Magnetism Magnetism is a force of attraction or repulsion that acts at a distance. It is due to a magnetic field, which is caused by moving electrically charged particles or is inherent in magnetic objects such as a magnet. A magnet is an object that exhibits a strong magnetic field and will attract materials like iron to it. Magnets have two poles, called the north (N) and south (S) poles. Two magnets will be attracted by their opposite poles, and each will repel the like pole of the other magnet. Magnetism has many uses in modern life. Magnetic field A magnetic field consists of imaginary lines of flux coming from moving or spinning electrically charged particles. Examples include the spin of a proton and the motion of electrons through a wire in an electric circuit. Names of poles Magnetic field or lines of flux of a moving charged particle The lines of magnetic flux flow from one end of the object to the other. By convention, we call one end of a magnetic object the N or North-seeking pole and the other the S or South-seeking pole, as related to the Earth's North and South magnetic poles. The magnetic flux is defined as moving from N to S. Magnetic forces: The magnetic field of an object can create a magnetic force on other objects with magnetic fields. That force is what we call magnetism. - 1 -
Attraction When two magnets or magnetic objects are close to each other, there is a force that attracts the poles together. Force attracts N to S Magnets also strongly attract ferromagnetic materials such as iron, nickel and cobalt. Repulsion When two magnetic objects have like poles facing each other, the magnetic force pushes them apart. Force pushes magnetic objects apart Magnets can also weakly repel diamagnetic materials. Magnets A magnet is an object or material that attracts certain metals, such as iron, nickel and cobalt. It can also attract or repel another magnet. All magnets have North-seeking (N) and South-seeking (S) poles. When magnets are placed near each other, opposite poles attract and like poles repel each other. Various electrical devices make use of magnets. Types of magnets There are permanent magnets, temporary magnets and electromagnets. - 2 -
Permanent magnets A permanent magnet is one that will hold its magnetic properties over a long period of time. Magnetite Magnetite is a magnetic material found in nature. It is a permanent magnet, but it is relatively weak. Alloys Most permanent magnets we use are manufactured and are a combination or alloy of iron, nickel and cobalt. Rare-earth permanent magnets are a special type of magnet that can have extreme strength. Temporary magnets A temporary magnet is one that will lose its magnetism. For example, soft iron can be made into a temporary magnet, but it will lose its magnetic power in a short while. Electromagnet By wrapping a wire around an iron or steel core and running an electrical current through the wire, you can magnetize the metal and make an electromagnet. If the core is soft iron, the magnetism will diminish as soon as the current is turned off. This feature makes electromagnets good for picking up and dropping objects. Typically DC electricity is used, but AC current will also result in an electromagnet. Various shapes The magnet can be made into various shapes. The bar magnet is the most common configuration. Bar magnet Magnets also can be square, spherical, shaped like a horseshoe, and even shaped like a donut. Horseshoe magnet - 3 -
If you put an iron plate across the N and S poles of a horseshoe magnet that would essentially "short circuit" the effect of the magnetism, such that its strength would not be very great. As soon as the plate was removed, the magnet would regain its full strength. That method is sometimes used in magnets that are temporary to help keep their magnetic properties for a longer time. Electromagnetism An electromagnet is an object that acts like a magnet, but its magnetic force is created and controlled by electricity thus the name electromagnet. Making an electromagnet If you wrap a wire around an iron core, such as a nail, and you send electrical current through the wire, the nail will become highly magnetized. You can verify that by picking up small objects or by showing its effect on a compass. This is called an electromagnet. Insulated wire Creating a simple electromagnet using a nail Note that the wire must be an insulated wire. A bare wire would cause an electrical short and the current would then run through the nail or metal core. In some electromagnets, like in an electric motor, the wire will look like bare copper, but it is insulated with a thin coating of a clear material. Also, if the wire is thin, it may get warm from the resistance to the electricity passing through it. Wire in a coil Wrapping the wire in a coil concentrates and increases the magnetic field, because the additive effect of each turn of the wire. - 4 -
