Magentism | General Awareness for SSC CGL PDF Download

Introduction

Magnetism is a fundamental concept in physics that explores the properties and behavior of magnets, which are materials that can attract certain substances like iron. This field of study not only covers the types and characteristics of magnets but also delves into the principles of magnetic fields and forces.

Magnet

• A magnet is a material which can attract iron objects. 
• A natural magnet is an ore of iron (Fe₃O₄) called magnetite or lodestone. 
• A magnet which is prepared artificially, is called an artificial magnet.
  For examples 
• A bar magnet, a horse-shoe magnet etc.
• A freely suspended magnet always aligns itself into NorthSouth direction. Like magnetic poles repel and unlike magnetic poles attract each other. 
• A current-carrying coil containing a soft iron core, is called an electromagnet. 
• An electromagnet is utilised in electric bell, telegraph receiver, telephone diaphragm, transformer, dynamo etc. 
• Permanent magnets are made of steel and temporary magnet or electromagnets are made of soft iron because steel cannot magnetised easily but when it is magnetised one time, cannot be demagnetised easily. The soft iron can be magnetised or demagnetised easily.

Properties of Magnet

• Attractive property A magnet can attract small pieces of magnetic substances like iron, steel, cobalt, nickel etc. The attraction is maximum at poles. Unlike poles attract and like poles repel.
• Directive property A magnet, when suspended freely, aligns itself approximately along geographical N-S line. 
• Magnetic poles exist in pairs If a magnet is cut into two equal parts transverse to its length, then N and S-poles of the magnet do not get separated.

Magnetic Field

• The space in the surrounding of a magnet or a current carrying conductor in which its magnetic effect can be experienced, is called magnetic field. 
• Magnetic lines of force is an imaginary line drawn in magnetic field at which a magnetic North pole will move, if it is free to do so. 
• A tangent drawn at any point of an magnetic line of force represents the direction of magnetic field at that point. 
• The magnetic flux linked with a surface is equal to the total number of magnetic lines of force passing through that surface normally. Its unit is weber.
  Magnetic flux, φ = B. A = BA cos θ

Magnetic Force Acting on a Charge Moving in Uniform Magnetic Field

• The magnetic force on a moving charge in a magnetic field is given by
  F = Bqv sin θ
  where, B = magnetic field
  q = charge
  v = speed
  θ = angle between the direction of motion and magnetic field.

Magnetic Force Acting on a Current-Carrying Conductor Placed in Uniform Magnetic Field

• If a conductor carrying element I is placed in a magnetic field, the magnetic force on it is given by
  F = Bil sin θ
  where, l = electric current flowing through the conductor
  l = length of the conductor
  θ = angle between the direction of current and magnetic field. 
• The direction of this force can be find out by Fleming’s left hand rule which is given below.
  
• If we stretch the thumb, then the fore finger and the middle finger of left hand in such a way that all three are perpendicular to each other and if fore finger represents the direction of magnetic field, middle finger represents the direction of current flowing through the conductor, then thumb will represent the direction of magnetic force.

Earth’s Magnetism

• The earth has its own magnetic field and it resembles that of a magnetic dipole located at the centre of the earth. The pole near the geographic North of the earth is called the magnetic North pole. Similarly, the pole near the geographic South pole is called the magnetic South pole. 
• The Earth’s magnetic field diverts charged particle coming from space towards its poles and saves living beings from being severely harmed. 
• Magnetic compass A magnetic needle which always direct in North-South (N-S) direction. 
• Neutral point A point in a magnetic region where the net magnetic field is zero.

Magnetic storm 

• Local disturbances in the earth’s magnetic field which can damage telecommunication which are probably caused by lump of charged particles emanating from the sun is known as magnetic storm.

