For other uses, see Solenoid (disambiguation).
Magnetic field created by a solenoid

A solenoid is a 3-dimensional coil. A coil is a series of loops A coiled coil is a structure where the coil itself is in turn also looping In physics, the term solenoid refers to a loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electrical current is passed through it. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets. An electromagnet is a type of Magnet in which the Magnetic field is produced by the flow of an electric current. The term solenoid refers specifically to a magnet designed to produce a uniform magnetic field in a volume of space (where some experiment might be carried out).

In engineering, the term solenoid may also refer to a variety of transducer devices that convert energy into linear motion. Engineering is the Discipline and Profession of applying technical and scientific Knowledge and A transducer is a device usually electrical, electronic, Electro-mechanical, Electromagnetic, Photonic, or Photovoltaic In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός The term is also often used to refer to a solenoid valve, which is an integrated device containing an electromechanical solenoid which actuates either a pneumatic or hydraulic valve, or a solenoid switch, which is a specific type of relay that internally uses an electromechanical solenoid to operate an electrical switch; for example, an automobile starter solenoid, or a linear solenoid, which is an electromechanical solenoid. A solenoid valve is an Electromechanical Valve for use with Liquid or Gas controlled by running or stopping an Electric current Pneumatics, Pressurized gas to affect mechanical motion Pneumatic power is used in Industry, where it is common to have factory units plumbed for Compressed For the mechanical technology see Hydraulic machinery and Hydraulic cylinder Hydraulics is a topic of science and Engineering A relay is an electrical Switch that opens and closes under the control of another Electrical circuit. A starter solenoid (or starter relay is the part of an Automobile which relays a large Electric current to the starter motor, which in turn sets the engine

## Magnetic field

This is a derivation of the magnetic field around a solenoid that is long enough so that fringe effects can be ignored. In the diagram to the right, we immediately know that the field points in the positive z direction inside the solenoid, and in the negative z direction outside the solenoid.

A solenoid with 3 Ampèrian loops

We see this by applying the right hand grip rule for the field around a wire. For the related yet different principle relating to electromagnetic coils see Right-hand rule. If we wrap our right hand around a wire with the thumb pointing in the direction of the current, the fingers show how the field behaves. Since we are dealing with a long solenoid, all of the components of the magnetic field not pointing upwards cancel out by symmetry. Outside, a similar cancellation occurs, and the field is only pointing downwards.

Now consider loop "c". By Ampère's law, we know that the path integral of B around this loop is zero, since no current passes through it (and where it can be assumed that the circuital electric field passing through the loop is constant under such conditions such as a constant, or constantly changing current through the solenoid). We have shown above that the field is pointing upwards inside the solenoid, so the horizontal portions of loop "c" doesn't contribute anything to the integral. Thus the integral up side 1 is equal to the integral down side 2. Since we can arbitrarily change the dimensions of the loop and get the same result, the only physical explanation is that the integrands are actually equal, that is, the magnetic field inside the solenoid is constant. A similar argument can be applied to loop "a" to conclude that the field outside the solenoid is constant.

A solenoid with a looping magnetic field line

An intuitive argument can be used to show that the field outside the solenoid is actually zero. Magnetic field lines only exist as loops, they cannot diverge from or converge to a point like electric field lines can. The magnetic field lines go up the inside of the solenoid, so they must go down the outside so that they can form a loop. However, the volume outside the solenoid is much greater than the volume inside, so the density of magnetic field lines outside is greatly reduced. Recall also that the field outside is constant. In order for the total number of field lines to be conserved, the field outside must go to zero as the solenoid gets longer.

Now we can consider loop "b". Take the path integral of B around the loop, with the height of the loop set to h. The horizontal components vanish, and the field outside is zero, so Ampère's Law gives us:

Bh = μ0NI

From which we get:

$B = \mu_0 \frac{N I}{h}$

This equation is for a solenoid with no core. The inclusion of a usually metal core, such as iron, increases the magnitude of the magnetic field of the solenoid when it is unchanged (same current, length, number of coils). Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 This expressed by the formula

$b = \kappa \mu_0 \frac{N I}{h}$

κ is the permeability constant of the material that the core is made of. κμ0 is the relative permeability (μ) of the core material such that:

$B = \mu \frac{N I}{h}$

## Rotary Voice Coil

This is a rotational version of a solenoid. In Multiphase flow in Porous media, relative permeability is a dimensionless measure of the effective permeability of each phase Typically the fixed magnet is on the outside, and the coil part moves in an arc controlled by the current flow through the coils. Rotary voice coils are widely employed in devices such as disk drives. Disk storage is a general category of a Computer storage mechanisms in which data is recorded on planar round and rotating surfaces ( disks, discs, or

## Electromechanical solenoids

A 1920 explanation of a commercial solenoid used as an electromechanical actuator

Electromechanical solenoids consist of an electromagnetically inductive coil, wound around a movable steel or iron slug (termed the armature). Steel is an Alloy consisting mostly of Iron, with a Carbon content between 0 Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 The coil is shaped such that the armature can be moved in and out of the center, altering the coil's inductance and thereby becoming an electromagnet. In Electrical circuits, any Electric current i produces a Magnetic field and hence generates a total Magnetic flux \Phi acting The armature is used to provide a mechanical force to some mechanism (such as controlling a pneumatic valve). Although typically weak over anything but very short distances, solenoids may be controlled directly by a controller circuit, and thus have very low reaction times.

