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      Class 12 PHYSICS – JEE

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      • Class 12 PHYSICS – JEE
      CoursesClass 12PhysicsClass 12 PHYSICS – JEE
      • 1.Electrostatics (1)
        8
        • Lecture1.1
          Charge, Coulombs Law and Coulombs law in Vector form 41 min
        • Lecture1.2
          Electric Field; Electric Field Lines; Field lines due to multiple charges 42 min
        • Lecture1.3
          Charge Distribution; Finding Electric Field due to Different Object 01 hour
        • Lecture1.4
          Solid angle; Area Vector; Electric Flux; Flux of closed surface; Gauss Law 47 min
        • Lecture1.5
          Finding E Using Concept of Gauss law and Flux 01 hour
        • Lecture1.6
          Chapter Notes – Electrostatics (1)
        • Lecture1.7
          NCERT Solutions – Electrostatics
        • Lecture1.8
          Revision Notes Electrostatics
      • 2.Electrostatics (2)
        7
        • Lecture2.1
          Work done by Electrostatic Force; Work done by man in E-Field; Electrostatic Potential Energy 49 min
        • Lecture2.2
          Finding Electric Potential, Equipotential Surface and Motion in Electric Field 01 hour
        • Lecture2.3
          Electric Dipole and Dipole in Uniform and Non-uniform Electric field 01 hour
        • Lecture2.4
          Analysis of charge on conductors; Potential due to induced charge 58 min
        • Lecture2.5
          Conductors with cavity- Case 1: Empty cavity, Case 2: Charge Inside Cavity 41 min
        • Lecture2.6
          Connecting Two Conductors; Grounding of conductor; Electric field just outside conductor; Electrostatic pressure; Self potential Energy 54 min
        • Lecture2.7
          Chapter Notes – Electrostatics (2)
      • 3.Current Electricity (1)
        9
        • Lecture3.1
          Current, Motion of Electrons in Conductor; Temp. Dependence of Resistor 26 min
        • Lecture3.2
          Circuit Theory and Kirchoffs Laws 31 min
        • Lecture3.3
          Some Special Circuits- Series & Parallel Circuits, Open Circuit, Short Circuit 26 min
        • Lecture3.4
          Wheatstone Bridge, Current Antisymmetric 21 min
        • Lecture3.5
          Equivalent Resistance- Series and parallel, Equipotential Points, Wheatstone Bridge 25 min
        • Lecture3.6
          Current Antisymmetric, Infinite Ladder, Circuit Solving, 3D circuits 20 min
        • Lecture3.7
          Chapter Notes – Current Electricity
        • Lecture3.8
          NCERT Solutions – Current Electricity
        • Lecture3.9
          Revision Notes Current Electricity
      • 4.Current Electricity (2)
        4
        • Lecture4.1
          Heating Effect of Current; Rating of Bulb; Fuse 19 min
        • Lecture4.2
          Battery, Maximum power theorem; Ohmic and Non Ohmic Resistance; Superconductor 31 min
        • Lecture4.3
          Galvanometer; Ammeter & Voltmeter and Their Making 44 min
        • Lecture4.4
          Potentiometer and its applications ; Meter Bridge; Post Office Box; Colour Code of Resistors 32 min
      • 5.Capacitor
        6
        • Lecture5.1
          Capacitor and Capacitance; Energy in Capacitor 38 min
        • Lecture5.2
          Capacitive Circuits- Kirchoff’s Laws; Heat Production 01 hour
        • Lecture5.3
          Equivalent Capacitance; Charge on both sides of cap. Plate 52 min
        • Lecture5.4
          Dielectric Strength; Polar and Non-Polar Dielectric; Equivalent Cap. with Dielectric 01 hour
        • Lecture5.5
          Inserting and Removing Dielectric- Work (Fringing Effect), Force; Force between plates of capacitor 38 min
        • Lecture5.6
          Revision Notes Capacitor
      • 6.RC Circuits
        3
        • Lecture6.1
          Maths Needed for RC Circuits, RC circuits-Charging Circuit 19 min
        • Lecture6.2
          RC circuits-Discharging Circuit, Initial & Steady State, Final (Steady) State, Internal Resistance of Capacitor 44 min
        • Lecture6.3
          Revision Notes RC Circuits
      • 7.Magnetism and Moving Charge
        16
        • Lecture7.1
          Introduction, Vector Product, Force Applied by Magnetic Field, Lorentz Force, Velocity Selector 40 min
        • Lecture7.2
          Motion of Charged Particles in Uniform Magnetic Field 40 min
        • Lecture7.3
          Cases of Motion of Charged Particles in Uniform Magnetic Field 56 min
