Magnetism and Electromagnetism
Overview
Magnetism and Electromagnetism is a core chapter in ITI Electrician Semester 2 (NCVT). It covers properties of magnets, magnetic fields, laws of magnetism, electromagnetic induction, and related concepts like permeability, reluctance, and flux. Understanding these principles is essential for working with motors, generators, transformers, and other electrical machines. This chapter regularly contributes 4–6 marks in NCVT theory papers.
Typed Notes
Properties of Magnets and Magnetic Terms
A magnet has two poles — North and South. Like poles repel; unlike poles attract (Law of Magnetism). The region around a magnet where its effect is felt is called the magnetic field. Magnetic field lines (lines of force) emerge from the North pole and enter the South pole externally. They never cross each other and always form closed loops. Key terms: Magnetic flux (Φ) — total field lines through a surface, unit Weber; Flux density (B) — flux per unit area, unit Tesla; Magnetomotive Force (MMF) — NI (Ampere-Turns), drives flux through a magnetic circuit.
Electromagnetic Induction — Faraday's and Lenz's Laws
Michael Faraday (1831) discovered that a changing magnetic field induces an EMF in a conductor. Faraday's First Law states that EMF is induced whenever flux linkage changes. Second Law states: e = N × (dΦ/dt), where N = number of turns. Lenz's Law (Heinrich Lenz) states the induced EMF opposes the change in flux — this gives the negative sign in e = −N(dΦ/dt). Lenz's Law is an application of the Law of Conservation of Energy. Self-induction occurs when a coil's own changing current induces EMF in itself; mutual induction occurs between two separate coils — the principle of a transformer.
Fleming's Rules and Force on a Conductor
Fleming's Right-Hand Rule (Generator Rule): Stretch thumb, index finger, and middle finger of the right hand mutually at right angles. Thumb = motion of conductor, Index = field direction (N to S), Middle = direction of induced current. Fleming's Left-Hand Rule (Motor Rule): Same hand position with left hand. Thumb = direction of force/motion, Index = field direction, Middle = current direction. The force on a current-carrying conductor in a magnetic field is F = BIL sinθ (Newtons), where B = flux density, I = current, L = length of conductor, θ = angle between conductor and field. Maximum force when θ = 90°.
Magnetic Circuit — Permeability, Reluctance, and Ohm's Law of Magnetism
A magnetic circuit is a closed path followed by magnetic flux. Ohm's Law of Magnetism: Flux (Φ) = MMF / Reluctance, i.e., Φ = NI/S. This is analogous to I = V/R in electric circuits. Permeability (μ) measures how easily a material allows magnetic flux — μ = μ₀ × μᵣ, where μ₀ = 4π × 10⁻⁷ H/m (permeability of free space) and μᵣ = relative permeability (dimensionless; for iron μᵣ ≈ 200–8000). Reluctance S = l/(μ₀μᵣA). High permeability means low reluctance. Hysteresis loss occurs due to repeated magnetisation and demagnetisation of core material — reduced by using silicon steel.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Magnetic Field and Poles
- Like poles repel, unlike attract
- Field lines N → S
- Magnetic flux, weber
Compass ki needle geographic north point karti hai — earth ka magnetic field.
02Electromagnet (Solenoid)
- Current-carrying coil = magnet
- Polarity flips with current direction
- Core: soft iron strengthens field
Electric bell aur relay dono electromagnet par kaam karte hain.
03MMF, Flux and Reluctance
- MMF = N×I ampere-turns
- Reluctance = magnetic resistance
- Ohm's law for magnetic circuits
Relay coil turns badha do — kam current mein bhi armature attract hoga.
04B-H Curve and Hysteresis
- B vs H, saturation point
- Hysteresis loop area = energy loss
- Soft iron = narrow loop
Transformer core soft silicon steel ka — kam hysteresis loss.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Magnetic flux (Φ) is measured in Weber (Wb); 1 Wb = 10^8 Maxwell. Flux density (B) is measured in Tesla (T) — B = Φ/A.
- 02Faraday's First Law: Whenever magnetic flux linking a conductor changes, an EMF is induced. Second Law: Induced EMF is proportional to the rate of change of flux linkage.
- 03Lenz's Law: Induced current always opposes the cause that produces it — this is the basis of back-EMF in motors.
- 04Fleming's Right-Hand Rule gives direction of induced EMF (generators); Fleming's Left-Hand Rule gives direction of force on a conductor (motors).
- 05Reluctance (S) is the opposition to magnetic flux — S = l/(μ₀μᵣA); unit is AT/Wb (Ampere-Turns per Weber). It is the magnetic equivalent of resistance.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
State Faraday's Second Law of Electromagnetic Induction and write its formula.
Differentiate between Fleming's Left-Hand Rule and Fleming's Right-Hand Rule.
What is reluctance in a magnetic circuit? Give its unit and formula.
What is Lenz's Law? How is it related to the Law of Conservation of Energy?
Calculate the force on a conductor of length 0.5 m carrying a current of 10 A placed at 90° in a magnetic field of flux density 2 T.
Magnetism and Electromagnetism covers magnetic flux, flux density, MMF, and reluctance in magnetic circuits. Faraday's Laws explain induced EMF; Lenz's Law gives its direction opposing flux change. Fleming's Right-Hand Rule applies to generators and Left-Hand Rule to motors. Permeability and reluctance govern how well a material supports magnetic flux. These concepts directly underpin the working of motors, generators, and transformers tested in NCVT ITI Electrician papers.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.The unit of magnetic flux density (B) is:
Q2.Which law states that induced EMF opposes the change in flux that produces it?
Q3.Fleming's Left-Hand Rule is used to find the direction of force in a:
Q4.The value of permeability of free space (μ₀) is:
Mark karo — dashboard par progress dikhega.