Magnetic Circuit
A magnetic circuit is a closed loop or collection of loops designed to confine and direct a magnetic field, similar to how an electrical circuit channels electric current. Central to many electronic devices, magnetic circuits facilitate the operation of electromagnets, switches, and various forms of machinery, including telephones and loudspeakers. They play a critical role in scientific instruments like electron microscopes and spectrometers. Magnets, whether natural or man-made, exhibit properties of attraction and repulsion, with electromagnets providing adjustable strengths for diverse applications. The effectiveness of a magnetic circuit is influenced by factors such as reluctance, permeability, and magnetomotive force (MMF), which collectively determine the flow of magnetic flux. Various circuit designs exist, including series and composite configurations, and they may incorporate air gaps to regulate magnetic flow. Understanding these principles is essential for the design and function of many modern devices, from electric motors to transformers, underscoring the significance of magnetic circuits in technology and industry.
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Magnetic Circuit
A magnetic circuit is a closed loop or group of loops that confine a magnetic field. Similar to the way an electric circuit contains electrical energy, a magnetic circuit contains magnetic flux. There are a number of other similarities between electrical circuits and magnetic circuits, including the fact that both are used in electronic devices. Magnetic circuits are important to the function of electromagnets and in many types of switches. They are key components in everyday items such as telephones and in devices that help generate or enhance sound, such as recording machines and loudspeakers. They also help to further research and science, in equipment such as electron microscopes and spectrometers. Magnetic circuits are also found in many electric motors.
Background
Magnets are natural or man-made objects that exert a force that draws certain metal objects toward them. The name comes from the Greek word magnes, which was the name of a city in ancient Greece. The word was applied to a rock known as lodestone, which the Greeks observed had the properties of attracting some rocks and repelling others.
Each magnet has two poles, commonly referred to as north and south. Each pole will attract certain elements. These include iron and substances made with iron, such as steel, as well as cobalt and nickel. Magnets will also attract each other when opposite poles are placed together and repel each other when two of the same poles—north/north or south/south—are put together.
Some magnets are permanent magnets and always have the same strength to their magnetic field. Natural magnets are generally permanent magnets, as are the bar magnets and horseshoe-shaped magnets often used in classroom experiments. For many applications, it is more desirable to have a magnet that can have different strengths. In these cases, electromagnets are used. An electromagnet is made by wrapping a material that can conduct electricity around a core of a magnetic material. The core is then electrified; the electrical charge increases or decreases the magnetic field, depending on how much electrical current is applied.
This results in a magnet that can change strengths almost instantaneously, which can be very useful in electronic devices. For instance, when an electronic alarm bell rings, it is often the result of the action of an electromagnet. Pulses of electrical current are sent through the system and an electromagnet reacts to the pulses by attracting a flexible bar to which a striker is attached. As the current pulses on and off, the bar is attracted and released, causing the striker to hit the bell over and over.
In addition to having the advantage of adjustable magnetism, the magnetic field of an electromagnet can also be reversed by means of adjusting the electrical current. This makes them very useful in many types of magnetic circuits. Some magnetic circuits also include air gaps, or spaces between the metal parts of the magnet, that help to regulate the function of the circuit.
Overview
A magnetic circuit is one that contains magnetic flux. Magnetic flux is a term used to refer to the number of lines of a magnetic field that can be found within the specific area in question. In the case of a magnetic circuit, this would be the loop or loops that make up the circuit. This is similar to the way electrical current is measured, by how much of the current is traveling through a particular device or wire.
There are several different types of magnetic circuits. These include series and series-parallel circuits. These circuits provide multiple paths for the flux to follow either through a series of circuits or through a circuit that has multiple air gaps or other features that line up in parallel to each other. These air gaps or other additions to the circuit help to regulate the flow of the magnetic flux. Circuits can also be homogenous in design, meaning that the material that the flux passes through is all the same, or composite, meaning there are two or more materials involved, such as a metal form interrupted by an air gap, that the flux goes through in the circuit.
The strength of the flux is called the magnetomotive force, or MMF. This is calculated by multiplying the amount of current applied to the magnet by the number of times, or turns, the current-carrying wire is wrapped around the electromagnet. The result in the amount of flux, which is reported in weber units, or, in many situations, in milliwebers or microwebers, depends on the size of the magnetic circuit.
There are several laws that govern the function of magnetic circuits. These are related to factors such as the reluctance of a circuit to produce flux (similar to resistance in electrical circuits) and permeability, or the willingness of a circuit to allow the flux to move through. Ohm's law, which is similar to the law that applies to electrical circuits, says that reluctance can be determined by dividing the MMF by the flux. It is named after German physicist Georg Ohm, who discovered the corresponding law for electrical circuits. It is sometimes referred to as Hopkinson's law, after British electrical engineer John Hopkinson, who originally applied it to magnetic circuits.
Several other laws pertaining to electrical circuits were also adapted to use for magnetic circuits, including two by another German physicist, Gustav Kirchhoff. These laws, which originally applied to voltage, have been used to help determine the magnetomotive force and the amount of flux in a magnetic circuit. Other important measurements used in magnetic circuitry include flux density—the concentration of the magnetic field at any point in the circuit—and hysteresis—the amount of time between a change in magnetic field strength and the flux density.
Designs using magnetic circuits also have to allow for leakage of the magnetic field. This is not as much of a problem with an electrical circuit, since the current can be more readily contained through the use of materials that conduct electricity and insulate against it. Magnetic fields, on the other hand, cannot be contained in the same way and can sometimes be weakened by leakage.
Magnetic circuits are used in a wide variety of objects. Many transformers, electronic relays, and contactors include magnetic circuits. Generators and motors of many sizes also need magnetic circuits to function. They are important to the function of speakers used in sound systems and with musical instruments, and are also incorporated into computers.
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