Magnetism
Magnetism is an interaction by which certain objects, called magnetic objects, exert forces on one another without physical contact. A magnetic object is surrounded by a magnetic field that becomes weaker with distance. A second object placed in that field experiences a magnetic force determined by the field at its location. Humans have known about naturally magnetic materials for thousands of years; for example, lodestone is a naturally magnetised form of the iron oxide mineral magnetite and was used as an early navigational aid.
What is a magnetic field?
A magnetic field is a region of space in which a magnet or any magnetic material experiences a non-contact magnetic force. Magnetic fields are produced by moving electric charges. At the atomic level, electrons moving in atoms (orbital motion and intrinsic spin) produce tiny magnetic fields. The vector sum of these microscopic fields determines whether a material shows a net magnetic field.
Sources of magnetism in materials
Inside any material, many electrons produce magnetic fields. In most materials the directions of these microscopic fields are randomly oriented and cancel one another, so the material shows no net magnetism. In some materials, particularly ferromagnetic materials (such as iron, nickel and cobalt), groups of atoms form regions called domains. Within a domain, the magnetic moments of atoms are aligned, producing a net magnetic moment for that region. When many domains align in the same direction, the object shows a macroscopic magnetic field and behaves as a magnet.
Domains and magnetisation
Each domain contains many atoms whose magnetic moments point the same way. In an unmagnetised ferromagnetic sample, domains are oriented randomly so their net effect cancels. Magnetisation can be produced by applying an external magnetic field or by stroking the material with a magnet; this tends to make more domains point in one direction, producing a net magnetic field for the whole object. If the alignment remains after removing the external field, the material is said to be magnetised permanently.
Permanent magnets and poles
A permanent magnet is a material in which most domains remain aligned without the need for an external field. Every permanent magnet has two opposite ends called north (N) and south (S) poles. If a magnet is cut into pieces, each piece becomes a smaller magnet with both an N and an S pole. Magnetic poles always occur in pairs; isolated single magnetic poles (magnetic monopoles) are not observed in ordinary materials.
Attraction and repulsion of poles
Like poles repel and unlike poles attract. That is, N-N or S-S interactions are repulsive, while N-S interactions are attractive.
Representing magnetic fields: field lines and flux
Magnetic fields are often shown by drawing magnetic field lines. Field lines:
- are drawn so that arrows indicate the direction of the magnetic field at each point (by convention outside a magnet the lines point from the north pole to the south pole);
- never cross;
- are closer together where the field is stronger and farther apart where the field is weaker.
The number of field lines passing through a given two-dimensional surface is called the magnetic flux through that surface; flux is used to quantify the strength of the magnetic field across that surface.
Simple experiments and investigations
Investigation: How to detect magnetism in objects
- Find two paper clips. Put the paper clips close together and observe what happens.
- What happens to the paper clips?
- Are the paper clips magnetic?
- Take a permanent bar magnet and rub it once along one of the paper clips. Remove the magnet and bring the rubbed paper clip close to the other paper clip. Observe whether the untouched paper clip experiences a force and whether that force is attractive or repulsive.
- Rub the same paper clip a few more times with the bar magnet in the same direction as before. Put the paper clip close to the other one and observe any difference from step 2.
- Is there any difference compared with step 2?
- If there is a difference, what is the reason for it?
- Is the paper clip which was rubbed repeatedly now magnetised?
- What is the difference between the two paper clips at the level of their atoms and electrons?
- Find a metal knitting needle, metal ruler, or another metal object. Rub the bar magnet along the knitting needle a few times in the same direction. Put the knitting needle close to the paper clips and observe what happens.
- Does the knitting needle attract the paper clips?
- What does this tell you about the material of the knitting needle - is it ferromagnetic?
- Repeat the experiment with objects made from different materials. Record which materials appear to be ferromagnetic and which do not. Present your results in a table.
Investigation: Visualising a bar magnet's field
Place a bar magnet under a non-magnetic, flat surface and put a sheet of paper over it. Sprinkle iron filings uniformly on the paper and gently tap or shake the paper so the filings can align with the field. Observe and sketch the pattern. The iron filings line up along the magnetic field lines and reveal the shape of the field in the plane of the paper.
