Introduction
The magnetic field of a single-pole magnet is often likened to the one-eyed person in the proverbial room—you cant ignore its presence. Such a simple device presents complex behaviours that depend on the material, shape, size and orientation of the object. Magnetic field distributions are used to measure and characterise the performance of electrical equipment, and are also useful in many other situations such as aerospace engineering and medical diagnostics (MRI).
In this report, we will investigate the magnetic field of a single-pole magnet in order to understand the effects of orientation on its field. We will make use of forces on a test mass to determine the characteristics of the field.
Magnetic poles and field structure
A single-pole magnet has two poles, a north pole and a south pole, located at opposite ends of the magnet. If the magnet is left alone, its magnetic field will extend outward in evenly spaced (dipolar) symmetric lines about the poles, radiating straight outward linking the north and south poles, as can be seen in Figure 1.
The force at the end of each magnetic field line is equal in magnitude and opposite in direction, creating attraction. The force is strongest near the poles, and fades away as the distance from the pole increases. The attraction between unlike magnetic poles will draw these poles towards each other while two like poles will repel each other.
The intensity of the magnetic field is proportional to the current through the magnet. The current is related to the voltage by Ohms law, so increasing the voltage will increase the field strength.
Figure 1: Dipolar magnetic field of a single-pole magnet.
Orientation of the magnet
The orientation of the magnet, the way it is pointed, can have an effect on the distribution of the magnetic field. When the pole of the magnet is orientated parallel to the vertical dipole, the field is vertical to the ground and usually a uniform intensity (Figure 2). As the pole is tilted away from vertical, the field becomes tilted and its intensity decreases.
When the pole of the magnet is oriented perpendicular to the vertical dipole, the field is horizontal. The field strength is not uniform as it is when vertical, and can be strongest on either the left or right side of the pole.
Figure 2: Vertical and horizontal orientations of a single-pole magnet.
Forces on a test mass
By recording the forces on a test mass in the magnetic field of a single-pole magnet, we can measure the characteristics of the field. The test mass is suspended in the field by a non-magnetic backing so that it will be free to respond to the field.
When the test mass is brought close to the pole, the force of attraction on the mass increases. At a certain point, the attraction of the pole will be greater than the gravitational force, and the mass will be pulled into the pole and remain there.
At different distances from the pole, the force of attraction on the mass will decrease. This decrease follows a parabolic pattern, and for any given distance the force is equal in both directions, towards and away from the pole. The force of attraction will also depend on the orientation of the pole—for a pole oriented horizontally, the force will be greatest when the mass is on the right side of the pole, and weakest when it is on the left.
Conclusion
In this report, we have discussed the characteristics of the magnetic field of a single-pole magnet. We have seen that the field is dipolar, and that its strength is related to the current through the magnet and decreases with distance from the pole. We have also seen that the orientation of the pole affects the field in both strength and pattern, and that it can be measured using forces on a test mass. This knowledge can be used to characterise the performance of various electrical and magnetic devices, as well as for medical diagnostics and other applications.
