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Magnetism

 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. By wrapping insulated wire around a piece of iron and then running electrical current through the wire, the iron becomes magnetized. This happens because a magnetic field is created around a wire when it has electrical current running through it. Creating a coil of wire concentrates the field. Wrapping the wire around an iron core greatly increases the strength of the magnetic field. Questions you may have include: How can you make an electromagnet? What factors are involved in electromagnetism? How does an iron core affect the strength? 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. Creating a simple electromagnet using a nail Insulated wire 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. Turn on and off The most interesting feature of the electromagnet is that when the electrical current is turned off, the magnetism is also turned off. This is especially true if the core is made of soft iron, which quickly loses its magnetism. Hardened steel may retain its magnetism, so you can't use the most valuable feature of an electromagnet. Being able to turn the magnetism on and off has lead to many amazing inventions and applications. How electromagnetism works When electricity passed through a wire, a magnetic field is created around the wire. Looping the wire increases the magnetic field. Adding an iron core greatly increases the effect and creates an electromagnet. You can create an electromagnet without the iron core. That is usually called a solenoid. Magnetic field When DC electricity is passed through a wire, a magnetic field rotates around the wire in a specific direction. Magnetic field rotating around wire Compass can show field Connecting a wire to a battery and placing a compass near the wire can demonstrate a magnetic field. When the current is turned on, the compass-needle will move. If you reverse the direction of the current, the needle will move in the opposite direction. Right hand rule To find the direction the magnetic field is going, you can use the "right-hand rule" to determine it. If you take your right hand and wrap it around the wire, with your thumb pointing in the direction of the electrical current (positive to negative), then your fingers are pointing in the direction of the magnetic field around the wire. Try it with the picture above. 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. 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. Using AC electricity If AC electricity is used, the electromagnet has the same properties of a magnet, except that the polarity reverses with the AC cycle. Note that it is not a good idea to try to make an AC electromagnet. This is because of the high voltage in house current. Using a wire around a nail would result in a blown fuse in the AC circuit box. There is also the potential of an electric shock. 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 104 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. Effect of iron core When the coil is wrapped around an iron core, the strength of the electromagnetic field is much greater than the same coil without the iron core. This is because the atoms in the iron line up to amplify the magnetic effect. The orientation of the atoms in the iron is called its domain. Current When you increase the current, the magnetic strength increases, but it is not exactly linear as it is with the coil by itself. The characteristics of the core cause the curve of magnetic strength versus current to be an s-shaped hysteresis curve. The shape of this curve depends on how well the material in the core becomes magnetized and how long it remains magnetized. Soft iron loses its magnetism readily, while hard steel tends to retain its magnetism. In conclusion By wrapping a wire around an iron core and applying an electric current through the wire, you create an electromagnet. This device is magnetic only when the current is flowing. The iron core greatly increases the magnetic strength.