How Does a Magnetic Circuit Work in Physics?
One or more closed-loop pathways carrying a magnetic flux create a magnetic circuit.
Permanent magnets or electromagnets produce the flux, which is limited to the route by magnetic cores made of ferromagnetic materials like iron, though there may be air holes or other materials in the path. Magnetic circuits are used in a variety of applications to effectively channel magnetic fields like electric motors, generators, galvanometers, transformers, relays, lifting electromagnets, SQUIDs, and magnetic recording heads.
The equations of the magnetic field of an unsaturated ferromagnetic substance and the equations of an electrical circuit have a one-to-one correspondence of a "magnetic circuit."
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The magnetic fields of involved devices like transformers can be easily overcome using the methods for electrical circuits using this principle.
Some Examples of Magnetic Circuits are:
horseshoe magnet with iron keeper (low-reluctance circuit).
horseshoe magnet with no keeper (high-reluctance circuit).
electric motor (variable-reluctance circuit).
some types of pickup cartridge (variable-reluctance circuits).
Magnetic Flux
The number of magnetic field lines that travel through the cross-sectional region of a magnetic component determines the magnetic flux through that component.
This is the net sum, which equals the number of people who pass through in one direction minus the number of people who pass through in the opposite direction. The product of the magnetic field and the area element determines the flux by an element of area perpendicular to the magnetic field direction. A scalar product of the magnetic field and the area element vector defines magnetic flux in general.
It has a magnetic nucleus, for starters. The heart can be made of a single material, such as sheet steel, or it can be made up of several parts with an air space between them. At least one pair of wire spins, i.e. a coil built around the core, surrounds the core. Transformers have several sets of turns (in the simplest case, one for the primary and another for the secondary).
Where,
B: the magnetic flux density in teslas.
Φ: magnetic flux in webers.
Circuit Models
The resistance–reluctance model, which draws parallels between electrical and magnetic circuits, is the most common method of describing a magnetic circuit. This model works well with systems with only magnetic components, but it has significant flaws when it comes to modelling systems with both electrical and magnetic components.
Let’s discuss the several types of circuit.
Thermal Circuit Breaker
Thermal circuit breakers for machinery (CBE) are particularly well-suited to protecting motors and transformers against current overload. When the internal bimetal is deflected, the circuit breaker trips. The current flow that heats the bimetal causes it to deflect, resulting in thermal inertia of the bimetal. A thermal circuit breaker with an iron reed, precious metal contacts, and interconnecting terminals is known as a thermal circuit breaker. When an overcurrent occurs, heat is generated as the current flows through the reed switch circuit causing the reed to deflect and snap open.
Thermal Magnetic Circuit Breaker
A thermal magnetic circuit breaker is the most common model in American homes today. These are circuit breakers that detect electrical faults using two components.
The first is an electromagnet, which is vulnerable to massive electrical current spikes.
Electrical surges can cause short-circuiting, which can cause significant harm to your expensive electrical equipment (like a clothes dryer or air conditioner) or big devices (think DVD player or desktop computer).
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The electromagnet reacts quickly to such dangerous conditions by interrupting the flow of energy, protecting the appliances. It is also called a magnetic circuit breaker.
Thermal magnetic circuit breakers are common because they can easily restrict short-circuiting and then resume normal power flow until the surge has passed.
Reed Switch Circuit
A reed switch circuit is an electronic switch that is activated by a magnetic field. It was invented in 1922 by a senior professor at Leningrad Electrotechnical University and later evolved at Bell Telephone Laboratories in 1936 by W. B. Ellwood into the reed relay. It consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope in its simplest and most familiar shape. The contacts are normally open, closing when a magnetic field is applied, or normally closed, opening when a magnetic field is applied.
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A permanent magnet or an electromagnetic coil may be used to trigger the switch, resulting in a reed relay. The contacts in the reed switch circuit revert to their original location when the magnetic field is withdrawn.
A reed switch circuit is commonly used to monitor the opening of a door or window for a security alarm.
Coupled Circuits
The transition may be triggered by a permanent magnet or an electromagnetic coil, resulting in a reed relay. When the magnetic field is removed from the reed switch circuit, the contacts return to their original position. For a security warning, a reed switch circuit is typically used to detect the opening of a door or window.
Classification of Coupled Circuits
We can classify a couple of circuits into the following two categories.
Electrically Coupled circuits.
Magnetically Coupled circuits.
Electrically Coupled Circuits
When there is a physical relationship between two coils, electrical coupling happens (or inductors). This pairing may be positive or oppositional. It is determined by whether the new enters through the dotted terminal or exits through the dotted terminal.
Consider the following electric circuit, which consists of two series-connected inductors.