        • Lecture7.4
          Force on a Current Carrying Wire on Uniform B and its Cases, Questions and Solutions 59 min
        • Lecture7.5
          Magnetic Field on Axis of Circular Loop, Magnetic field due to Moving Charge, Magnetic Field due to Current 52 min
        • Lecture7.6
          Magnetic Field due to Straight Wire, Different Methods 40 min
        • Lecture7.7
          Magnetic Field due to Rotating Ring and Spiral 41 min
        • Lecture7.8
          Force between Two Current Carrying Wires 36 min
        • Lecture7.9
          Force between Two Current Carrying Wires 58 min
        • Lecture7.10
          Miscellaneous Questions 55 min
        • Lecture7.11
          Solenoid, Toroid, Magnetic Dipole, Magnetic Dipole Momentum, Magnetic Field of Dipole 54 min
        • Lecture7.12
          Magnetic Dipole in Uniform Magnetic Field, Moving Coil Galvanometer, Torsional Pendulum 01 hour
        • Lecture7.13
          Advanced Questions, Magnetic Dipole and Angular Momentum 56 min
        • Lecture7.14
          Chapter Notes – Magnetism and Moving Charge
        • Lecture7.15
          NCERT Solutions – Magnetism and Moving Charge
        • Lecture7.16
          Revision Notes Magnetism and Moving Charge
      • 8.Magnetism and Matter
        10
        • Lecture8.1
          Magnetic Dipole, Magnetic Properties of Matter, Diamagnetism; Domain Theory of Ferro 47 min
        • Lecture8.2
          Magnetic Properties of Matter in Detail 39 min
        • Lecture8.3
          Magnetization and Magnetic Intensity, Meissner Effect, Variation of Magnetization with Temperature 55 min
        • Lecture8.4
          Hysteresis, Permanent Magnet, Properties of Ferro for Permanent Magnet, Electromagnet 31 min
        • Lecture8.5
          Magnetic Compass, Earth’s Magnetic Field 20 min
        • Lecture8.6
          Bar Magnet, Bar Magnet in Uniform Field 49 min
        • Lecture8.7
          Magnetic Poles, Magnetic Field Lines, Magnetism and Gauss’s Law 32 min
        • Lecture8.8
          Chapter Notes – Magnetism and Matter
        • Lecture8.9
          NCERT Solutions – Magnetism and Matter
        • Lecture8.10
          Revision Notes Magnetism and Matter
      • 9.Electromagnetic Induction
        14
        • Lecture9.1
          Introduction, Magnetic Flux, Motional EMF 01 min
        • Lecture9.2
          Induced Electric Field, Faraday’s Law, Comparison between Electrostatic Electric Field and Induced Electric Field 43 min
        • Lecture9.3
          Induced Current; Faraday’s Law ; Lenz’s Law 56 min
        • Lecture9.4
          Faraday’s Law and its Cases 50 min
        • Lecture9.5
          Advanced Questions on Faraday’s Law 37 min
        • Lecture9.6
          Cases of Current Electricity 59 min
        • Lecture9.7
          Lenz’s Law and Conservation of Energy, Eddy Current, AC Generator, Motor 01 hour
        • Lecture9.8
          Mutual Induction 53 min
        • Lecture9.9
          Self Inductance, Energy in an Inductor 34 min
        • Lecture9.10
          LR Circuit, Decay Circuit 01 hour
        • Lecture9.11
          Initial and Final Analysis of LR Circuit 38 min
        • Lecture9.12
          Chapter Notes – Electromagnetic Induction
        • Lecture9.13
          NCERT Solutions – Electromagnetic Induction
        • Lecture9.14
          Revision Notes Electromagnetic Induction
      • 10.Alternating Current Circuit
        8
        • Lecture10.1
          Introduction, AC/DC Sources, Basic AC Circuits, Average & RMS Value 46 min
        • Lecture10.2
          Phasor Method, Rotating Vector, Adding Phasors, RC Circuit 35 min
        • Lecture10.3
          Examples and Solutions 21 min
        • Lecture10.4
          Power in AC Circuit, Resonance Frequency, Bandwidth and Quality Factor, Transformer 51 min
        • Lecture10.5
          LC Oscillator, Question and Solutions of LC Oscillator, Damped LC Oscillator 53 min
        • Lecture10.6
          Chapter Notes – Alternating Current Circuit
        • Lecture10.7
          NCERT Solutions – Alternating Current Circuit
        • Lecture10.8
          Revision Notes Alternating Current Circuit
      • 11.Electromagnetic Waves
        4
        • Lecture11.1
          Displacement Current; Ampere Maxwell Law 14 min
        • Lecture11.2
          EM Waves; EM Spectrum; Green House Effect; Ozone Layer 36 min
        • Lecture11.3
          Chapter Notes – Electromagnetic Waves