Coiled wire increases magnetic field A coil of wire used to create a magnetic field is called a solenoid. Iron core Wrapping the wire around an iron core greatly increases the magnetic field. If you put a nail in the coil in the drawing above, it would result in an electromagnet with the a north seeking pole on the "N" side. Strength of electromagnetic field The strength of the electromagnetic field is determined by the amount of current, number of coils of wire, and the distance from the wire. Unit The unit of magnetic force is called the tesla (T). Near a strong magnet the force is 1-T. Another unit used is the gauss, where 10 4 gauss (10,000) equals 1 tesla. Current The strength of the magnetic field is proportional to the current in the wire. If you double the current, the magnetic force is doubled. Since Voltage = Current x Resistance (V = I*R), you can double the current in a wire by doubling the voltage of the source of electricity. Turns of coil If you wrap the wire into a coil, you increase the magnetic force inside the coil, proportional to the number of turns. In other words, a coil consisting of 10 loops has 10 times the magnetic force as a single wire with the same current flowing through it. Likewise, a coil of 20 loops has 2 times the magnetic force than one with 10 loops. Varies with distance The magnetic force decreases with distance. It varies inversely proportional to the square of the distance. For example the force at 2 cm. from a wire is 1/4 that of at 1 cm., and the force at 3 cm. is 1/9 the force at 1 cm. - 5 -
Generating Electrical Current Electrical current can be generated by moving a metal wire through a magnetic field. Electrical generators rotate a coil of wires through a magnetic field. An electric motor is very similar to a generator, except that power is provided to turn the rotors. Generator in action The following animation shows an AC generator in action. As one side of the loop moves to the other pole of the magnetic field, the current in it changes direction. The two slip rings of the AC generator allow the current to change directions and become alternating current. Can be used as motor Simple generator Note that when a generator has its wire wound around an iron core, it can also be used as an electric motor. Instead of rotating the loops through a magnetic field to create electricity, a current is sent through the wires, creating electromagnets. The outer magnets will then repel the electromagnets and rotate the shaft as an electric motor. Electromagnetic Devices Electromagnets are used in a number of everyday devices. One useful characteristic of an electromagnet is the fact that you can vary its magnetic force by changing the amount and direction of the current going through the coils or windings around it. Loudspeakers and tape recorders are devices that apply this effect. Some electromagnets can be very strong and its power can be readily turned off and on. Junk yard electromagnets, common doorbells and electromagnetic locks are examples. Electromagnets can also be used to create continual motion when opposed by other electromagnets or permanent magnets. Examples include electric motors and maglev trains. - 6 -
If you examine the back area of a loudspeaker, you should be able to see the permanent magnet and coil of wire for the electromagnet. Some loudspeakers use an electromagnet without the iron core, which is called a solenoid. Electric Motor Electric motors are everywhere! In your house, almost every mechanical movement that you see around you is caused by an AC (alternating current) or DC (direct current) electric motor. Let's start by looking at the overall plan of a simple two-pole DC electric motor. A simple motor has six parts, as shown in the diagram below: Armature or rotor Commutator Brushes Axle Field magnet DC power supply of some sort An electric motor is all about magnets and magnetism: A motor uses magnets to create motion. If you have ever played with magnets you know about the fundamental law of all magnets: Opposites attract and likes repel. So if you have two bar magnets with their ends marked "north" and "south," then the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other (and similarly, south will repel south). Inside an electric motor, these attracting and repelling forces create rotational motion. - 7 -
Parts of an electric motor. The motor being dissected here is a simple electric motor that you would typically find in a toy: The axle holds the armature and the commutator. The armature is a set of electromagnets, in this case three. The armature in this motor is a set of thin metal plates stacked together, with thin copper wire coiled around each of the three poles of the armature. The two ends of each wire (one wire for each pole) are soldered onto a terminal, and then each of the three terminals is wired to one plate of the commutator. The figures below make it easy to see the armature, terminals and commutator: - 8 -
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