Magnetism

Magnetism

This Revision Bite is about magnetism. It has these parts in it: Bar magnets Magnetic fields The Earth's magnetic field Electromagnets Using electromagnets

Bar magnets

Bar magnets are permanent magnets. This means that their magnetism is there all the time and cannot be turned on or off. They have two poles:

• north pole (short for north-seeking pole)
• south pole (short for south-seeking pole).

The north pole is normally shown as N and the south pole is normally shown as S.

A bar magnet

Magnets are made from magnetic materials. These are metals that can be magnetised or will be attracted to a magnet. Most materials are not magnetic, but iron, cobalt and nickel are magnetic. Steel is mostly iron, so steel is magnetic too.

Attract and repel

If you bring two bar magnets together, there are two things that can happen:

• if you bring a north pole and a south pole together, they attract and the magnets may stick together;
• if you bring two north poles together, or two south poles together, they repel and the magnets push each other away.

We say that unlike poles attract, and like poles repel.

Remember that unmagnetised iron, steel, cobalt and nickel objects will be attracted to either pole of a magnet. This means you can only show that an object is a magnet if it repels a known magnet.

Magnetic fields

Magnets create magnetic fields. These cannot be seen. They are the space around a magnet where magnetic materials are affected by forces, and so may be attracted or repelled.

Although we cannot see magnetic fields, we can detect them using iron filings. The tiny pieces of iron line up in the magnetic field.

It would be difficult to draw the results from the sort of experiment seen in the photograph, so we draw simple magnetic field lines instead. In the diagram, note that:

• the field lines come out of N and go into S
• the field lines are more concentrated at the poles.

The magnetic field is strongest at the poles, where the field lines are most concentrated.

Attraction and magnetic fields

Iron filings can show the magnetic field between two magnets where unlike poles face each other, and so attract.

If we look at the field lines, we see that they go from the north pole of one magnet to the south pole of the other magnet.

Magnetic field lines between two attracting magnets

Repulsion and magnetic fields

Iron filings can also show the magnetic field between two magnets where like poles face each other, and so repel.

If we look at the field lines, we see that they do not go from one magnet to the other, and they do not cross.

Magnetic field lines between two repelling magnets

The Earth's magnetic field

The Earth does not contain a giant bar magnet, but it behaves as if it does. The Earth has a magnetic north pole and a magnetic south pole. Compasses work because bar magnets can line up in the Earth's magnetic field and point north. Make sure you understand these things:

• the north pole of a bar magnet is actually called the 'north-seeking pole', and it points to the Earth's magnetic north pole;
• the south pole of a bar magnet is actually called the 'south-seeking pole', and it points to the Earth's magnetic south pole.

Compasses point towards the Earth's magnetic north pole

Small compasses called plotting compasses can be used to show the magnetic field around a bar magnet, instead of using iron filings. The needle in each compass turns and lines up along the field lines.

A line of plotting compasses follows a field line

If we arrange some plotting compasses around a bar magnet, we see that the needles point away from the magnet's north pole and towards its south pole.

Plotting compasses in a circle

When you do this experiment, it is important to keep iron or steel objects away, otherwise the compass needles might point to them instead

Electromagnets

A magnetic field is produced when an electric current flows through a coil of wire. This is the basis of the electromagnet. We can make an electromagnet stronger by doing these things:

• wrapping the coil around an iron core
• adding more turns to the coil
• increasing the current flowing through the coil.

How to make a simple electromagnet

The magnetic field around an electromagnet is just the same as the one around a bar magnet. It can, however, be reversed by turning the battery around. Unlike bar magnets, which are permanent magnets, the magnetism of electromagnets can be turned on and off just by closing or opening the switch.

Using electromagnets

Many objects around you contain electromagnets. They are found in electric motors and loudspeakers. Very large and powerful electromagnets are used as lifting magnets in scrap yards to pick up, then drop, old cars and other scrap iron and steel.

An electromagnet in a scrap yard

Electric bell

Electric bells like the ones used in most schools also contain an electromagnet.

• When the current flows through the circuit, the electromagnet makes a magnetic field.
• The electromagnet attracts the springy metal arm.
• The arm hits the gong, which makes a sound and the circuit is broken.
• The electromagnet is turned off and the springy metal arm moves back.
• The circuit is complete again.

ELECTRICAL ENGINEERING FOR HEALTH

Magnetic Resonance Imaging for Medical Diagnostics

Today's medical professionals use a variety of diagnostic tools to probe the condition of biological entities. One such technique is that of magnetic resonance imaging (MRI).

MRI works in the following manner. The patient is first placed in a strong magnetic field. The paramagnetic atoms become aligned with the direction of the field. The aligned hydrogen atoms take on resonance characteristics; that is, they can absorb energy and re-emit the energy as electromagnetic radiation at a specific frequency. A short pulse of radiofrequency (RF) waves is then sent to the body. The aligned protons (hydrogen) are excited and become deflected. Once the RF pulse is stopped, the protons realign with the magnetic field while emitting RF waves that are measured. The emitted RF waves contain information on both the physical and chemical characteristics and spatial distribution of body materials. For instance, an image representing the density of hydrogen nuclei within the body is produced.

The electromagnet experiment serves to illustrate just a fraction of the physics behind MRI. The electromagnet interacts with metal objects on a macroscopic scale whereas the object of MRI is to take advantage of interactions on an atomic scale.

The electrical engineer is involved in the development of medical diagnostic procedures in a variety of ways, including: Radiofrequency (RF) waves are studied in electromagnetics Instrumentation is developed to measure the emissions Signal processing techniques are used to analyze and enhance the images