        • Lecture11.4
          Revision Notes Electromagnetic Waves
      • 12.Photoelectric Effect
        5
        • Lecture12.1
          Recalling Basics; Photoelectric Effect 50 min
        • Lecture12.2
          Photo-electric Cell 35 min
        • Lecture12.3
          Photon Flux; Photon Density; Momentum of Photon; Radiation Pressure- Full Absorption, Full Reflection; Dual nature 52 min
        • Lecture12.4
          Chapter Notes – Photoelectric Effect
        • Lecture12.5
          Revision Notes Photoelectric Effect
      • 13.Ray Optics (Part 1)
        12
        • Lecture13.1
          Rays and Beam of Light, Reflection of Light, Angle of Deviation, Image Formation by Plane Mirror 01 hour
        • Lecture13.2
          Field of View, Numerical on Field of Line, Size of Mirror 42 min
        • Lecture13.3
          Curved Mirrors, Terms Related to Curved Mirror, Reflection of Light by Curved Mirror 40 min
        • Lecture13.4
          Image Formation by Concave Mirror, Magnification or Lateral or Transverse Magnification 01 hour
        • Lecture13.5
          Ray Diagrams for Concave Mirror 45 min
        • Lecture13.6
          Image Formation by Convex Mirror; Derivations of Various Formulae used in Concave Mirror and Convex Mirror 01 hour
        • Lecture13.7
          Advanced Optical Systems, Formation of Images with more than one Mirror 24 min
        • Lecture13.8
          Concept of Virtual Object, Formation of Image when Incident ray are Converging, Image Characteristics for Virtual Object, 55 min
        • Lecture13.9
          Newton’s Formula, Longitudinal Magnification 23 min
        • Lecture13.10
          Formation of Image when Two Plane Mirrors kept at an angle and parallel; Formation of Image by two Parallel Mirrors. 43 min
        • Lecture13.11
          Chapter Notes – Ray Optics
        • Lecture13.12
          NCERT Solutions – Ray Optics
      • 14.Ray Optics (Part 2)
        13
        • Lecture14.1
          Refractive Index, Opaque, Transparent, Speed of Light, Relative Refractive Index, Refraction and Snell’s Law, Refraction in Denser and Rarer Medium 42 min
        • Lecture14.2
          Image Formation due to Refraction; Derivation; Refraction and Image formation in Glass Slab 57 min
        • Lecture14.3
          Total Internal Reflection, Critical Angle, Principle of Reversibility 01 hour
        • Lecture14.4
          Application of Total Internal Reflection 45 min
        • Lecture14.5
          Refraction at Curved Surface, Image Formation by Curved Surface, Derivation 56 min
        • Lecture14.6
          Image Formation by Curved Surface, Snell’s Law in Vector Form 01 hour
        • Lecture14.7
          Lens, Various types of Lens, Differentiating between various Lenses; Optical Centre, Derivation of Lens Maker Formula 01 hour
        • Lecture14.8
          Lens Formula, Questions and Answers 39 min
        • Lecture14.9
          Property of Image by Convex and Concave Lens; Lens Location, Minimum Distance Between Real Image and Object 01 hour
        • Lecture14.10
          Power of Lens, Combination of Lens, Autocollimation 35 min
        • Lecture14.11
          Silvering of Lens 44 min
        • Lecture14.12
          Cutting of Lens and Mirror, Vertical Cutting, Horizontal Cutting 49 min
        • Lecture14.13
          Newton’s Law for Lens and Virtual Object 01 hour
      • 15.Ray Optics (Part 3)
        6
        • Lecture15.1
          Prism, Angle of Prism, Reversibility in Prism 51 min
        • Lecture15.2
          Deviation in Prism, Minimum and Maximum Deviation, Asymmetric, Thin Prism, Proof for formula of Thin Prism 59 min
        • Lecture15.3
          Dispersion of Light, Refractive Index, Composition of Light, Dispersion through Prism 01 hour
        • Lecture15.4
          Rainbow Formation, Scattering of Light, Tyndall Effect, Defects of Image, Spherical Defect, Chromatic Defect, Achromatism. 57 min
        • Lecture15.5
          Optical Instruments, The Human Eye, Defects of Eye and its Corrections 01 hour
        • Lecture15.6
          Microscope & Telescope 02 hour
      • 16.Wave Optics
        21
        • Lecture16.1
          Introduction to Wave Optics 11 min
        • Lecture16.2
          Huygens Wave Theory 14 min
        • Lecture16.3
          Huygens Theory of Secondary Wavelets 10 min
        • Lecture16.4
          Law of Reflection by Huygens Theory 10 min
        • Lecture16.5
