An electromagnet is a type of magnet in which the magnetic field is produced by the flow of an electric current.
Electromagnets are magnets that are only magnetic when there is a coil of wire with electricity running through it. This is called a solenoid. The strength of the magnet is proportional to the current flowing in the circuit. Electromagnets are used for a variety of purposes. In a simple example, an electromagnet can pick up pieces of metal, iron, steel, nickel, and cobalt. The electricity running through the wire is called a current. The current is a flow of electrons, negatively charged particles.
Electromagnets can be made stronger by adding more coils to the copper wire, or adding an iron core through the coils (for example a nail). You can also increase the current to make the magnetism stronger.
British electrician William Sturgeon invented the electromagnet in 1825.
An electromagnet is very beneficial because it can be deactivated (turned off) easily, whereas a permanent magnet cannot be deactivated and will continue to affect its immediate environment. Iron stops being an electromagnet very quickly, but steel takes time to wear off. To make an electromagnet, you can wind copper wire around a steel rod. Then connect the two ends of the wire to the + (positive) and - (negative) side of the battery.
Electromagnets are used in everyday items such as burglar alarms, electric relays and fire bells. Their ability to change from the state of non-magnetic to magnetic just by passing an electric current through it allows it to be used in many different items.
Topics of Interest
An electromagnet is a type of magnet whose magnetic field is produced by the flow of electric current. The magnetic field disappears when the current ceases.
Invention and history
British electrician William Sturgeon invented the electromagnet in 1825. The first electromagnet was a horseshoe-shaped piece of iron that was wrapped with a loosely wound coil of several turns. When a current was passed through the coil, the electromagnet became magnetized and when the current was stopped, the coil was de-magnetized. Sturgeon displayed its power by lifting nine pounds with a seven-ounce piece of iron wrapped with wires through which the current of a single cell battery was sent.
Sturgeon could regulate his electromagnet; this was the beginning of using electrical energy for making useful and controllable machines and laid the foundations for large-scale electronic communications.
The simplest type of electromagnet is a coiled piece of wire. A coil forming the shape of a straight tube (similar to a corkscrew) is called a solenoid; a solenoid that is bent so that the ends meet is a toroid. Much stronger magnetic fields can be produced if a "core" of paramagnetic or ferromagnetic material (commonly soft iron) is placed inside the coil. The core concentrates the magnetic field that can then be much stronger than that of the coil itself.
Magnetic fields caused by coils of wire follow a form of the right-hand rule (for conventional current or left hand rule for electron current). If the fingers of the left hand are curled in the direction of electron current flow through the coil, the thumb points in the direction of the field inside the coil. The side of the magnet that the field lines emerge from is defined to be the north pole.
The material of the core of the magnet (usually iron) is composed of small regions called magnetic domains that act like tiny magnets. Before the current in the electromagnet is turned on, the domains in the iron core point in random directions, so their tiny magnetic fields cancel each other out, and the iron has no large scale magnetic field. When a current is passed through the wire wrapped around the iron, its magnetic field penetrates the iron, and causes the domains to turn, aligning parallel to the magnetic field, so their tiny magnetic fields add to the wire's field, creating a large magnetic field that extends into the space around the magnet. The larger the current passed through the wire coil, the more the domains align, and the stronger the magnetic field is. Finally all the domains are lined up, and further increases in current only cause slight increases in the magnetic field: this phenomenon is called saturation.
When the current in the coil is turned off, most of the domains lose alignment and return to a random state and the field disappears. However some of the alignment persists, because the domains have difficulty turning their direction of magnetization, leaving the core a weak permanent magnet. This phenomenon is called hysteresis and the remaining magnetic field is called remanent magnetism. The residual magnetization of the core can be removed by degaussing.
Electromagnets and permanent magnets
The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be rapidly manipulated over a wide range by controlling the amount of electric current. However, a continuous supply of electrical energy is required to maintain the field.
As a current is passed through the coil, small magnetic regions within the material, called magnetic domains, align with the applied field, causing the magnetic field strength to increase. As the current is increased, all of the domains eventually become aligned, a condition called saturation. Once the core becomes saturated, a further increase in current will only cause a relatively minor increase in the magnetic field. In some materials, some of the domains may realign themselves. In this case, part of the original magnetic field will persist even after power is removed, causing the core to behave as a permanent magnet. This phenomenon, called remanent magnetism, is due to the hysteresis of the material. Applying a decreasing AC current to the coil, removing the core and hitting it, or heating it above its Curie point will reorient the domains, causing the residual field to weaken or disappear.
In applications where a variable magnetic field is not required, permanent magnets are generally superior. Additionally, permanent magnets can be manufactured to produce stronger fields than electromagnets of similar size.
When a magnetic field higher than the ferromagnetic limit of 1.6 T is needed, superconducting electromagnets can be used. Instead of using ferromagnetic materials, these use superconducting windings cooled with liquid helium, which conduct current without electrical resistance. These allow enormous currents to flow, which generate intense magnetic fields. Superconducting magnets are limited by the field strength at which the winding material ceases to be superconducting. Current designs are limited to 10–20 T, with the current (2009) record of 33.8 T. The necessary refrigeration equipment and cryostat make them much more expensive than ordinary electromagnets. However, in high power applications this can be offset by lower operating costs, since after startup no power is required for the windings, since no energy is lost to ohmic heating. They are used in particle accelerators, MRI machines, and research.
Uses of electromagnets
Electromagnets are widely used in many electric devices, including:
- Motors and generators
- Relays, including reed relays originally used in telephone exchanges
- Electric bells
- Magnetic recording and data storage equipment: tape recorders, VCRs, hard disks
- Particle accelerators
- Magnetic locks
- Magnetic separation of materials
- Industrial lifting magnets
- Electromagnetic suspension used for MAGLEV trains
Did you know that...
Electromagnet attracts paper clips when current is applied creating a magnetic field, loses them when current and magnetic field are removed.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)