Investigation: Field patterns for two bar magnets
Place two bar magnets close together with different arrangements of poles (like poles facing each other; unlike poles facing each other; side by side). Cover with paper, sprinkle iron filings, and observe the patterns. Opposite poles produce field lines that converge between the magnets; like poles produce field lines that diverge between them.
Types of magnetic behaviour in materials
Materials respond to magnetic fields in different ways:
- Ferromagnetism: Shown by iron, nickel, cobalt and some alloys. These materials can be strongly magnetised and can retain magnetisation. They show domains whose moments align strongly with one another.
- Paramagnetism: Materials such as aluminium or platinum are weakly attracted by a magnetic field; their magnetism appears only while the external field is present and disappears when it is removed.
- Diamagnetism: Materials such as copper or bismuth produce a magnetic response opposite to an applied field and are weakly repelled by a magnet. Diamagnetism is very weak in most materials.
Retentivity and related terms
Retentivity is the ability of a ferromagnetic material to retain magnetisation after the external field is removed. Some ferromagnetic materials retain magnetism strongly (good permanent magnets); others are easily magnetised but also easily demagnetised (soft iron used in transformer cores).
Note (briefly): heating a ferromagnetic material above its Curie temperature removes its ferromagnetic ordering; upon cooling below that temperature the material may regain ferromagnetism, depending on circumstances. (The Curie temperature is a well-established physical property for each ferromagnetic material.)
The compass and navigation
A compass contains a small magnetised needle free to rotate. In the Earth's magnetic field the needle aligns with the field, indicating the magnetic north-south direction. This allowed early navigators to find approximate directions. The needle points toward the Earth's magnetic north pole, which is not exactly coincident with the geographic (true) north pole; the difference is called magnetic declination.
The Earth's magnetic field
The Earth behaves approximately like a giant magnet with a magnetic axis and two magnetic poles. The Earth's magnetic field is believed to be generated by electric currents produced by the motion of conducting liquid metal in the planet's outer core. The magnetic poles do not exactly coincide with the geographic poles and they drift slowly with time. Over geological time scales the Earth's magnetic field has reversed polarity many times (the magnetic north and south poles have swapped places).
Magnetosphere and space-weather effects
The region around Earth where charged particles are influenced by the planet's magnetic field is called the magnetosphere. The magnetosphere protects the surface from many high-energy charged particles from the Sun (the solar wind). When the Sun ejects large clouds of charged particles and magnetic fields (coronal mass ejections), interactions with the Earth's magnetosphere can cause geomagnetic storms that affect satellites, communications and power grids.
Aurorae
The spectacular aurorae (Northern and Southern Lights) occur when charged particles guided by the Earth's magnetic field collide with atoms and molecules in the upper atmosphere near the polar regions, producing visible light (often green and red emissions). Aurorae are most commonly seen at high latitudes near the geomagnetic poles.
Practical notes and applications
- Permanent magnets are used in motors, loudspeakers, magnetic storage and many everyday devices.
- Soft ferromagnetic materials (low retentivity) are used in transformer cores and electromagnets where magnetisation must be reversible.
- Compasses, magnetometers and satellites are used to measure and monitor the Earth's magnetic field and its changes.
Summary
- Magnets have two poles: north and south.
- Certain substances can be easily magnetised (ferromagnetic materials).
- Like poles repel; unlike poles attract.
- The Earth has a magnetic field that affects compass needles and protects the planet from charged particles from the Sun.
- A compass aligns with the Earth's magnetic field and helps determine direction.
- Aurorae are caused by charged particles from space interacting with the Earth's atmosphere along magnetic field lines near the poles.
End-of-chapter exercises
1. Describe what is meant by the term magnetic field.
2. Use words and pictures to explain why permanent magnets have a magnetic field around them. Refer to domains in your explanation.
3. What is a magnet?
4. What happens to the poles of a magnet if it is cut into pieces?
5. What happens when like magnetic poles are brought close together?
6. What happens when unlike magnetic poles are brought close together?