          Deriving Laws of Refraction by Huygens Wave Theory 10 min
        • Lecture16.6
          Multiple Answer type question on Huygens Theory 41 min
        • Lecture16.7
          Conditions of Constructive and Destructive Interference 22 min
        • Lecture16.8
          Conditions of Constructive and Destructive Interference 06 min
        • Lecture16.9
          Conditions of Constructive and Destructive Interference 23 min
        • Lecture16.10
          Incoherent Sources of Light 38 min
        • Lecture16.11
          Youngs Double Slit Experiment 12 min
        • Lecture16.12
          Fringe Width Positions of Bright and Dark Fringes 15 min
        • Lecture16.13
          Numerical problems on Youngs Double Slit Experiment 11 min
        • Lecture16.14
          Numerical problems on Youngs Double Slit Experiment 19 min
        • Lecture16.15
          Displacement of Interference Pattern 19 min
        • Lecture16.16
          Numerical problems on Displacement of Interference Pattern 28 min
        • Lecture16.17
          Shapes of Fringes 37 min
        • Lecture16.18
          Colour of Thin Films 59 min
        • Lecture16.19
          Interference with White Light 32 min
        • Lecture16.20
          Chapter Notes – Wave Optics
        • Lecture16.21
          NCERT Solutions – Wave Optics
      • 17.Atomic Structure
        6
        • Lecture17.1
          Thomson and Rutherford Model of Atom; Trajectory of Alpha particle; Bohr’s Model ; Hydrogen Like Atom; Energy Levels 58 min
        • Lecture17.2
          Emission Spectra, Absorption Spectra; De Broglie Explanation of Bohr’s 2nd Postulate; Limitations of Bohr’s Model 37 min
        • Lecture17.3
          Momentum Conservation in Photon Emission, Motion of Nucleus, Atomic Collision 58 min
        • Lecture17.4
          Chapter Notes – Atomic Structure
        • Lecture17.5
          NCERT Solutions – Atomic Structure
        • Lecture17.6
          Revision Notes Atomic Structure
      • 18.Nucleus
        6
        • Lecture18.1
          Basics- Size of Nucleus, Nuclear Force, Binding Energy, Mass Defect; Radioactive Decay 01 hour
        • Lecture18.2
          Laws of Radioactive Decay 36 min
        • Lecture18.3
          Nuclear Fission; Nuclear Reactor; Nuclear Fusion- Reaction Inside Sun 30 min
        • Lecture18.4
          Chapter Notes – Nucleus
        • Lecture18.5
          NCERT Solutions – Nucleus
        • Lecture18.6
          Revision Notes Nucleus
      • 19.X-Ray
        4
        • Lecture19.1
          Electromagnetic Spectrum, Thermionic Emission; Coolidge Tube – Process 1 22 min
        • Lecture19.2
          Coolidge Tube – Process 2; Moseley’s Law; Absorption of X-rays in Heavy Metal 39 min
        • Lecture19.3
          Chapter Notes – X-Ray
        • Lecture19.4
          Revision Notes X-Ray
      • 20.Error and Measurement
        2
        • Lecture20.1
          Least Count of Instruments; Mathematical Operation on Data with Random Error 18 min
        • Lecture20.2
          Significant Digits; Significant Digits and Mathematical Operations 30 min
      • 21.Semiconductors
        9
        • Lecture21.1
          Conductor, Semiconductors and Insulators Basics Difference, Energy Band Theory, Si element 21 min
        • Lecture21.2
          Doping and PN Junction 01 hour
        • Lecture21.3
          Diode and Diode as Rectifier 01 hour
        • Lecture21.4
          Voltage Regulator and Zener Diode and Optoelectronic Jn. Devices 01 hour
        • Lecture21.5
          Transistor, pnp, npn, Modes of operation, Input and Output Characteristics, , Current Amplification Factor 01 hour
        • Lecture21.6
          Transistor as Amplifier, Transistor as Switch, Transistor as Oscillator, Digital Gates 01 hour
        • Lecture21.7
          Chapter Notes – Semiconductors
        • Lecture21.8
          NCERT Solutions – Semiconductors
        • Lecture21.9
          Revision Notes Semiconductors
      • 22.Communication Systems
        5
        • Lecture22.1
          Basic working and terms; Antenna; Modulation and Types of Modulation 32 min
        • Lecture22.2
          Amplification Modulation, Transmitter, Receiver, Modulation index 40 min
        • Lecture22.3
          Chapter Notes – Communication Systems
        • Lecture22.4
          NCERT Solutions – Communication Systems
        • Lecture22.5
          Revision Notes Communication Systems

