Did You Know?
Some animals like mollusks and pigeons have magnetic minerals such as magnetite in their body. It is believed that these animals use these minerals to align themselves with the Earth's magnetic field, allowing them to navigate their surrounding area easily.
All substances which are capable of attracting ferric elements such as iron are called magnets. Bar magnets are rectangular, narrow objects that are made either of iron or steel composites or ferromagnetic materials which generate magnetic fields. These are the most commonly available variety of magnet shapes. All bar magnets have two poles, i.e., north and south poles. For identification purposes, the north pole is painted red, while the south pole is painted blue. However, in science laboratories, they are painted red all over, and the north pole is indicated with a white dot. Let's look at a few traits and characteristics displayed by bar magnets.
- Bar magnets are usually dark-gray or black in color, since they are commonly made of alnico, i.e., a mix of aluminum, nickel, and cobalt.
- They are commonly characterized by having a north and south pole at the opposite ends.
- These magnets keep their magnetism for very long periods of time.
- They are permanent magnets, i.e., they keep their magnetism even when removed from a magnetic field.
- Depending on the material that the magnets are made from, the average magnetization magnitude is around 100,000 A/m.
- Magnetism at the center is the weakest, while the parts which represent the poles display the strongest magnetic forces, because the field lines are most concentrated at the poles.
- Like poles repel and unlike poles attract.
- Bar magnets are widely used in school classrooms to study theories such as the law of magnetism, learning about magnetic and non-magnetic metals, and various practical experiments involving magnetism, mostly because they are inexpensive, and very easy to use.
- They can be used to magnetize objects such as metal pins and paper clips, by rubbing the magnet against the object.
- When a bar magnet is suspended from a string, it automatically aligns itself towards the magnetic north pole of the Earth. Hence, this can serve as a makeshift compass.
- These are used in a variety of industries for the automation of machines, collection of loose metals, and to help other magnets in retaining their magnetism.
- They can also be seen inside some varieties of electric bells.
- These magnets are also used in devices such as telephones, radios, and television sets.
- They are used as stirring rods for magnetic laboratory experiments.
- They are also used in many medical procedures.
Magnetic Field Lines Around a Bar Magnet
Magnetic fields are the changes in energy inside a space. The magnetic field lines around a bar magnet have certain properties:
- The magnetic lines always form closed, continuous lines on both sides of the magnet.
- These lines do not intersect each other, and move from the north towards the south, seeking the path of least resistance, due to which they form loops.
- The lines spread out more as they move farther away from the magnet's poles, i.e., from an area of high permeability to an area of low permeability.
- The magnetic field lines move from the south pole to the north pole within magnetic material, but move from the north pole to the south pole while moving through air.
- The magnetic lines of force travel more easily through iron than through air.
It is the basic law of magnetism that oppositely charged poles of magnets are attracted to each other, while similarly charged poles repel each other. This can be seen through the changes in magnetic field lines occurring, when a magnet is placed close to another. However, it is important to note that if a bar magnet is cut in half, both pieces will form separate magnets, and will have their own north and south poles. The magnetic field surrounding a bar magnet can be seen through a very simple experiment using iron filings, which we shall now learn about.
Bar Magnet and Iron Filings Experiment
- Fix a sheet of paper on a plain horizontal surface using an adhesive to stop it from moving around.
- Place a bar magnet at the center of the paper.
- Sprinkle iron filings around the magnet uniformly.
- Gently tap the paper.
- Observe the pattern in which the filings arrange themselves, representing the magnetic lines of force.
- Use a compass to find out the direction of the magnetic lines.
- You can redo the experiment with more than one magnet in different positions to find changes in the magnetic field due to the interaction of one magnet with another.
It can be seen that the iron filings align themselves with the lines of magnetic force. The lines show where the magnetic field leaves the magnet at one pole and reenters at the opposite pole along the length of the magnet. You will also notice that, although there are many poles all along the length of the magnet, most of them are concentrated towards the ends. It is important to note that, although the image shows the magnetic lines of force only in two dimensions, they are actually three-dimensional.
How are Electromagnets Different from Bar Magnets?
Although bar magnets and electromagnets show similar magnetic fields, and in some cases may also look similar to each other, there are a few prominent differences:
- While most bar magnets are blocks of ferromagnetic materials, electromagnets are mostly made of coils of wire, which may or may not surround a ferromagnetic core. Electromagnets behave like bar magnets when an electric current is passed through them.
- While bar magnets generate their own magnetic field, electromagnets depend on an external electric source to generate a magnetic field.
- Bar magnets are permanent magnets and generate a constant magnetic pull. On the other hand, the magnetism of electromagnets can be turned on or off by toggling the electricity supply, which makes it ideal for creating simple motors.
- While the power of a bar magnet is constant and is determined through the material it is made of, the magnetic force of an electromagnet can change, and is determined through the amount of electricity flowing through the coil.
How is the Earth Like a Bar Magnet?
Our planet has a giant magnetic field, which extends thousands of miles into space, and protects us from harmful radioactive solar winds. The Earth's magnetic field is very similar to that of a bar magnet. It has two poles?north and south?and lines of electromagnetic forces can be found running along its length in long loopy arcs from pole to pole. This magnetic field is thought to occur due to the thermal convection of molten metal which is continuously circulating deep within the Earth's core, creating powerful electric currents.
The magnetic poles of the Earth have reversed positions regularly in the past, and currently, the magnetic north pole is close to the geographical south pole. This position is estimated to reverse again, sometime within the next 10,000 years.