Magnetic circuit laws
Table of Contents
Magnetic circuit laws – field strength, Flux density, Magneto-Motive Force, Magnetic Reluctance, Residual Magnetism, Electromagnetic induction
Magnetic circuit laws
1. Magnetic flux(ϕ):
A magnet generates magnetic flux, which is the magnetic lines of force. It is denoted by ϕ and its unit is Weber.
1 weber = 108 lines of force
2. Magnetic field strength
Magnetic field strength is also known as field intensity, magnetic intensity, or magnetic field, and is represented by the letter H. The measure is ampere turns per metre.
H= Ampere turns / Length
H = NI/l AT/m
3. Flux density
Flux density, denoted by the symbol B, is the sum of the lines of force per square metre of the magnetic core’s cross-sectional area. Its SI unit (in the MKS system) is tesla (weber per metre square).
B= ϕ/a Wb/m2 or Tesla
ϕ -total flux in webers
A – area of the core in square metres
B – flux density in weber/metre square.
4. Magneto-Motive Force
Five variables – the current, number of turns, magnetic core material, length of core, and cross-sectional area of the core – affect the amount of flux density setup in the core. The magnetising effect will increase with more current and wire turns used in Magnetic circuit. The magnetomotive force (mmf), which is a term similar to electromotive force, is what we call this turn-current product (ernf).
MMF = NI ampere – turns
Where mmf is the magneto motive force in ampere turns
N is the number of turns, A.
5. Magnetic Reluctance
Reluctance, a term used to describe a property similar to electrical resistance in magnetic circuits (symbol S). If we denote mmf by ampere turns, the total flux is inversely proportional to the reluctance. We are able to write
S µ l/a
K = Constant of proportionality
= Reciprocal of absolute permeability of material
S = l / μa = l / μo µra A/Wb
Where, S – reluctance
I – length of the magnetic path in meters
μo- permeability of free space
µr – relative permeability
a – cross-sectional area
6. Residual Magnetism
When a material’s effective magnetising force is zero,. Magnetism is what is left in the material.
7. Magnetic Saturation
Magnetic saturation refers to the point beyond. Which a magnet’s strength cannot be increased.
8. End Rule
This rule states that if the coil is viewed from one end. And the current direction is in a clockwise direction, the opposite end is the South Pole. The North Pole is the end if the current direction is counterclockwise.
9. Len’s Law
When an emf is induced in a circuit electromagnetically. The current set up always opposes the motion or change in current which produces it.
10. Electromagnetic induction
Electromagnetic induction means the electricity induced by the magnetic field in Magnetic circuit
Faraday’s Laws of Electro Magnetic Induction
There are two laws of Faraday’s laws of electromagnetic induction. They are,
1) First Law
2) Second Law
Whenever a conductor cuts the magnetic flux lines. An emf is induced in the conductor.
The magnitude of the induced emf is equal to the rate of change of flux-linkages.
11. Fleming’s Right Hand Rule
This rule is use to find out the direction of dynamically induced emf. According to the rule hold out the right hand with the Index finger middle finger and thumb at the right angels to each others. If the index finger represents the direction of the lines of flux, the thumb points in the direction of motion then middle finger points in the direction of induced current.
When flux is changing with time and relative motion between the coils flux exist between both the coils or conductors and emf induces in both coil and the total induced emf e is given as
Inductance and Energy
Electric circuits frequently use coils wound around magnetic cores. An ideal circuit component known as inductance, which is defined as the flux linkage of the coil per ampere of its circuit, may serve as a representation for the coil.
Statically And Dynamically Induced Emf
Induced electro motive forces are of two types. They are,
i) Dynamically induced emf.
ii) Statically induced emf .
1. Statically Induced Emf
Statically Induced emf is of two types. They are
1 .Self induced emf
2. Mutually induced emf.
1.1 Self Induced emf
Self induction is the phenomenon where a change in a conductor’s current causes the conductor itself to produce an emf. To put it another way, when a conductor receives current, flux is created. Likewise, when the current is changed, the flux also changes. According to Faraday’s law, when the flux changes, an emf is induced. It’s known as self induction. The applied emf’s direction will always be reversed by the induced emf. The counter emf of self induction is the name given to the opposing emf that is thus created.
Uses of Self induction
.1. In the fluorescent tubes for starting purpose and to reduce the voltage.
2. In regulators, to give reduced voltage to the fans.
3. In lightning arrester.
4. In auto- transformers.
5. In smooth choke which is use in welding plant.
1.2 Mutually Induced EMF
It is the electromagnetic induction produced by one circuit in the near by second circuits Due to the variable flux of the first circuit cutting the conductor of the second circuit, that means when two coils or circuits are kept near to each other and if current is given to one circuit and it is changed, the flux produced due to that current which is linking both the coils or circuits cuts both the coils, an emf will be produced in both the circuits. The production of emf in second coil is due to the variation of current in first coil known as mutual induction.
1. It is use in ignition coil which is use in motor car.
2. It is also use in inductance furnace.
3. It is use for the principle of transformer
2. Dynamically induced EMF
Dynamically induced emf means an emf induced in a conductor when the conductor moves across a magnetic field. The Figure shows when a conductor “A”with the length “L” moves across a “B” wb/m2.
Flux density with “V” velocity,. Then the dynamically induced emf is induced in the conductor. This induced emf is utilize in the generator. The quantity of the emf can calculated using the equation
emf= Blv volt