Magnets

Types of Magnets

Magnets can be made from materials that allow the way their electrons spin to be changed so they spin in parallel with each other. Normally electrons spin in random directions, but when a material is magnetized the spin of each electron is coordinated to have the same direction of spin. This is why hitting a bar magnet with a hammer will demagnetize it, the impacts disrupt the spin of the electrons. It is possible to re-magnetize it by repeatedly running another magnet across it until the spins are realigned.

Some metals are easy to magnetize, especially iron, cobalt and nickel. Alloys made from these can also be magnetized, such as steel which is a mixture of iron with carbon and other elements. Oxygen can also be magnetized but this is only apparent when it is in its liquid form.

All magnets have two poles, a north pole and a south pole, because of the interaction of their magnetic fields, like poles repel each other, but unlike poles attract.

Magnets, figure 1

Two forms of Magnets

Materials capable of being magnetized can be either permanent magnets or temporary magnets.

Permanent magnets: These magnets retain their magnetism over long periods of time, the spin of their electrons have been aligned and remain in this configuration unless they are subject to an impact or excessive heating.

Applications:

● Speakers (Cobalt and nickel)

● Microphones (Cobalt and nickel)

● Built into some ipad cases to lock the device when the lid is closed.

● Small electric motors and generators

Temporary magnets: In this type of magnet the magnetic field can be turned on and off. The magnetic material is surrounded by a coil of wire, when a current is flowing in the wire this produces a magnetic field that is transferred to the material. These have the advantages of being very strong and easy to control. They can be turned on and off, they can also be set to different strength by controlling the electrical current in the coil.

Applications:

● Picking up and moving vehicles in a wrecking yard

● Separating metal items from domestic and commercial waste

● Commercial generator

● Large electric motors

● MRI (magnetic resonance imaging) machines in hospitals

Magnetic Fields and Interactions

All magnets, whether they are permanent or temporary, are surrounded by a magnetic field. This field flows out of the north pole and heads toward the south pole in every increasing loops. The concentration of the line indicates the strength of the field.

Magnets, figure 1

Where two poles are brought together a uniform field is formed, in which the lines of the field run directly from one pole to the other, as illustrated in this diagram.

Magnets, figure 2

The attraction and repulsion of the magnets is due to the interaction of their magnetic fields.

When two like poles are together their fields push against each other causing a force of repulsion.

Magnets, figure 3

When two opposite poles are together their fields align and pull together in the same direction causing attraction.

Plotting Magnetic Fields

Although magnetic fields are invisible, it is possible to detect the presence of a magnetic field using iron filings or a plotting compass.

A plotting compass is a small compass that will point SOUTH unlike an other compass. When placed next to a magnet the compass needle with rotate to point along the line of the local magnetic field. By moving the compass and recording the position and direction of the needle it is possible to map out the local magnetic field close to any magnet,

Magnets, figure 1

Plotting the local magnetic field with a plotting compass.

Magnets, figure 2

The needle will turn to point in the direction of the local __south __pole of the magnet and spin away from the local north pole of the magnet. This is designed to make it easier to visualize the direction of the lines of magnetism. Once several positions have been mapped the points can be joined up with arrows to indicate the shape of the field and the orientation of the north pole of the local magnetic field.

Earths Magnetic Field

The Earth is made up of four main layers, the innermost layers, known as the core have a high concentration of iron. The outer core is liquid, and due to the heating from the solid inner core the liquid iron outer core is constantly moving.

These convection current in the outer core result in a strong magnetic field which surrounds the whole Earth.

Magnets, figure 1

This magnetic field can be detected by any compass, conventional compases have a needle that points to the Earth’s North Pole. It is possible to plot the Earth’s magnetic field with a dipping compass. This is simply a compass that is held vertically rather than horizontally. By moving this dipping compass over the surface of the Earth the lines of magnetism can be followed and plotted.

This early dipping compass was used in 1581 to map parts of the Earth’s magnetic field for the first time. At the poles the compass needle will point at 90° to the surface, near to the equator and the compass will start to align with the horizon as the compass follows the curved lines of magnetism.

Magnets, figure 2

This mapping of the lines of magnetism provided evidence that the Earth is surrounded by a strong magnetic field, the pattern of the lines is the same as a bar magnet.

In a conventional compass the needle, normally made of steel, is a small magnet and the north pole of the compass needle aligned with the Earth poles. As opposites attract, what we term the North Pole geographically is in fact the south magnetic pole of the Earth’s magnetic field.

Why does the needle of a compass point north?
Your answer should include: Earth / Earth’s / south / magnetic pole
Explanation: The needle of the compass is made of a metal that can be magnetized, such as steel. The needle is attracted to the poles of the Earth’s magnetic field. The north pole of the compass is attracted to the Earth’s south magnetic pole at the geographic North Pole.