        Chapter Notes – Alternating Current Circuit

        A current which flows first in one direction in a circuit, called the positive direction, then in the reverse direction or negative direction is called alternating current. This cycle shown in figure is repeated again and again and has an average value of zero over a period. Since the variation of the current strength is sine wave as shown in figure, the value of current at any instant say t seconds after it had zero value will be given by the following expression:

        i=Imaxsinωt
        where i is the value of current at time t and Imax is the maximum value of the current and ω is the angular frequency ( =2πf, f being frequency).

        Both the emf and current undergo a complete cycle of changes, having positive and negative value, every time wt changes by 360o or 2π radians as shown in figure.

        Different Forms of a.c. emf
        The variation of A.C. quantity is sinusoidal hence can be expressed as:

        e=Emaxsinωt   as  ω=2πf, we have
        e=Emaxsin(2πft)  since  f=1/T, we have
        e=Emaxsin(2πTt)

        Phase of An A. C. Quantity
        Phase of an A. C. quantity represents the fraction of the time period of that alternating quantity that has elapsed since the current last passed through the zero position of reference. Phase is also expressed in terms of angle in radians. For example phase at A is T/4 second or π/2 radians. In electricity we are more concerned with the phase difference rather than absolute phase of an alternating quantity. Phase difference between quantities indicates the lag or lead of an alternating quantity with respect to other.A leading quantity is the quantity which reaches maximum (or zero) value earlier as compared to the other quantity, while lagging alternating quantity is one which reaches its zero or maximum value later than the other quantity. In figure two alternating quantities are shown quantity A leads B by an angle ϕ. Hence their equations are

        iA = Imax sin wt   and   iB = Imax sin (ωt + ϕ)

        Effective Virtual or R. M. S. value of a Quantity
        The r. m. s. (or effective apparent) value of an alternating current is the value of the direct current which produces same amount of heat in the same time in the same conductor. An alternating current is not steady but varies from instant to instant. If i1, i2, i3, …….. in are instantaneous currents r. m. s. value will be

        Imax=i21+i22+.........+i2nn−−−−−−−−−−√

        we can also expresses this value as

        Ir.m.s.=Imax/2–√=0.707Vmax

        Similarly   Vr.m.s.=Vmax/2–√=0.707Vmax

        Mean or Average Value of an Alternating Quantity
        The mean value or average value (Iav) of an alternating current is equal to the steady current which transfers across any circuit the same charge as is transferred by the alternating current during the same time.

        In case of sinusoidal or symmetrical alternating current, the average value over complete cycle is zero. Hence in these cases the average value is obtained by integrating the instantaneous values  of alternating quantity over half cycle only. The alternating currents average value is given by

        Iav=2Imaxπ     or        Iav=0.637Imax

        Form Factor

        The ratio of the root mean square (r.m.s.) to the average value of an alternating current gives an indication of the shape of the wave and is known as its form factor and is denoted by K i.e.
        Form factor K=Ir.m.s.Iav=0.707Imax0.637Imax=1.11
        Thus for sine wave form factor is 1.11

        Representation of AC Current and Voltage by Rotating Vectors

        A phasor is a vector which rotates about the origin with angular speed ω, as shown in the fig. given below:

        The vertical components of phasors V and I represent the sinusoidally varying quantities v and i. The magnitudes of phasors I and V represent the amplitudes or the peak values vm and im of these oscillating quantities. The projection of voltage and current phasors on vertical axis, i.e., vm sinωt and im sinωt respectively represent the value of current and voltage at that instant.

        A.C. Circuit containing Resistance Only (Resistive Circuit)

        In a circuit containing pure resistance (ohmic) only i.e. free from any inductance or capacitance in the circuit, the current and potential difference between any two points is given by i = v/R where i and v are instant current and voltage respectively and R is the resistance between the points. The emf applied isv=Vmaxsinωt
        and the current i=vmaxRsinωt, Imax=VmaxR
        or        i=Imaxsinωt
        This shows that the current and voltage are in phase. Graphically it is shown in figure (a) and vectorically in figure (b).
        The power in a circuit is given by P(=v×i)
        hence P=Imaxsinωt×Vmaxsinωt
        or         P=12ImaxVmax(1−cos2ωt)
        The power consists of a constant part ½ Imax Vmax/2 and a variable part
        ½ ImaxVmax cos 2ωt. The average value of variable part of power over a cycle is zero. Thus the average power for one cycle is

        P=12ImaxVmax=Imax2√×Vmax2√=Ir.m.s.Vr.m.s.

