Tutorial 1 - Magnetism Basics
To revise basic magnetism.
To link magnetism with electricity.
What are magnetic fields like?
How do magnetic fields arise?
What happens if a current flows in a wire?
Revision of Basic Magnetism
A magnetic material is one that experiences a force on it when it's in a magnetic field. If the material is not magnetised, it is attracted to whatever is causing the magnetic field (a magnet). If it is magnetised, it can be attracted or repelled by the magnet. Magnetism is a property that is found in three elements and some of their alloys:
These three metallic elements are found next to each other on the periodic table.
The magnetism we are interested in is called ferromagnetism. There are other forms of magnetism, but we won't consider them here. You can always look at an article on the web. Some minerals are ferrimagnetic. Their magnetism arises in a different manner to ferromagnetism.
You will have done this simple experiment in early secondary school. If we hang a magnet so that it hangs freely, it aligns itself with the Earth's magnetic field. You will remember that a bar magnet has two poles.
We always get magnetic poles in pairs; they are never found on their own. If we break up a magnet with a hammer (easily done, as the material is brittle), we find that the pieces always have both a north pole and a south pole. We will look more at this when we discuss the domain theory of magnetism.
The term magnet tends to be associated with permanent magnets, i.e. a magnet that does not lose its magnetism. A permanent magnet is made from a hard magnetic material. It is possible to have a magnet that loses its magnetism when it's not in contact with a magnet. This is a temporary magnet and is made from a material such as iron. Iron is described a soft magnetic material. Soft iron is not soft at all; it's hard and heavy, as you will find out when you drop it on your foot. Soft iron cores are used for electromagnets and transformers, both of which we will look at later.
A magnet has force field surrounding it called a magnetic field.
There are certain rules for drawing magnetic fields:
The picture shows the field lines in two dimensions, but the field itself is three-dimensional. You can show the presence of a magnetic field using iron filings by placing a piece of paper over a magnet and sprinkling the iron filings. It's a messy experiment, but shows the idea well. If you get the filings onto the magnet, you will never get them off!
The closer the field lines are together, the stronger the magnetic field strength. We will discuss this in Tutorial 2.
If we have two pieces of magnetised material, we observe the following:
We can see the effect on the field lines when two magnets attract:
The field lines go from north to south. At the edges they bow out. In the middle they are uniform and straight.
When the magnets repel, the picture is quite different:
The lines bend away from each other. Half way between there is a neutral point, where the force is zero. This is because the forces are vectors, i.e. have a value and a direction. If the force vectors are in the same direction, they add up. If they are in opposite directions, they take away. At the neutral point, the total of the forces is zero. Just like all other force fields, it doesn't mean that there are no forces; the directions and values of the forces sum to zero.
The field lines can be made more concentrated by bending the magnet into a horseshoe shape. This makes the magnet stronger:
A ring magnet has a field that looks like this, as seen from the side:
Picture from Wikipedia
A radial magnetic field can be made like this:
This is useful with coils in motors and meters.
What causes magnetism?
Earlier we said that if we break up a magnet, we get pieces that have a north and a south pole.
If we grind it up further, we still find that the pieces have north and south poles. We never get a pole on its own.
There are a number of theories as to how magnetism arises. We will use the ideas of domains, which are tiny little molecular magnets. They are magnetised due to a quantum phenomenon called electronic spin. In most elements the electrons are in pairs and the spins cancel each other out. However in the elements iron, cobalt, and nickel, the electron spins are thought to be unbalanced, due to there being shells where there are single electrons. The effect of this is that there are tiny electrical currents. When an electrical current flows, there is always a magnetic effect. We will consider a domain as a tiny molecular magnet.
If the material is not magnetised, the domains are arranged randomly like this:
If the material is partially magnetised, some of the domains are lined up, while others remain randomly oriented:
If the material is fully magnetised, all the domains are lined up:
It is not possible to make the magnet any more magnetised than this. The magnet is saturated.
Permanent magnets are made by placing a piece of hard material in a coil, and passing a big direct current through the coil. The domains are lined up and the material is magnetised.
The magnetism can be lost by placing the magnet in a coil carrying alternating current. The domains get jumbled up. Other ways of jumbling up the domains are:
What causes the domains?