Coil Places in Uniform Magnetic Field

• When a coil having number of turns N, each of area of crosssection A carrying current I is placed in a uniform magnetic field B, then a torque acts on it, which tries to rotate it.
  Torque, τ = NBIA sin θ
  where, θ is angle subtended between the direction of magnetic field and normal to plane of the coil. 
• In radial magnetic field, θ = 90°
  ∴ τ_max = NBIA

Moving Coil Galvanometer

• A moving coil galvanometer is used to detect the presence of current and the direction of current in any circuit. 
• When current is passed through a coil, suspended in a magnetic field, a torque acts on it. As coil rotates, a restoring torque acts on phospher bronze strip due to twist produce in it. In equilibrium, both torques become equal the pointer stops for a short moment and coil starts to rotate in opposite direction.

Ammeter and Voltmeter

• An ammeter is a device designed to measure electric current and must be connected in series within the circuit. 
• An ideal ammeter has zero resistance. To convert a galvanometer into an ammeter, you need to attach a low resistance in parallel with it.
• A voltmeter is a device used to measure potential difference between two points in an electric circuit. 
• The resistance of an ideal voltmeter is infinity. It is always connected in parallel. 
• A galvanometer can be converted into a voltmeter by connecting a high resistance in series. 
• A small resistance connected in parallel with the load resistance to reduce amount of electric current through resistor is called shunt.

Magnetic Substances

There are three types of magnetic substances Paramagnetic, Diamagnetic and Ferromagnetic.

Paramagnetic Substances

• Those substances which are feebly magnetised in the direction of magnetic field when placed in strong magnetic field, are called paramagnetic substances.
  For examples 
• Aluminium, platinum, chromium, manganese, solutions of salts of iron, nickel, oxygen etc. 
• These substances are attracted towards strong magnetic field in a non-uniform magnetic field. 
• The magnetism of these substances decreases with increase in temperature.

Diamagnetic Substances

• Those substances which are feebly magnetised in the opposite direction of magnetic field when placed in strong magnetic field are called diamagnetic substances.
  For examples 
• Gold, silver, zinc, copper, mercury, water, alcohol, air, hydrogen etc. 
• These substances are attracted towards weak magnetic field in a non-uniform magnetic field. 
• The magnetism produced in these substances does not change with increase or decrease in temperature.

Ferromagnetic Substances

• — Those substances which are strongly magnetised in the direction of magnetic field when placed in it, are called ferromagnetic substances.
  For examples 
• Iron, nickel, cobalt etc. 
• The magnetism produced in these substances decreases with increase in temperature and at a particular temperature, called Curie temperature. 
• At the Curie temperature, a paramagnetic substance becomes diamagnetic.
• The Curie law is, X_m ∝ 1/T
  where, X_m = magnetic susceptibility of a paramagnetic substance and T = temperature.
• Curie temperature for iron is 770°C and for nickel is 358°C.
• In 2016, noble prize is Physics has been awarded to three theoretical researchers for their insights into the odd behaviour of matter in unusual phases, like superconductors, superfluid films and some kinds of magnets. They discovered the strange changes that occur with very thin layers of matter or extreme magnetic fields.

Electromagnetic Induction (EMI)

Whenever the magnetic flux linked with an electric circuit changes, an emf is induced in the circuit. This phenomenon is called electromagnetic induction.

Faraday’s Laws of EMI

• Whenever the magnetic flux linked with a circuit changes, an induced emf is produced in it. 
• The induced emf loses as long as the change in magnetic flux continues. 
• The magnitude of induced emf is directly proportional to the rate of change in magnetic flux, i.e.,
  where, N = 1, 2, 3… constant of proportionality and negative sign indicates Lenz’s law.

Lenz’s Law 

• The direction of induced emf or induced current is always in such a way that it opposes the cause due to which it is produced. 
• Lenz’s law is in accordance with the conservation of energy.

Motional EMF 

• If a rod of length l moves in a magnetic field B with a velocity v, then induced emf produced in it given by
  E = (B × v ) l = Bvl sin θ
  where, θ = angle between the velocity v of the conductor and the magnetic field B.

Fleming’s Right Hand Rule

If we stretch the thumb, the fore finger and the middle finger of right hand in such a way that all three are perpendicular to each other, and if thumb represents the direction of motion, the fore finger represents the direction of magnetic field, then middle finger will represent the direction of induced current.