        A.C. Circuit Containing Inductor Only (Inductive Circuit)

        Consider a circuit having no ohmic resistance and only inductance L as shown in figure. When an A. C. current is passed through this circuit a magnetic flux is set up which induces alternating emf in the inductance which is  L(di/dt) and opposes the variation of current through it at every instant. If the applied voltage is vmax sin ωt we have

        Ldidt=Vmaxsinωt

        Integrating above relation we have

        i=Imaxsin(ωt−π2)    where   Imax=VmaxωL

        From the above relation it is clear that there is a phase difference of p/2 between voltage and current i.e. current lags voltage by π/2 radians. Graphically, the voltage and current wave are as shown in figure (a) and vectorically as shown in figure (b).The quantity ωL is called inductive reactance or reactance and denoted by
        XL(=ωL=2πfL). The power in an inductive circuit is given by

        P=−12VmaxImaxsin2ωt

        As there is no constant term in power expression, hence the average power in a pure inductive circuit is zero. This is called wattles power.

        A.C. Circuit Containing Capacitance Only (Capacitive Circuit)

        When an alternating voltage is applied to the plates of the capacitor, they are first charged in one direction and then in the opposite direction. If the applied voltage is v=Vmaxsinωt, the potential difference between the plates at any instant will be v=q/c=Vmaxsinωt and the current (dq/dt) is given by

        i=Imaxcosωt=Imaxsin(ωt+π2)

        where  Imax=Vmax(1/ωC)
        From above relation it is clear that current leads the applied voltage by 90o or π/2. The quantity 1/ωC is called reactance or capacitive reactance and denoted by XC.
        The voltage and current waves are as shown in figure (a) and vectorically as shown in figure (b).

        The power in the circuit is given by

        P=VmaxsinωtImaxcosωt=12VmaxImaxsin2ωt

        Thus the average power consumed in a pure capacitive circuit is zero.

        Resistance and inductance in series A. C. Circuit

        In a circuit containing a pure inductance L and a pure resistance R connected in series as shown in figure, when an A. C. voltage is applied in this circuit, the voltage drops on the resistance say vR(= iR in phase with current) and on the L say vL (= ωL), leading the current by 90o. These voltages are shown by the vector diagram in figure.The resultant voltage is given by

        v=v2L+v2R−−−−−−√
        and     i=vR2+X2L√

        The term  is called impedance of the circuit and denoted by Z (its units are ohm) i.e.
        Z=R2+X2L−−−−−−−√
        The triangle whose sides are proportional to the voltages are called voltage triangle (fig.(a)) and a triangle whose sides are proportional to R, XL and Z is called impedance triangle (fig.(b)).

        Average power in R and L in series circuit when A. C. current passes is given by
        P=Vr.m.s.Ir.m.s.cosϕ
        where ϕ is the angle by which vector v leads the vector i. The value of cos ϕ (=R/Z) is constant for a given circuit and is known as power factor. It is defined as the factor by which product of rms current and voltage should be multiplied to have true power in watt.

        If ϕ is 90o i.e. cos ϕ = 0, the ohmic resistance of the A. C. current is zero and the average power also remains zero, i.e. inspite of flow of current there is no dissipation of energy. The current in such a circuit is called wattles current. In practice we can not have a resistance free circuit and hence wattles current is not a reality.

        Resistance and Capacitance in Series A. C. Circuit

        Such a circuit having resistance, capacitance in series with an A. C. source is shown in figure.The voltage and current vectors are as shown in figure. The voltage and impedance triangles are shown in figure are given as

        v=v2R+v2c−−−−−−√      and     Z=R2+X2C−−−−−−−√

        where  XC=1/ω C is capacitive reactance.
        The power consumed p=v.i=Vr.m.s.Ir.m.s.cosϕ

        RLC Series A. C. Circuit

        A circuit having resistance R, inductance L and capacitance C in series with an A. C. source shown in figure.The voltage drop across resistance R be VR in phase with current, voltage drop across inductance L be VL leading the current by 90o and voltage drop across capacitance C be vc lagging the current by 90o. The resultant of vL and vC, (vL – vC) leads the current by 90o, provided (vL > vC) and will lag current by 90o if vC > vL. The resultant voltage is given by

        v=v2R+(vL−vC)2−−−−−−−−−−−−−√
        and the impedance is given by
        Z=R2+(ωL−1ωC)2−−−−−−−−−−−−−−√

        The voltage triangle and impedance triangle are shown in figure

        When ωL>1/ωC
        The net reactance ωL−1/ωC  is positive hence f is also positive as a result current will lag behind voltage or voltage leads the current.

