Topics
Circular Motion
- Angular Displacement
- Angular Velocity
- Angular Acceleration
- Angular Velocity and Its Relation with Linear Velocity
- Uniform Circular Motion (UCM)
- Radial Acceleration
- Dynamics of Uniform Circular Motion - Centripetal Force
- Centrifugal Forces
- Banking of Roads
- Vertical Circular Motion Due to Earth’s Gravitation
- Equation for Velocity and Energy at Different Positions of Vertical Circular Motion
- Kinematical Equations for Circular Motion in Analogy with Linear Motion.
Rotational Dynamics
- Rotational Dynamics
- Circular Motion and Its Characteristics
- Applications of Uniform Circular Motion
- Vertical Circular Motion
- Moment of Inertia as an Analogous Quantity for Mass
- Radius of Gyration
- Theorems of Perpendicular and Parallel Axes
- Angular Momentum or Moment of Linear Momentum
- Expression for Torque in Terms of Moment of Inertia
- Conservation of Angular Momentum
- Rolling Motion
- Overview: Rotational Dynamics
Gravitation
- Newton’s Law of Gravitation
- Periodic Time
- Kepler’s Laws
- Binding Energy and Escape Velocity of a Satellite
- Weightlessness
- Variation of ‘G’ Due to Lattitude and Motion
- Variation in the Acceleration>Variation in Gravity with Altitude
- Communication satellite and its uses
- Composition of Two S.H.M.’S Having Same Period and Along Same Line
Mechanical Properties of Fluids
- Fluid and Its Properties
- Thrust and Pressure
- Pressure of liquid
- Pressure Exerted by a Liquid Column
- Atmospheric Pressure
- Gauge Pressure and Absolute Pressure
- Hydrostatic Paradox
- Pascal’s Law
- Application of Pascal’s Law
- Measurement of Atmospheric Pressure
- Mercury Barometer (Simple Barometer)
- Open Tube Manometer
- Surface Tension
- Molecular Theory of Surface Tension
- Surface Tension and Surface Energy
- Angle of Contact
- Effect of Impurity and Temperature on Surface Tension
- Excess Pressure Across the Free Surface of a Liquid
- Explanation of Formation of Drops and Bubbles
- Capillarity and Capillary Action
- Fluids in Motion
- Critical Velocity and Reynolds Number
- Viscous Force or Viscosity
- Stokes’ Law
- Terminal Velocity
- Continuous and Discontinuous Functions
- Bernoulli's Equation
- Applications of Bernoulli’s Equation
- Overview: Mechanical Properties of Fluids
Kinetic Theory of Gases and Radiation
- Gases and Its Characteristics
- Classification of Gases: Real Gases and Ideal Gases
- Mean Free Path
- Expression for Pressure Exerted by a Gas
- Root Mean Square (RMS) Speed
- Interpretation of Temperature in Kinetic Theory
- Law of Equipartition of Energy
- Specific Heat Capacity
- Absorption, Reflection, and Transmission of Heat Radiation
- Perfect Blackbody
- Emission of Heat Radiation
- Kirchhoff’s Law of Heat Radiation and Its Theoretical Proof
- Spectral Distribution of Blackbody Radiation
- Wien's Displacement Law
- Stefan-boltzmann Law of Radiation
- Overview: Kinetic Theory of Gases and Radiation
Angular Momentum
- Definition of M.I., K.E. of Rotating Body
- Rolling Motion
- Physical Significance of M.I (Moment of Inertia)
- Torque and Angular Momentum
- Theorems of Perpendicular and Parallel Axes
- M.I. of Some Regular Shaped Bodies About Specific Axes
Thermodynamics
- Thermodynamics
- Thermal Equilibrium
- Measurement of Temperature
- Heat, Internal Energy and Work
- First Law of Thermodynamics
- Thermodynamic State Variables and Equation of State
- Thermodynamic Process
- Heat Engine
- Refrigerators and Heat Pumps
- Entropy and Second Law of Thermodynamics
- Carnot Cycle and Carnot Engine
- Overview: Thermodynamics
Oscillations
- Periodic and Oscillatory Motion
- Simple Harmonic Motion (S.H.M.)
- Differential Equation of Linear S.H.M.
- Projection of U.C.M.(Uniform Circular Motion) on Any Diameter
- Phase of K.E (Kinetic Energy)
- K.E.(Kinetic Energy) and P.E.(Potential Energy) in S.H.M.