Eddy Current

If a piece of metal is placed in a varying magnetic field or rotated with high speed in a uniform magnetic field, then induced current set up in the piece is like whirlpool of air, called eddy current, also known as foucault’s current.

Uses

• Eddy currents are used in dead beat galvanometer, induction furnaces, induction motor, speedometers of automobiles etc. 
• Eddy currents are used in diathermy for deep heat treatment of the human body.

Self and Mutual Induction

• The phenomenon of production of induced emf in a circuit due to change in current flowing in its own, is called self induction. 
• The magnetic flux linked with a coil
  φ = LI
  where, L = coefficient of self induction
  I = current.
  The unit of self induction is Henry (H).
• The coefficient of self induction of a coil is equal to the numerical value of induced emf in the coil when the rate of change of current through the coil is unity. 
• The phenomenon of production of induced emf in a circuit due to change in magnetic flux in its neighbouring circuit, is called mutual induction. 
• The coefficient of mutual induction (M) of two coils is equal to the numerical value of induced emf in one coil when the rate of change of current through second coil is unity. 
• In case of mutual induction, flux linked is given by φ = MI . 
• Its unit is Henry (H).

Alternating Current

An electric current whose magnitude and direction changes continuously is called alternating current.
Alternating current is represented as
I = I₀ sin ωt
where, I₀ = peak value of alternating current
ω = angular frequency of alternating current.

• Alternating voltage is given by
  V = V₀ sin ωt
  where, V₀ = peak value of alternating voltage
  ω = angular frequency of alternating voltage. 
• The frequency of alternating current in India is 50 Hz. 
• Mean or average value of AC is zero for one complete cycle. But its average value for a half-cycle is given by
  
• Root mean square value of AC is given by
  
• Similarly, root mean square value of alternating voltage is given by
  
• An AC ammeter and AC voltmeter read root mean square value of alternating current and alternating voltage respectively.

AC Generator or Dynamo

• It is a device which converts mechanical energy into alternating current. 
• Its working is based on electromagnetic induction. 
• The induced emf produced by the AC generator is given by
  e = NBAω sin ωt = e₀ sin ωt
  There  are four main parts of an AC generator 
• Armature It is a rectangular coil of insulated copper wire having a large number of turns. 
• Field magnets These are two pole pieces of a strong electromagnet. 
• Slip rings These are two hollow metallic rings. 
• Brushes These are two flexible metals or carbon rods which remain slightly in contact with slip rings.

DC Motor

It is a device which converts electrical energy into mechanical energy.

• An DC generator or dynamo contains slip rings or commutator inspite of slip rings. 
• Its working is based on the fact that when a current carrying coil is placed in uniform magnetic field, a torque acts on it. 
• Torque acting on a current carrying coil placed in uniform magnetic field
  τ = NBIA sin θ 
• When armature coil rotates, a back emf (induced emf) is produced in the coil.
• Efficiency of a motor ( η) = Back emf/Applied emf = E/V

Transformer

• It is a device which can change a low voltage current into a high voltage current and vice-versa. 
• Its working is based on mutual induction.
  There are two types of transformers

Step-up Transformer

• It converts a low voltage current into a high voltage current. 
• In this transformer, N_s > N_p, E_s > E_p
  and I_p < I_s

where,
N_s = number of turns in the secondary coil
N_p = number of turns in the primary coil
E_s = emf induced in the secondary coil
E_p = emf induced in the primary coil
l_p = current in the primary coil
I_s = current in the secondary coil.

Step-down Transformer

• It converts a high voltage current into a low voltage current. In this transformer,
  N_s < N_p, E_s < E_{p
  and Ip > Is}
• _{Transformation ratio (K)}
• _{For step-up transformer, K > 1}
• _{For step-down transformer, K < 1}
• _{The main energy losses in a transformer are given below
  – Iron loss 
  – Flux loss 
  – Hysteresis loss
  – Humming loss (ohmic loss)}