        When ωL=1/ωC
        The net reactance is zero hence ϕ=0 and the voltage and current are in phase and impedance is equal to the resistance.

        When ωL<1/ωC
        The net reactance ωL−1/ωC  is negative hence f is also negative and the current leads the applied emf.

        Series resonance circuit

        The effective reactance is inductive or capcitive depending upon XL > XC or XL < XC. The inductive reactance XL­ is directly proportional to the frequency while the capacitive reactance is inversely proportional to the frequency. At certain frequency both reactances become equal and this frequency is called resonance frequency. At resonance frequency XL = XC or ωL=1/ωC i.e.

        ωr=1LC−−−√          or         fr=12π1LC−−−√ Hz

        The current and voltages are in phase as Z = R. Such circuits are called acceptor circuits. The ratio of vL or vC with applied voltage at resonant frequency is called Q-factor or voltage magnification and is given by

        Q=1RLC−−√

        LCR-parallel circuit at resonance

        It resistance (R), capacitor (C) and inductor (L) are arranged as shown in circuits.At resonance condition, the resonance frequency  fr=12π1LC−R2L2−−−−−−−√

        At resonance, the impedance of the circuit is maximum according to the following graph

        At resonance, current flowing through the circuit will be minimum

         

         

        Quality factor (q)

        Qualitative measurement of sharpness of current peak at resonance condition in LCR circuit

        Q=XLR=XCR=frΔf
        Q=ωrLR=1ωrCR
        another value of Q factor    Q=1RLC−−√

        Different types of ac circuits

        TRANSFORMERS

        One of the great advantages of ac over dc of for electric power distribution is that it is much easier to step voltage levels up and down with ac that with dc. For long-distance power transmission it is desirable to use as high a voltage and as small a current as possible, this reduces i2R losses in the transmission lines, and smaller wires can be used, saving on material costs.
        The key components of the transformer are two coils or windings, electrically insulated from each other but wound on the same core. The design of a simple transformer is as shown in the figure.The core is typically made of a material, such as iron, with a very large relative permeability μR. This keeps the magnetic field lines due to a current in one winding almost completely within the core. Hence almost all of these field lines pass through the other winding, maximizing the mutual inductance of the two windings. The winding to which power is supplied is called primary, the winding from which power is delivered is called the secondary .The circuit symbol for a transformer with an iron core, such as those used in power distribution systems, is

        Here’s how a transformer works. The ac source causes an alternating current in the primary, which sets up an alternating flux in the core, this induces an emf in each winding, in accordance with Faraday’s law. The induced emf in the secondary gives rise to an alternating current in the secondary, and this delivers energy to the device to which the secondary is connected. All currents and emf’s have the same frequency as the ac source .

        Let us see how the voltage across the secondary can be made larger or smaller in amplitude than the voltage across the primary. We neglect the resistance of the windings and assume that all the magnetic field lines are confined to the iron core, so at any instant the magnetic flux ΦB is the same in each turn of the primary and secondary windings. The primary winding has N1 turns, and the secondary winding has N2 turns. When the magnetic flux changes because of changing currents in the two coils, the resulting induced emf’s are

        ε1=−N1dΦBdt            and  ε2=−N2dΦBdt

        The flux per turn ΦB is the same in both the primary and the secondary, the above equations show that the induced emf per turn is the same in each. The ratio of the primary emf ε1 to the secondary emf ε2 is therefore equal at any instant to the ratio of primary to secondary turns.

        ε1ε2=N1N2

        Since ε1 and ε2 both oscillate with the same frequency as the ac source, the above equation also gives the ratio of the amplitudes or of the rms values of the induced emf’s. If the windings have zero resistance, the induced emf’s ε1 and ε2 are equal to the terminal voltages across the primary and the secondary, respectively; hence

        V2V1=N2N1       (terminal voltages of transformer primary and secondary)

        where V1 and V2 are either the amplitudes or the rms values of the terminal voltages. By choosing the appropriate turns ratio N2/N1, we may obtain any desired secondary voltage from a given primary voltage.

