Solenoid

FIELDS OF STUDY: Electromagnetism

ABSTRACT: A solenoid is an electromagnetic device made up of a coiled conducting wire. An applied electrical current produces a strong magnetic field in a solenoid, according to Ampère’s law. The fine electronic control of solenoids and electromagnets makes them suitable for a very broad range of applications.

PRINCIPAL TERMS

• Ampère’s law: the rule stating that the strength of a magnetic field about a current-carrying conductor is directly proportional to the magnitude of the current.
• electromagnet: a device that becomes magnetic due to the presence of an electric current.
• ferromagnetic: describing material that can be made permanently magnetic by the presence of a magnetic field, such as iron and other ferrous (ironlike) metals.
• inductance: the ability to generate a magnetic field when electrical current is flowing.
• permeability: the ability with which a magnetized material supports a magnetic field.
• right-hand rule: if a wire is grasped in the right hand with the thumb pointing in the direction of current flow, the fingers will point in the direction of the magnetic field around the wire.
• turns: the number of times that a conductor is wrapped around to form a helical coil.

Electricity and Magnetism

A solenoid combines the relationship between electricity and magnetism in a way that performs useful functions. In its simplest form, a solenoid consists just of a wire looped into the shape of a helical coil, like a spring. Electrical current flowing through the wire generates a magnetic field, which is what makes the solenoid so useful. Magnets made of a ferromagnetic material such as iron, nickel, or cobalt maintain their magnetism as permanent magnets. A solenoid, on the other hand, is an electromagnet. This means that it is magnetic due to an electric current and only as long as that current operates.

André-Marie Ampère (1775–1836) identified that an electrical current flowing in a conductor causes a magnetic field around the conductor. The right-hand rule can always be used to determine the direction of this magnetic field. According to Ampère’s law, the strength of the magnetic field is directly proportional to the magnitude of the current. This is echoed in Faraday’s law of magnetic induction, which states that a magnet moving past a conductor induces a voltage and current in the conductor and that the magnitude of these depends on changes in the magnetic field strength as it passes the conductor. The induced electrical current produces a magnetic field around the wire. Lenz’s law states that this induced magnetic field opposes the original magnetic field. When the conductor, usually a wire, is wrapped into a helical coil, the magnetic fields about the individual turns of wire add together to form a single large magnetic field. This is a solenoid.

As electrical current flows through the wire of a solenoid, it generates a magnetic field according to Ampère’s law. The magnetic field is produced at each point along the length of the coiled wire, just as it would be if the wire were straight. The helical shape causes the magnetic field around the adjacent coils of wire to align, forming a tube of concentrated magnetic flux lines within the coil. The flux lines surrounding the outside of the solenoid are also aligned in the opposite direction, but are widely dispersed. The magnetic field surrounding the outside of an active solenoid is thus much weaker than the internal magnetic field, to the extent that it is negligible. A solenoid’s internal magnetic field has much the same characteristics as a permanent bar magnet, but only when electric current is flowing through the wire. Often the wire is coiled around a metal core to increase the strength of the magnetic field produced.

Electromagnets and Inductors

By itself, the coil of a solenoid produces a magnetic field that creates an induced voltage opposite the direction of current flow in the circuit. This is the basic principle of an electronic component called an inductor. These are readily identifiable on any circuit board as they typically are constructed by wrapping a length of fine wire around a small cardboard tube. The purpose of an inductor is to provide a buffer against fluctuations in the voltage applied to the circuit so that the voltage remains constant. When an increase in voltage occurs, it increases the current flowing through the circuit, and this causes the inductor to produce a greater voltage in the opposite sense. Similarly, when applied voltage decreases, so does the counter voltage produced by the inductor. The inductance of an inductor depends on the number of turns of wire, the area of the inside of the coil, the length of the coil, and the permeability of the core material. Inductance is measured in henries (H), a standard unit named for Joseph Henry (1797–1878), according to the formula

L is the inductance in henries, ℓ is the length of the solenoid in meters, N is the number of turns of coiled wire, A is the cross-sectional area of the solenoid coil in square meters (A = πr²), and μ is the permeability of the core material.

Inductors and inductance coils typically are constructed around a material that is not magnetizable, such as air, plastic, or cardboard. When the solenoid coil is wrapped instead about a core material that is magnetizable, such as an iron rod, an electromagnet is formed. While the material within the core may be permanently magnetized, its magnetic field strength is greatly enhanced by that of the solenoid while current is flowing.

The magnetic flux in the core of an electromagnet depends greatly on the magnetic permeability of the core material. Electricity and magnetism are related at the subatomic level through unpaired electrons in the atoms of the material. Ferromagnetic metals acquire greatly enhanced magnetic properties when acted upon by a solenoid and conduct magnetism much as they would conduct electricity.

The flow of electricity through a conductor is subject to electrical resistance. If the material’s resistance is completely eliminated, the material becomes a superconductor. In that state, an induced electric current will continue to flow indefinitely after the magnetic induction field is removed. Electrons moving through a superconducting medium move in pairs like a laser beam rather than individually moving through numerous collisions down the length of the conductor. Since there is no energy lost to collisions, there is no resistance to the movement of the electrons and no heat loss due to friction. Superconducting magnets can be based on the geometry of solenoids.

Solenoids in Action

The simplicity of a solenoid lends itself to a broad array of applications. Solenoids allow the creation of basic electromagnets and inductors. These devices can be controlled electronically with very high precision and are found in many places. One example is the starter motor of an internal combustion engine in a car or truck, which uses a solenoid to take a small current from the ignition switch and relay a stronger current from the battery to start the engine. Scrap yards also often make use of large electromagnetic hoists to move ferrous metals about efficiently. The current is turned on to activate the solenoid’s magnetic field, allowing materials to be attached to the magnet. When they have been moved to the desired spot, the current is switched off and the materials are released.

Another common example of a solenoid is a simple electromagnetic lock or switch system. This can be produced using a solenoid and a rod that can be drawn into the core of the solenoid when current is flowing. Such a system is encountered whenever someone has to be remotely let in at an apartment building. A signal from a control panel typically closes a circuit that shunts electrical current into a solenoid. This causes it to generate a magnetic field and draw the lock pin into its open position. When the signal is released, the solenoid ceases to be active and allows a spring to pull the lock pin back into position. Basically all electromagnetic switch systems work this way. The security of such systems typically depends upon the manner in which the control circuit must be accessed, ranging from a simple push button to complex ID protocols.

Solenoids and their magnetic properties are also used in a number of very powerful analytical techniques. Among the most important is nuclear magnetic resonance (NMR) spectrometry. Chemists have used this method to analyze the structure of molecules since its development in the 1950s. The technique involves placing a homogeneous sample of a compound in solution into a strong magnetic field and detecting the energy levels that it absorbs when irradiated with a variable electromagnetic field. The energy and patterns of the absorption depend entirely on the three-dimensional structure of the molecule being examined, and provide very precise information about that structure.

The methodology of nuclear magnetic resonance was expanded with the development of superconducting magnets and magnetic resonance imaging (MRI) for medical diagnosis. The images are obtained by immersing the patient inside a strong magnetic field and plotting the patterns of energy absorption through irradiation by a variable electromagnetic field, just as in NMR spectrometry. The technique has been used routinely to examine living persons, ancient Egyptian mummies, and large zoo animals. Control of the stability and uniformity of the magnetic field is critical to these applications. Superconducting magnets made possible by solenoids help achieve this ability.

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