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Magnetism
Concept sheet | Science and Technology
Magnetism is a characteristic property of a substance that has the ability to be attracted or repelled by a magnet.
To better understand magnetism, we can study the behaviour of magnets and the magnetic field they generate.
What Is a Magnet?
A magnet is an object that can attract or repel other magnets and can attract substances made from iron |\text{(Fe)},| cobalt |\text{(Co)},| nickel |\text{(Ni)},| or an alloy of these metals.
Although magnets vary in shape and size, they always have a north pole and a south pole.
Attraction and Repulsion Between Two Poles
When magnets are placed close together, we notice the following effects:
- Two opposite poles attract each other
- Two like poles repel each other
Opposite poles (south-north) attract each other.
Like poles (north-north) repel each other.
The attraction or repulsion between two poles is due to the magnetic field exerted by the magnets.
What Is a Magnetic Field?
A magnetic field is the space in which the magnetic forces (attraction/repulsion) are exerted by a magnet or a moving electric charge.
The magnetic field of a magnet is not visible to the naked eye, but it can be represented by arrows called magnetic field lines.
- Outside the magnet, the magnetic field lines run from the north pole to the south pole.
- The closer the magnetic field lines are to each other, the stronger the magnetic field is.
- Magnetic field lines do not cross.
The Magnetic Field of a Bar Magnet
The Magnetic Field of a U-Shaped Magnet
One of the things magnetic field lines help us understand is the attraction or repulsion between two poles.
Opposite poles (north-south) attract each other.
Like poles (south-south) repel each other.
How Compasses Behave Inside a Magnetic Field
Because a compass’ needle is a magnet, we can see the effect a magnetic field has on it.
Move the compass around the bar magnet and observe how the needle behaves.
In the previous animation, we can see that:
- The north pole of the compass is attracted by the south pole of the magnet
- The south pole of the compass is attracted by the north pole of the magnet
- The magnetic field lines around the magnet have the same orientation as the north pole of the compass
Earth’s Magnetic Field
Earth’s magnetic field, also known as the magnetosphere, is the magnetic field exerted by Earth’s outer core.
The Earth’s outer core is made up of iron |\text{(Fe)}| and nickel |\text{(Ni)}| in a liquid state, among other things. Their convection movements generate electrical currents and a magnetic field similar to that of a bar magnet.
If we imagine Earth’s outer core as a bar magnet, its south pole would be aligned with a point historically called the North Magnetic Pole. This means that the so-called North Magnetic Pole actually behaves like a south magnetic pole.
A compass needle naturally follows the lines of Earth’s magnetic field.