- Composition of Two S.H.M.’S Having Same Period and Along Same Line
- Some Systems Executing Simple Harmonic Motion
Oscillations
- Oscillations
- Explanation of Periodic Motion
- Linear Simple Harmonic Motion (S.H.M.)
- Differential Equation of Linear S.H.M.
- Acceleration (a), Velocity (v) and Displacement (x) of S.H.M.
- Amplitude (A), Period (T) and Frequency (N) of S.H.M.
- Reference Circle Method
- Phase in S.H.M.
- Graphical Representation of S.H.M.
- Composition of Two S.H.M.’S Having Same Period and Along Same Line
- The Energy of a Particle Performing S.H.M.
- Simple Pendulum
- Angular S.H.M. and It's Differential Equation
- Damped Oscillations
- Free Oscillations, Forced Oscillations and Resonance Oscillations
- Periodic and Oscillatory Motion
- Overview: Oscillations
Elasticity
- Eneral Explanation of Elastic Property
- Stress and Strain
- Hooke’s Law
- Elastic Energy
- Elastic Constants and Their Relation
- Determination of ‘Y’
- Behaviour of Metal Wire Under Increasing Load
- Application of Elastic Behaviour of Materials
Surface Tension
- Molecular Theory of Surface Tension
- Surface Tension
- Capillarity and Capillary Action
- Effect of Impurity and Temperature on Surface Tension
Superposition of Waves
Wave Optics
- Concept of Wave Optics
- Nature of Light
- Light as a Wave
- Huygens Principle
- Reflection of Light at a Plane Surface
- Refraction of Light at a Plane Boundary Between Two Media
- Polarisation of Light
- Interference
- Diffraction of Light
- Resolving Power
- Overview: Wave Optics
Wave Motion
- Wave Motion Introduction
- Simple Harmonic Progressive Waves,
- Reflection of Transverse and Longitudinal Waves
- Change of Phase
- Principle of Superposition of Waves
- Formation of Beats
- Beats
Stationary Waves
- Study of Vibrations in a Finite Medium
- Formation of Stationary Waves on String
- Study of Vibrations of Air Columns
- Free and Forced Vibrations
- Forced Oscillations and Resonance
Electrostatics
- Concept of Electrostatics
- Application of Gauss' Law
- Electric Potential and Potential Difference
- Electric Potential Due to a Point Charge
- Equipotential Surfaces
- Electrical Energy of Two Point Charges and of a Dipole in an Electrostatic Field
- Free and Bound Charges
- Combination of Capacitors
- Displacement Current
- Energy Stored in a Charged Capacitor
- Van De Graaff Generator
- Uniformly Charged Infinite Plane Sheet and Uniformly Charged Thin Spherical Shell (Field Inside and Outside)
- Overview: Electrostatics
Current Electricity
Kinetic Theory of Gases and Radiation
- Concept of an Ideal Gas
- Assumptions of Kinetic Theory of Gases
- Derivation for Pressure of a Gas
- Degrees of Freedom
- Derivation of Boyle’s Law
- Thermal Equilibrium
- First Law of Thermodynamics
- Heat Engine
- Temperature and Heat
- Qualitative Ideas of Black Body Radiation
- Wien's Displacement Law
- Green House Effect
- Stefan's Law
- Maxwell Distribution
- Specific Heat Capacities - Gases
- Law of Equipartition of Energy
Wave Theory of Light
Magnetic Fields Due to Electric Current
- Magnetic Fields Due to Electric Current
- Magnetic force
- Cyclotron
- Helical Motion
- Magnetic Force on a Wire Carrying a Current
- Force on a Closed Circuit in a Magnetic Field
- Torque on a Current-Loop in a Uniform Magnetic Field
- Magnetic Dipole Moment
- Magnetic Potential Energy of a Dipole
- Magnetic Field Due to a Current-carrying Conductor: Biot-savart's Law
- Force of Attraction Between Two Long Parallel Wires
- Magnetic Field Produced by a Current in a Circular Arc of a Wire
- Applications of Biot-Savart's Law > Magnetic Field at the Axis of a Circular Current-carrying Loop
- Magnetic Lines for a Current Loop
- Ampere’s Circuital Law
- Applications of Ampere’s Circuital Law > Magnetic Field of a Toroidal Solenoid
- Overview: Magnetic Fields Due to Electric Current
Magnetic Materials
- Magnetic Materials
- Torque Acting on a Magnetic Dipole in a Uniform Magnetic Field
- Origin of Magnetism in Materials
- Magnetisation and Magnetic Intensity
- Magnetic Properties of Materials
- Classification of Magnetic Materials
- Hysteresis: Retentivity and Coercivity
- Permanent Magnet
- Magnetic Shielding
- Overview: Magnetic Materials
Interference and Diffraction
- Interference of Light
- Conditions for Producing Steady Interference Pattern
- Interference of Light Waves and Young’s Experiment
- Analytical Treatment of Interference Bands
- Measurement of Wavelength by Biprism Experiment
- Fraunhofer Diffraction Due to a Single Slit
- Rayleigh’s Criterion
- Resolving Power of a Microscope and Telescope
- Difference Between Interference and Diffraction
Electromagnetic Induction
- Electromagnetic Induction
- Faraday's Laws of Electromagnetic Induction
- Lenz's Law
- Flux of a Vector Field
- Motional Electromotive Force (e.m.f.)
