Magnetism and magnetic fields Revision Notes

Permanent Magnets: Key Characteristics

Permanent magnets are materials that create their own magnetic field. This field is always present, meaning the magnet is always 'on' and cannot be easily turned off. They are typically made from 'hard' magnetic materials like steel (an alloy of iron) that retain their magnetism once magnetised. The magnetic domains within permanent magnets are permanently aligned, giving them persistent magnetic properties. Examples include the magnets found in compasses, refrigerator doors, and loudspeakers.

Induced Magnets: Key Characteristics

Induced magnets are materials that become magnetic only when they are placed within an external magnetic field. Their magnetism is temporary; it exists only while the external magnetic field is present and vanishes once the external field is removed. 'Soft' magnetic materials, such as iron, nickel, and cobalt, are good examples of materials that can become induced magnets. When placed near a permanent magnet, the magnetic domains in an induced magnet align temporarily, causing it to exhibit magnetic properties.

Interaction Between Permanent and Induced Magnets

When a permanent magnet is brought close to an induced magnetic material, the induced material becomes temporarily magnetised. The pole of the induced magnet closest to the permanent magnet will always be the opposite polarity. This interaction always results in an attractive force between the permanent magnet and the induced magnet. For instance, if a North pole of a permanent magnet is brought near an iron nail, the nearest end of the nail will become a South pole, and the further end a North pole, attracting the nail.

Distinguishing Features Summary

The fundamental difference lies in the origin and persistence of their magnetism. Permanent magnets possess intrinsic, long-lasting magnetism. Induced magnets only exhibit magnetism when influenced by an external magnetic field, and this magnetism is temporary. Permanent magnets can attract or repel other magnets, but they will always attract induced magnetic materials. Induced magnetic materials can only be attracted by other magnets and do not repel.

The Principle of Electromagnetic Induction (Higher)

Electromagnetic induction describes the generation of an electrical potential difference (and thus, a current if the circuit is closed) across a conductor due to a change in the magnetic field around it. This change can be caused by the movement of a conductor through a magnetic field, or by a changing magnetic field flux through a stationary conductor. The process converts mechanical energy (if moving the conductor or magnet) or changes in magnetic field energy into electrical energy.

Induced Potential Difference and Current (Higher)

When a magnetic field changes around a conductor, a potential difference is induced across its ends. If the conductor forms part of a complete circuit, this induced potential difference will drive an induced current. The magnitude of the induced potential difference is directly proportional to the rate at which the magnetic field lines are cut or the magnetic flux changes. Factors influencing the magnitude include the strength of the magnetic field, the speed of movement, and the length of the conductor within the field.

Lenz's Law and Opposition to Change (Higher)

A critical aspect of electromagnetic induction is that the induced current always flows in a direction that generates a magnetic field opposing the original change that caused it. This principle is known as Lenz's Law. For example, if a magnet is moved towards a coil, the induced current in the coil will create a magnetic field that repels the incoming magnet. If the magnet is moved away, the induced field will attract it. This opposition ensures the conservation of energy, as work must be done against this opposing force to induce the current.

Applications of Electromagnetic Induction (Higher)

Electromagnetic induction is fundamental to the operation of many technologies. Electrical generators, for instance, use the mechanical rotation of coils within magnetic fields to induce electrical current. Transformers, essential for changing AC voltages, rely on a changing magnetic field in one coil inducing a potential difference in another. Induction cooking and metal detectors also utilise principles of electromagnetic induction.

Magnetic Field Lines: Direction

Magnetic field lines are a visual tool used to represent the magnetic field around a magnet. By convention, magnetic field lines always emerge from the North pole of a magnet and enter the South pole. Inside the magnet, they run from South to North, forming continuous closed loops. The direction of the magnetic field at any point is shown by an arrow on the field line, which indicates the direction a free-moving magnetic north pole would experience a force. A compass needle, which is a small magnet, points along these field lines.

Magnetic Field Strength

The strength of a magnetic field is indicated by the spacing of the magnetic field lines. Where the magnetic field lines are drawn closer together, the magnetic field is stronger. Conversely, where the magnetic field lines are spaced further apart, the magnetic field is weaker. The magnetic field is strongest at the poles of a magnet, as this is where the field lines are most concentrated and closely packed. As you move away from the poles, the field lines diverge and become less dense, indicating a decrease in field strength.

Plotting Magnetic Field Patterns

The pattern of magnetic field lines can be plotted using a compass. Place the compass near a pole of the magnet and mark the direction its north pole points. Move the compass to this new mark and repeat the process, drawing a series of dots. Connecting these dots forms a magnetic field line. Repeating this for different starting points allows you to map out the entire field pattern. Iron filings can also be used to visualise field lines; when sprinkled on a sheet above a magnet, they align themselves along the field lines, showing the pattern.

Key Characteristics of Magnetic Fields

Magnetic fields are regions where magnetic forces are exerted. The key characteristics are: 1. They always form continuous closed loops. 2. They never cross each other. 3. Their direction is always from North to South outside the magnet. 4. Their density (how close they are) indicates the strength of the field. The further from the magnet, the weaker the field becomes, as the lines spread out. The field is strongest at the poles.

What are Permanent Magnets?

Permanent magnets are materials that generate their own persistent magnetic field. Unlike electromagnets, which require an electric current to produce a magnetic field, permanent magnets maintain their magnetism without external influence once magnetised. Common examples include bar magnets and horseshoe magnets. Every permanent magnet has distinct poles, traditionally labelled as North (N) and South (S). These poles are where the magnetic field lines emerge from and enter into the magnet, respectively, and where the magnetic force is strongest.

Magnetic Poles: North and South

Each permanent magnet possesses two poles: a North-seeking pole and a South-seeking pole. The 'North-seeking' pole is so named because if a magnet is freely suspended, this pole will orient itself to point roughly towards the Earth's geographic North Pole (which is actually a magnetic South pole). Conversely, the 'South-seeking' pole points towards the Earth's geographic South Pole (a magnetic North pole). It's impossible to isolate a single magnetic pole; if you break a magnet in half, each piece will form its own North and South poles.

The Rule of Magnetic Interaction

The primary rule governing how permanent magnets interact is: 'Like poles repel, unlike poles attract.' This means that a North pole will repel another North pole, and a South pole will repel another South pole. However, a North pole will attract a South pole. This attractive or repulsive force acts without direct contact, across a distance, and its strength increases as the magnets are brought closer together. This fundamental principle underpins many magnetic phenomena and applications.