IMPORTANT FORMULAS

• Maximum number of electrons that can be accommodated in a shell = 2n²
  where 'n' is the number of the given shell from the nucleus.


• MIRROR FORMULA:
  
  (1/v) + (1/u) = (1/f) = (2/R)
  
  Where u, v, f and R are object distance, image distance, focal length of mirror and radius of curvature of mirror, respectively.


• LENS FORMULA:
  
  (1/v) - (1/u) = (1/f)
  
  Where u, v and f are object distance, image distance and focal length of lens, respectively.


• MAGNIFICATION:
  
  m = (h'/h) = (-v/u) for mirror
  
  m = (h'/h) = (v/u) for lens
  
  Where m, h' and h are magnification, image height and object height, respectively.


• η₂₁ = (sin i/sin r)
  
  Where η₂₁, i and r are refractive index of medium 2 with respect to medium 1, angle of incidence and angle of refraction, respectively.


• η₂₁ = (v₁/v₂)
  
  Where η₂₁, v₁ and v₂ are refractive index of medium 2 with respect to medium 1, speed of light in medium 1 and speed of light in medium 2, respectively.


• η_m = (c/v)
  
  Where η_m, c and v are absolute refractive index of a medium, speed of light in vacuum and speed of light in the medium, respectively.


• P = (1/f)
  
  Where P and f are power of lens in dioptre and focal length of lens in metre, respectively.


• I =(Q/t)
  
  Where I, Q and t are current in ampere, charge in coulomb and time in seconds, respectively.


• V = (W/Q)
  
  Where V, W and Q are voltage(potential difference) in volt, work done in joule and charge in coulomb, respectively.


• V = IR
  
  Where V, I and R are voltage in volt, current in ampere and resistance in ohm, respectively.


• R = ρ(l/A)
  
  Where R, ρ, l and A are resistance in ohm, resistivity in ohm-meter, length of the conductor in meter and cross-sectional area of conductor in m², respectively.


• R = R₁ + R₂ + R₃ +....
  
  Where R is the equivalent resistance of R₁, R₂, R₃,...connected in series.


• R = (1/R₁) + (1/R₂) + (1/R₃) +....
  
  Where R is the equivalent resistance of R₁, R₂, R₃,...connected in parallel.


• H = VIt = I²Rt
  
  Where H, V, I, R and t are heat produced in joule, voltage in volt, current in ampere, resistance in ohm and time in second, respectively.


• P = VI = I²R = (V²/R)
  
  Where P, V, I and R are power in watt, voltage in volt, current in ampere and resistance in ohm, respectively.


• 1 kWh = 3.6 x 10⁶ joule
  
  [kWh is kilowatthour also called 'unit' of electrical energy]

IMPORTANT LAWS

1. Law of Conservation of Mass : Mass can neither be created nor be destroyed, i.e. the total mass in a closed system remains unchanged.


2. Dobereiner's Law of Triads : When three elements with similar properties are arranged in the order of increasing atomic masses, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements.


3. Newland's Law of Octaves : If the elements are arranged in the order of increasing atomic mass, a similar set of properties is observed in every eighth element.


4. Mendeleev's Periodic Law : The properties of elements are the periodic function of their atomic masses.


5. Modern Periodic Law : The properties of elements are the periodic function of their atomic number.


6. Laws of Reflection :
   • The angle of incidence is always equal to the angle of reflection.
   • The incident ray, the normal to the reflecting surface at the point of incidence and the reflected ray, all lie in the same plane.


7. Laws of Refraction :
   • The incident ray, the refracted ray and the normal to the surface separating the two mediums at the point of incidence, all lie in the same plane.
   • The ratio of the sine of angle of incidence to the sine of angle of refraction is a constant, for the light of given colour and for the given pair of mediums.(better known as Snell's law)


8. Ohm's Law : The electric current flowing through a conducting wire is directly proportional to the potential difference across its terminals provided its temperature remains the same.


9. Joule's Law of Heating : The heat produced in a resistor is (i) directly proportional to the square of the current for a given resistance, (ii) directly proportional to the resistance for a given current, and (iii) directly proportional to the time for which the current flows through the resistor.


10. Right-Hand Thumb Rule : Hold (imagine to hold) a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current, then the fingers would wrap around the conductor in the direction of the field lines of the magnetic field.


11. Maxwell's Cork-Screw Rule : If we consider ourselves driving a cork-screw in the direction of the current, then the direction of the cork-screw is the direction of the magnetic field.


12. Fleming's Left-Hand Rule : Stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.


13. Fleming's Right-Hand Rule : Stretch the thumb, forefinger and middle finger of your right hand such that they are mutually perpendicular. If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of the conductor, then the middle finger will show the direction of the induced current.


14. Law of Conservation of Energy : Energy can neither be created nor be destroyed,i.e. total energy of a closed system remains unchanged.


15. 10% Law : About 10% of the available chemical energy of one trophic level of a food-chain is transferred to the next higher trophic level.