the compass needle deflected and turned perpendicular to the wire.

This experiment revealed a fundamental connection between electricity and magnetism - a phenomenon we now call electromagnetism. Ørsted had discovered that electric current creates its own magnetic field.

Magnetic field around a straight wire

When electric current flows through a straight wire, it produces a circular magnetic field around the wire. The field lines form concentric circles centred on the wire. The strength of this magnetic field:

• Is proportional to the magnitude of the current
• Decreases as you move farther from the wire

The right-hand grip rule

To determine the direction of the magnetic field around a current-carrying wire, physicists use a mnemonic called the right-hand grip rule:

This rule works for any straight current-carrying wire and helps predict the field pattern.

Applying the arrow convention to current

We can also apply the arrow convention to show current direction:

• Dot: Current coming out of the page
• Cross: Current going into the page

When we combine this with the right-hand grip rule, we can determine the magnetic field pattern around wires oriented perpendicular to the page.

Magnetic fields from current-carrying wires

Different configurations of current-carrying wires produce different magnetic field patterns.

Parallel wires

When two current-carrying wires run parallel to each other, they exert forces on one another due to their magnetic fields:

• If currents flow in the same direction, the wires attract each other
• If currents flow in opposite directions, the wires repel each other

This interaction occurs because each wire produces its own magnetic field, and these fields interact with the current in the neighbouring wire.

Current loops

When a wire is formed into a loop and current flows through it, the magnetic field becomes concentrated in the centre of the loop. The field lines pass through the loop and circle back around the outside.

This configuration creates a stronger, more concentrated magnetic field than a straight wire carrying the same current.

Solenoids

A solenoid is a coil of wire with many loops wound closely together. When electric current flows through a solenoid, it creates a magnetic field pattern very similar to that of a bar magnet - with a north pole at one end and a south pole at the other.

Electromagnets

In 1823, William Sturgeon discovered that placing an iron rod inside a solenoid dramatically increased the magnetic field strength. This invention is called an electromagnet.

An electromagnet consists of:

• A coil of wire (solenoid)
• An iron or steel core inside the coil
• An electric current flowing through the wire

The iron core can increase the magnetic field strength by up to 1000 times compared to the same solenoid without the core. This makes electromagnets extremely powerful and useful.

Advantages of electromagnets

Remember!