When electricity flows through a conductor, a magnetic field is produced. The intensity of this magnetic field is proportional to the distance from the conductor and by the amount of current flowing through it.
When you wrap a coil of wire around a magnet, the magnetic field pushes and pulls the electrons in the wire creating an electrical current. The direction of this current is governed by the magnetic field and can be used to make electromagnets or power electrical devices using the electricity induced in the wire.
The Magnetic Field
Inside a magnet, half the atoms have electrons that spin in one direction and half have electrons that spin in the opposite direction. This creates a magnetic field around the magnet and produces a north and south pole. This is the reason why you can cut a bar magnet in half to get two new smaller magnets with their own north and south poles!
The Magnetic Field Lines
Unlike electric field lines, magnetic field lines do not have starting points or stops. They point from the North Pole of a magnet to its South Pole. This is because the magnetic polarity of a magnet must have both a North and South Pole.
A magnetic field is generated when electrons in metals (such as copper and aluminum) are moved from one end of the magnet to the other. In fact, the magnetic field of a magnet is stronger in the center between the two conductors than at the ends. This is because the path length of the loops of magnetic flux are greater closer to the conductor than farther away.
This can be seen in the orientation of iron filings sprinkled on a piece of paper above a horseshoe magnet. The iron filings point in the direction of the magnetic field, indicating that the horseshoe magnet generates a strong magnetic field in this region.
The Magnetic Field and Electromagnetic Waves
When a magnetic field is created, it also produces electromagnetic waves, or radio waves that shoot out in all directions. These waves are very weak and can be heard, but they cannot be felt by humans or animals.
The magnetic and electric fields are correlated, meaning that a changing magnetic field produces corresponding changes in the electric fields. This relationship is known as Maxwell’s equations and forms part of classical electrodynamics.
Electromagnets can be made by wrapping a coil of wire around a magnet or by placing a wire in a changing magnetic field and inducing an electromotive force, or emf. The emf, or voltage, is caused by the changing magnetic field.
How to Measure the Magnetic Field
The first step to measuring a magnetic field is to map out where its lines of magnetic flux are. This is easy to do with the aid of a compass or a magnet.
A compass is a simple device that orients itself by drawing a corresponding line on the ground with its needle. The compass needles align themselves to the total magnetic field around the magnet, which is equal to the sum of the magnetic field from the earth plus the magnetic field from the wire.