Electromagnet

An electromagnet is a magnet in which the magnetic field is produced due to the flow of electric current. The magnetic field disappears when the current is turned off.

electromagnet

An electric current flowing in a wire creates a magnetic field around the wire. To concentrate the magnetic field, in an electromagnet the wire is wound into a coil with many turns of wire lying side by side. The magnetic field of all the turns of wire passes through the center of the coil, creating a strong magnetic field there. A coil forming the shape of a straight tube or helix is called a solenoid. Much stronger magnetic fields can be produced if a ferromagnetic material, such as soft iron, is placed inside the coil. The ferromagnetic core increases the magnetic field to thousands of times the strength of the field of the coil alone, due to the high magnetic permeability ( μ ) of the ferromagnetic material. This is called as ferromagnetic-core or iron-core electromagnet. The direction of the magnetic field through a coil of wire can be found from a form of the right-hand rule. If the fingers of the right hand are curled around the coil in the direction of current flow through the windings, 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 main advantage of an electromagnet over a  permanent magnet is that the magnetic field can be rapidly varied over a wide range by controlling the amount of electric current. But, a continuous supply of electrical energy is required to maintain the field.

Side effects in electromagnets

• Ohmic heating: The power consumed in a DC electromagnets is due to the resistance of winding, and is dissipated as heat. This heating is called ohmic heating and in large electromagnets require cooling water circulating through pipes in the windings to carry off the waste heat. Since power dissipation, P = I²R, increases with the square of the current but only increases approximately linearly with the number of windings, so the power lost in the windings can be minimized by reducing I and increasing the number of turns N . For example halving I and doubling N, halves the power loss. This is the one reason most electromagnets have windings with many turns of wire.

•  Lorentz forces: In powerful electromagnets, the magnetic field exerts a force on each turn of the windings, due to the Lorentz force (qv *B) acting on the moving charges within the wire. The Lorentz force is perpendicular to both the axis of the wire and the magnetic field. It has two effects on an electromagnet's windings:

• The field lines within the axis of the coil exert a radial force on each turn of the windings, tending to push them outward in all directions. This causes a tensile stress in the wire.
• The leakage field lines between each turn of the coil exert a repulsive force between adjacent turns, tending to push them apart.

• Core losses: In alternating current (AC) electromagnets, used in transformers, inductors, AC motors and generators, the magnetic field is constantly changing. This causes energy losses in their magnetic cores that are dissipated as heat in the core. The losses occurs due two processes:  

• Eddy currents:  From Faraday's law of induction, the changing magnetic field induces circulating electric currents inside nearby conductors, called eddy currents. The energy in these currents is dissipated as heat in the electrical resistance of the conductor, so they are a cause of energy loss. Eddy currents are closed loops of current that flow in planes perpendicular to the magnetic field. The energy dissipated is proportional to the area enclosed by the loop. To prevent them, the cores of AC electromagnets are made of stacks of thin steel sheets, or lamination, oriented parallel to the magnetic field, with an insulating coating on the surface.
• Hysteresis losses: Reversing the direction of magnetization of magnetic domains in the core material each cycle causes energy loss, because of the coercivity of the material. These loses are called as Hysterisis. To minimize this loss, magnetic cores used in transformers and other AC electromagnets are made up of "soft" or "low coercivity" material, such as silicon steel or soft ferrite.

 Uses of electromagnets

Electromagnets are widely used in electric and electromechanical devices, including:

• Motors and generators,
• Transformers,
• Electric bells and buzzers,
• Loudspeakers and earphones,
• Actuators,
• Magnetic recording and data storage equipment: tape recorders, VCRs, hard disks,
• Scientific instruments such as MRI machines and mass spectrometers,
• Particle accelerators,
• Magnetic locks,
• Magnetic separation equipment, used for separating magnetic from nonmagnetic material,
• Industrial lifting magnets,
• Electromagnetic suspension used for MAGLEV trains.