Consider an iron atom. It has a nucleus that consists of 26 protons and 30 neutrons.
In the electron shells there 26 electrons that are arranged in 4 shells. The outer shell electrons are responsible for the conduction of electricity in iron, as well as its chemical properties. The electrons in all the shells are moving about, thus forming a current, but this is not responsible for the magnetic properties. Instead electrons have a quantum property called spin, and the spin of an electron can be thought of as a tiny circular electric current. The electrons can spin in either direction. It is found that, in the K, L, and M shells, equal numbers of electrons spin clockwise and anticlockwise. Therefore the shells are magnetically neutral.
In the M shell, 9 out of the 14 electrons spin in one direction, while 5 of the 14 spin in the other direction. 5 of the 9 electrons have their magnetic effect cancelled. The result of this is that 4 of the 9 electrons produce a net magnetic field.
This magnetic effect influences neighbouring atoms, and small groups of atoms take up the same orientation. They form a dipole that is responsible for the domain.
A similar effect is observed in cobalt and nickel. In all other atoms the electron spins are balanced, so they are magnetically neutral.
The Earth's Magnetic Field
Magnetite or lodestone is an iron containing mineral that has magnetic properties. Ancient seafarers noticed that a piece of lodestone placed on a floating dish would orient itself in the same direction. They used this as a crude compass which was essential for navigation. The lodestone had lined up with the Earth's Magnetic Field. Many migrating birds are thought to have a tiny piece of magnetic material in their brains that acts as a compass, enabling them to navigate with remarkable accuracy.
The Earth's magnetic field looks like this:
The Earth's magnet is shown as a simple bar magnet in the diagram above. It arises in the core of the Earth which is made of a high proportion of iron. The inner core is solid and the outer core is liquid, in which there are thought to be titanic electric currents flowing. The south pole of the bar magnet is at the northern hemisphere (and vice versa). The magnet's axis is NOT the same as the axis of rotation of the Earth. Currently it is about 10 degrees to the west of the north south axis. The field lines are at 90 degrees to the ground above the pole. They are parallel to the ground near the equator. The angle to the ground can be measured with an inclinometer. The picture shows an inclinometer used in a school physics labs. Survey instruments are much more sophisticated.
The Earth's magnetic field protects us from the harmful effects of charged particles that are given off by the Sun. The interaction between the charged particles and the Earth's magnetic field is evidenced by the auroras that are seen near the poles.
The earth's magnetic field varies. It changes about 6.3 % every century, and geologists have found that there are periods when there is no magnetic field at all. This could cause trouble for migratory animals, and expose the Earth to high levels of radiation.
Other planets and stars also have magnetic fields. The picture from NASA shows the Sun's magnetic field:
Pictures from NASA
The Sun's magnetic field is shown clearly in the picture as a stream of charged particles follows the magnetic field lines.
The Magnetic Effect of an Electric Current
Electricity and magnetism cannot be separated. If an electric current flows, there is always a magnetic field. The wire that carries the current can be any metal; it does NOT have to be made of a magnetic material. A changing magnetic field causes a voltage. If there is a complete circuit, then a current will flow. We will look at this effect in a later tutorial.
The magnetic field of a single wire looks like this:
The direction of the field is determined by the Corkscrew Rule or Screwdriver Rule. (In the days when this was worked out, the early physicists were too posh to bother with a screwdriver, but would have used a corkscrew...)
If we have two parallel wires and currents flowing in the same direction, we see this:
In the middle, the magnetic fields are in the opposite directions, so they cancel out. The resulting pattern is this:
If the currents are in opposite directions, the pattern observed is this:
There is a resultant magnetic field that is shown by the green arrow.
The diagrams show a three dimensional picture in two dimensions. We can show the directions of the current more easily using dot and cross diagrams:
The current is shown vertical to the page (or screen - let's get up to date!). The magnetic field can be represented in the same way.
The magnetic field of a coil of a single turn of wire is like this:
If we have a coil of several turns of wire, the result is a solenoid:
The field is like that of a bar magnet:
If the current goes clockwise, we get a south pole. If the current goes anti-clockwise, it's a north pole.
The magnetic field of a wire interacts with another magnetic field like this:
It results in a force that we will look at in the next tutorial.