The force applied to the armature is proportional to the change in inductance of the coil with respect to the change in position of the armature, and the current flowing through the coil. The force applied to the armature will always move the armature in a direction that increases the coil's inductance.

The magnetic field inside a solenoid is given by:

$B=\mu_0 n I=\mu_0 \frac{NI}{h}$

where $\mu_0=4\pi \times 10^{-7}$ henries per meter, B is the magnetic field magnitude in teslas, n is the number of turns per meter, I is the current in amperes, N is the number of turns and h is the length of the solenoid in meters. The ampere, in practice often shortened to amp, (symbol A is a unit of Electric current, or amount of Electric charge per second See also: Electromagnet. An electromagnet is a type of Magnet in which the Magnetic field is produced by the flow of an electric current.

Electromechanical solenoids are commonly seen in electronic paintball markers, pinball machines, dot matrix printers and fuel injectors. PaintBallMarkerjpg|right|thumb|250px|Spyder VS2 Paintball Marker Pinball is a type of coin-operated Arcade game where a player attempts to score points by manipulating one or more Metal balls on a playfield inside a Glass A dot matrix printer or impact matrix printer refers to a type of Computer printer with a print head that runs back and forth on the page and prints by impact striking Fuel injection is a system for mixing fuel with air in an Internal combustion engine.

## Pneumatic solenoid valves

A pneumatic solenoid valve is a switch for routing air to any pneumatic device, usually an actuator of some kind. A solenoid valve is an Electromechanical Valve for use with Liquid or Gas controlled by running or stopping an Electric current An actuator is a mechanical device for moving or controlling a mechanism or system A solenoid consists of a balanced or easily movable core, which channels the gas to the appropriate port, coupled to a small linear solenoid. The valve allows a small current applied to the solenoid to switch a large amount of high pressure gas, typically up to 100 psi (7 bar, 0. 7 MPa, 0. 7 MN/m²). Some solenoids are capable of operating at far greater pressures. Pneumatic solenoids may have one, two, or three output ports, and the requisite number of vents. The valves are commonly used to control a piston or other linear actuator.

The pneumatic solenoid is akin to a transistor, allowing a relatively small signal to control a large device. In Electronics, a transistor is a Semiconductor device commonly used to amplify or switch electronic signals It is also the interface between electronic controllers and pneumatic systems.

## Hydraulic solenoid valves

Hydraulic solenoid valves are in general similar to pneumatic solenoid valves except that they control the flow of hydraulic fluid (oil), often at around 3000 psi (210 bar, 21 MPa, 21 MN/m²). A solenoid valve is an Electromechanical Valve for use with Liquid or Gas controlled by running or stopping an Electric current Hydraulic machinery uses solenoids to control the flow of oil to rams or actuators to (for instance) bend sheets of titanium in aerospace manufacturing. Hydraulic machinery are machines and tools which use Fluid power to do work Solenoid-controlled valves are often used in irrigation systems, where a relatively weak solenoid opens and closes a small pilot valve, which in turn activates the main valve by applying fluid pressure to a piston or diaphragm that is is mechanically coupled to the main valve.

Transmission solenoids control fluid flow through an automatic transmission and are typically installed in the transmission valve body. A transmission solenoid is an electro-hydraulic valve that controls fluid flow into and throughout an Automatic transmission.

## Automobile starter solenoid

In a car or truck, the starter solenoid is part of an automobile ignition system. A starter solenoid (or starter relay is the part of an Automobile which relays a large Electric current to the starter motor, which in turn sets the engine Also called a starter relay, the starter solenoid receives a large electric current from the car battery and a small electrical current from the ignition switch. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. A car battery is a type of Rechargeable battery that supplies electric energy to an Automobile. A key is a device which is used to open a lock. A typical key consist of two parts the blade, which slides into the Keyway of the lock and distinguishes When the ignition switch is turned on (when the key is turned to start the car), the small electrical current forces the starter solenoid to close a pair of heavy contacts, thus relaying the large electrical current to the starter motor. An automobile self-starter (commonly "starter motor" or simply "starter" is an Electric motor that initiates rotational motion in a car's Internal

Starter solenoids can also be built into the starter itself, often visible on the outside of the starter. If a starter solenoid receives insufficient power from the battery, it will fail to start the motor, and may produce a rapid 'clicking' or 'clacking' sound. The internal combustion engine is an engine in which the Combustion of Fuel and an Oxidizer (typically air occurs in a confined space called a This can be caused by a low or dead battery, by corroded or loose connections in the cable, or by a broken or damaged positive (red) cable from the battery. Corrosion means the breaking down of essential properties in a material due to Chemical reactions with its surroundings Any of these will result in some power to the solenoid, but not enough hold the heavy contacts closed, so the starter motor itself never spins, and the engine is not rotated (does not start).
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