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Magnetically Coupled Circuits
Magnetically Coupled Circuits occurs, when there is no physical connection between two coils (or inductors). This pairing may be supportive or antagonistic. It is determined by whether the new enters through the dotted terminal or exits through the dotted terminal. Two loops, with or without connections, influence each other by the magnetic field produced by one of them in magnetically coupled circuits. The transformer is designed to step up or down ac voltages or currents using the magnetic coupling principle.
Consider the following electrical equivalent circuit of the transformer. It has two coils and these are called primary and secondary coils.
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Series Magnetic Circuit
The magnetic circuit with a variety of components of various dimensions and materials bearing the same magnetic field is known as a series magnetic circuit. The flux in a series magnetic circuit is constant during the circuit. An integrated magnetic circuit made up of two magnetic materials of various permeabilities and an air gap.
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Parallel Magnetic Circuit
A parallel magnetic circuit is described as a magnetic circuit with two or more paths for the magnetic flux. Its behaviour is similar to that of a parallel electric circuit. The parallel magnetic circuit is made up of multiple dimensional areas and components, each with a different number of paths.
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FAQs on Magnetic Circuit: Meaning, Flux & Examples
1. What is a magnetic circuit?
A magnetic circuit is the closed path followed by magnetic flux lines. It is typically composed of ferromagnetic materials like iron, which have high permeability, to guide and concentrate the magnetic field. Key components include a core (the path for the flux), a winding (a coil of wire that generates the flux when current passes through it), and sometimes an air gap. It is the fundamental principle behind devices like transformers and motors.
2. What is the difference between a magnetic circuit and an electric circuit?
While analogous, magnetic and electric circuits have key differences:
Flow: In an electric circuit, current (flow of electrons) flows. In a magnetic circuit, magnetic flux is established.
Driving Force: The driving force in an electric circuit is the Electromotive Force (EMF), measured in Volts (V). In a magnetic circuit, it is the Magnetomotive Force (MMF), measured in Ampere-turns (AT).
Opposition: Resistance (R) opposes the flow of current. Reluctance (S) opposes the establishment of magnetic flux.
Energy: Energy is consumed in an electric circuit due to resistance (I²R loss). In a magnetic circuit, energy is only required to establish the flux, not to maintain it (assuming a DC field).
3. What are Magnetomotive Force (MMF) and magnetic flux?
Magnetomotive Force (MMF) is the force that establishes the magnetic flux in a magnetic circuit. It is produced by a current flowing through a coil of wire. The formula for MMF is MMF = N × I, where N is the number of turns in the coil and I is the current in Amperes. Its unit is the Ampere-turn (AT).
Magnetic Flux (Φ) is the total number of magnetic field lines passing through a given area. It is the magnetic equivalent of electric current. The SI unit for magnetic flux is the Weber (Wb).
4. What is magnetic reluctance and why is it an important concept?
Magnetic reluctance (S) is the opposition that a material presents to the establishment of magnetic flux. It is analogous to resistance in an electric circuit. The formula is S = l / (μA), where 'l' is the length of the path, 'A' is the cross-sectional area, and 'μ' is the magnetic permeability of the material. Reluctance is important because it determines how much MMF is needed to create a certain amount of flux. Materials with low reluctance (like iron) are used as cores to guide flux easily, while materials with high reluctance (like air) are used in air gaps to store magnetic energy or control the flux path.
5. How does Ohm's Law apply to a magnetic circuit?
Ohm's Law for electric circuits states that Current = Voltage / Resistance (I = V/R). There is a direct analogy in magnetic circuits, often called Hopkinson's Law or Ohm's Law for magnetic circuits. It states that the magnetic flux is directly proportional to the magnetomotive force and inversely proportional to the reluctance. The formula is:
Magnetic Flux (Φ) = Magnetomotive Force (MMF) / Reluctance (S)
This relationship is crucial for analysing and designing magnetic circuits, just as Ohm's Law is for electric circuits.
6. What are some real-world examples of devices that use magnetic circuits?
Magnetic circuits are fundamental to many electrical and electronic devices. Common examples include:
Transformers: To transfer electrical energy between circuits through a shared magnetic circuit.
Electric Motors and Generators: To convert between electrical and mechanical energy using the interaction of magnetic fields.
Inductors and Chokes: To store energy in a magnetic field or to filter signals.
Relays and Solenoids: To create mechanical motion (like flipping a switch or moving a valve) from an electrical signal.
Loudspeakers: To convert electrical signals into sound waves using a moving coil in a magnetic field.
7. How does introducing an air gap change the behaviour of a magnetic circuit?
Introducing an air gap significantly increases the total reluctance of the magnetic circuit. This is because air has a much lower magnetic permeability (μ) compared to ferromagnetic core materials. According to the formula Φ = MMF / S, a higher reluctance (S) means that a much larger Magnetomotive Force (MMF) is required to produce the same amount of magnetic flux. While this may seem inefficient, air gaps are intentionally designed into devices like motors and inductors to prevent core saturation and to store magnetic energy, which is essential for their operation.