- Induced Emf in a Stationary Coil in a Changing Magnetic Field
- Generators
- Back Emf and Back Torque
- Induction and Energy Transfer
- Eddy Currents or Foucault Currents
- Self Inductance
- Energy Stored in a Magnetic Field
- Energy Density of a Magnetic Field
- Mutual Inductance
- Transformers
- Overview of Electromagnetic Induction
Electrostatics
- Mechanical Force on Unit Area of a Charged Conductor
- Energy Density of a Medium
- Concept of Condenser
- The Parallel Plate Capacitor
- Capacity of Parallel Plate Condenser
- Effect of Dielectric on Capacitance
- Energy of Charged Condenser
- Condensers in Series and Parallel,
- Van-deGraaff Generator
AC Circuits
- AC Circuits
- Values of Alternating Current
- Phasors
- AC Voltage Applied to a Resistor
- AC Voltage Applied to an Inductor
- AC Voltage Applied to a Capacitor
- AC Voltage Applied to a Series LCR Circuit
- Power in AC Circuit
- LC Oscillations
- Electric Resonance
- Sharpness of Resonance: Q Factor
- Choke Coil
- Overview: AC Circuits
Current Electricity
- Meter Bridge
Dual Nature of Radiation and Matter
Magnetic Effects of Electric Current
Magnetism
Structure of Atoms and Nuclei
- Structure of the Atom and Nucleus
- Thomson’s Atomic Model
- Geiger-marsden Experiment
- Lord Rutherford’s Atomic model
- Atomic Spectra
- Neils Bohr’s Model of an Atom
- Atomic Nucleus
- Constituents of a Nucleus
- Isotopes
- Atomic and Nuclear Masses
- Size of the Nucleus
- Mass Defect and Binding Energy
- Binding Energy Curve
- Forms of Energy > Nuclear Energy
- Nuclear Binding Energy
- Radioactive Decays
- Law of Radioactive Decay
- Overview: Structure of Atoms and Nuclei
Semiconductor Devices
Electromagnetic Inductions
- Electromagnetic Induction
- Self Inductance
- Mutual Inductance
- Transformers
- Need for Displacement Current
- Coil Rotating in Uniform Magnetic Induction
- A.C. Generator
- Reactance and Impedance
- LC Oscillations
- Inductance and Capacitance
- Resonant Circuits
- Power in AC Circuit
- Lenz’s Law and Conservation of Energy
Electrons and Photons
Atoms, Molecules and Nuclei
- Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom
- Bohr’s Model for Hydrogen Atom
- Hydrogen Spectrum
- Atomic Masses and Composition of Nucleus
- Radioactivity
- Law of Radioactive Decay
- Atomic Mass, Mass - Energy Relation and Mass Defect
- Nuclear Binding Energy
- Nuclear Fusion
- de-Broglie Relation
- Wave Nature of Matter
- Wavelength of an Electron
- Davisson and Germer Experiment
- Continuous and Characteristics X-rays
- Mass Defect and Binding Energy
Semiconductors
- Energy Bands in Solids
- Extrinsic Semiconductor
- Applications of n-type and p-type Semiconductors
- Special Purpose P-n Junction Diodes
- Semiconductor Diode
- Voltage Regulator
- I-V Characteristics of Led
- Transistor and Characteristics of a Transistor
- Transistor as an Amplifier (Ce-configuration)
- Transistor as a Switch
- Oscillators
- Digital Electronics and Logic Gates
Communication Systems
Estimated time: 11 minutes
CBSE: Class 12
Introduction
Niels Henrik David Bohr (1885–1962) was a Danish physicist who explained the hydrogen atom's spectrum using quantum ideas. He also contributed to clarifying conceptual problems in quantum mechanics and proposed the complementary principle.