        Step up and Step down Transformers

        If N2 > N1, then V2 > V1 and we have a step-up transformer, if N2 < N1  then V2 < V1, and we have a step down transformer. At a power generating station, step-up transformers are used, the primary is connected to the power source, and the secondary  is connected to the transmission lines, giving the desired high voltage for transmission. Near the consumer, step-down transformers lower the voltage to a value suitable for use in home or industry.
        If the secondary circuit is completed by a resistance R, then the amplitude or rms value of the current in the secondary circuit is I2 = V2/R. From energy consideration the power delivered to the primary equals that taken out of the secondary (since there is no resistance in the windings), so

        V1I1=V2I2        (currents in transformer primary and secondary)

        We can combine the above equations and the relation

        I2=V2/R  to eliminate V2 and I2; we obtain
        V1I1=R(N2/N1)2

        This shows that when the secondary circuit is completed through a resistance R, the result is the same as if the source had been connected directly to a resistance equal to R divided by the square of the turns ratio, (N2/N1)2. In other words, the transformer “transforms” not only voltages and currents, but resistances as well. More generally, we can regard a transformer as “transforming” the impedance of the network to which the secondary circuit is completed.

        Efficiency of a Transformer

        In an ordinary transformer , there is some loss of energy due to primary resistance, hysteresis in the core, eddy currents in the core etc. The efficiency of a transformer is defined as

        η=outputpowerinputpower

        Efficiencies of the order of 99% can be easily achieved.

        Application 1


        A radio set operates at 6 V DC. A transformer with 18 turns in the secondary coil is sued to step down the input 220 V AC emf to 6 V AC emf. this AC emf is then rectified by another circuit to give 6 V DC which is fed to the radio. Find the number of turns in the primary.

        Solution:

        We have,

        ∣∣ε2ε1∣∣=N2N1
        or       N1=∣∣ε1ε2∣∣N2=2206×18=660

        LC OSCILLATIONS

        The ability of an inductor and a capacitor to store energy leads to the important phenomenon of electrical oscillations. Figure (a) shows a capacitor with initial charge Qo connected to an ideal inductor having no resistance. All the energy in the system is stored in the electrical field:

        UE = Q2o2C

        At t = 0, the switch is closed and the capacitor begins to discharge (see b). As the current increases, it sets up a magnetic field in the inductor, and so part of the energy is stored in the magnetic field, UB = 12LI2. When the current reaches its maximum value Io, as in Fig. (c), all the energy is in the magnetic field : UB = 12LI20. The capacitor now has no energy, which means Q = 0. Thus, I =0 when Q = Q0 and Q = 0 when I = I0. The current now starts to charge the capacitor, as in figure (d). In figure (e), the capacitor is fully charged, but with polarity opposite to its initial state in figure (a). The process just described will now repeat itself till the system reverts to its original state. Therefore the energy in the system oscillates between the capacitor and the inductor. As the block-spring system shown in the diagram suggests, the current and the charge in fact undergo simple harmonic oscillations. We will pursue the analogy later.

                  

                 

             

        The oscillations in an LC circuit are analogous to the oscillation of a block at the end of a spring. The figure depicts one-half of a cycle.

        Consider the situation depicted in Fig (b) and redrawn in figure (f). The current is increasing (dIdt>0), which means the induced emf in the inductor has the polarity shown, so
        Vb < Va.

        Fig(f) At the instant depicted the current is increasing, so the polarity of the induced emf in the inductor is an shown.,

        According to Kirchoff’s loop rule,

                                    QC−LdIdt=0

        In order to relate the current in the wire to the charge on the capacitor, we note that the current causes the charge Q on the capacitor to decrease, so

        I = –dQdt

        With this, the loop rule becomes

        d2Qdt2+1LCQ=0

        Fig. (f)  At the instant depicted the current is increasing, so the polarity of the induced emf in the inductor is an shown.,

        This has the same form as equation for simple harmonic oscillation.

        d2xdt2+ω2x=0

        The charge therefore oscillates with a natural angular frequency

        wo = 1LC√

        Application 2

        In an LC circuit, as in fig. (f), L = 40 mH, C = 20 µF, and the maximum potential difference across the capacitor is 80 V. Find

        (a)  the maximum charge on C

        (b)  the angular frequency of the oscillation

        (c)  the maximum current

        (d) the total energy

        Solution

        (a)     Qo = CVo = (2 x 10-5 F ) (80 V) = 1.6 x 10-3 C

        (b)     The angular frequency is

        wo = 1LC√=1(4×10−2H)(2×10−5F)√ = 1120 rad/s

        (c)     The maximum current is

        Io = woQo = (1120 rad/s) (1.6 x 10-3 C) = 1.79 A

        (d)    The total energy is simply the initial energy of the capacitor

        U =  Q2o2C=6.4×10−2 J

        Prev LC Oscillator, Question and Solutions of LC Oscillator, Damped LC Oscillator
        Next NCERT Solutions – Alternating Current Circuit

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