Bohr modified Rutherford’s atomic model by adding ideas from the newly developing quantum hypothesis. In 1913, he concluded that classical mechanics and classical electromagnetism were not sufficient to explain atomic structure and atomic spectra.
CBSE: Class 12
Limitations of Rutherford's Atomic Model
Rutherford’s model assumes that the atom, with a central nucleus and revolving electrons, is stable, similar to a sun-planet system. However, the nucleus-electron system is governed by Coulomb’s law of force, not gravitational force.
According to classical electromagnetic theory:
- An electron revolving in a circular orbit is constantly accelerated.
- An accelerating charged particle emits electromagnetic radiation.
- Therefore, the electron should continuously lose energy.
- It should spiral inward and eventually fall into the nucleus.
- Such an atom would not be stable.
Also, if the revolving electron emitted radiation continuously:
- The frequency of emitted radiation would equal the frequency of revolution,
- The frequency would change continuously as the electron spiralled inward,
- And the atom would emit a continuous spectrum.
This contradicts the observed line spectrum. Therefore, classical ideas were not sufficient to explain atomic structure.
CBSE: Class 12
Bohr’s Theory
Bohr combined classical and early quantum concepts and presented his theory in the form of three postulates.
Bohr’s First Postulate
An electron in an atom can orbit in certain stable orbits without emitting radiant energy, contrary to the predictions of electromagnetic theory.
According to this postulate:
- Each atom has certain definite stable states,
- Each possible state has a definite total energy,
- And these states are called the atom's stationary states.
Bohr’s Second Postulate
The electron revolves around the nucleus only in those orbits for which the angular momentum is an integral multiple of h/2π, where h is Planck’s constant.
Thus, the angular momentum of the orbiting electron is quantised:
L = \[\frac {nh}{2π}\]
Where n is an integer.
Bohr’s Third Postulate
An electron may make a transition from one specified non-radiating orbit to another orbit of lower energy. When this happens, a photon is emitted with energy equal to the difference between the initial and final states.
The frequency of the emitted photon is given by:
hν = Ei − Ef
where Ei and Ef are the energies of the initial and final states, and Ei > Ef.
CBSE: Class 12
Radius of the nth Orbit
For the hydrogen atom, the radius of the nth possible orbit is obtained using Bohr’s second postulate of the quantisation of angular momentum.
The radius is:
\[r_n=\left(\frac{n^2}{m}\right)\left(\frac{h}{2\pi}\right)^2\left(\frac{4\pi\varepsilon_0}{e^2}\right)\]
This expression gives the radius of the allowed orbit corresponding to the quantum number n.
CBSE: Class 12
Total Energy of the Electron
The total energy of the electron in the stationary states of the hydrogen atom is:
\[E_n=-\frac{me^4}{8n^2\varepsilon_0^2h^2}\]
Substituting values, this becomes:
\[E_n=-\frac{2.18\times10^{-18}}{n^2}\mathrm{J}\]
Since atomic energies are often expressed in electron volts, and 1 eV = 1.6 × 10-19 J, this can be written as:
\[E_n=-\frac{13.6}{n^2}\mathrm{eV}\]
The negative sign of the total energy means that the electron is bound to the nucleus. Energy is required to remove an electron from a hydrogen atom to infinity.
CBSE: Class 12
Example
Classical Electromagnetic Theory and Frequency of Emitted Light
According to classical electromagnetic theory, the initial frequency of the light emitted by an electron orbiting a proton in the hydrogen atom can be calculated from the orbital frequency.
Given:
- Radius of orbit = 5.3 × 10-11 m.
- Velocity of electron = 2.2 × 106 m s-1.
Using:
ν = \[\frac{v}{2\pi r}\]
ν ≈ 6.6 × 1015 Hz
Thus, according to classical electromagnetic theory, the initial frequency of the emitted light is 6.6 × 1015 Hz.
CBSE: Class 12
Key Points: Bohr’s Model for Hydrogen Atom
- Bohr accepted Rutherford’s nuclear model but modified it using quantum ideas.
- Classical mechanics and electromagnetism could not explain atomic-scale behaviour fully.
- Only certain orbits are allowed for the electron in the hydrogen atom.
- These orbits have definite total energy.
- Electron transitions between energy levels lead to photon emission.
- The energy of the hydrogen atom is negative because the electron is bound to the nucleus.
