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D’Alembert Principle

It is alternative form of Newton’s second law of motion which has been named after its introducer, French physicist Polymath Jean Le Rond D’Alembert. It states that difference between forces acting on a system of mass of particles & time derivatives of momentum of system itself along any virtual displacement consistent with constraints of system is zero.        

Daltons Law of Partial Pressure

Law proposed by John Dalton in 1801 which states that “the total pressure exerted by the mixture of non-reactive gases is equal to the sum of partial pressures of individual gases.

Damped Vibration

When pendulum vibrates in air medium, there are frictional forces and consequently energy is dissipated in each vibration. Thus amplitude decreases continuously with time, and finally oscillations die out. Such vibrations are called damped vibrations. 



It is phenomenon of dying out of mechanical or electrical oscillations due to external forces.

Daniel Cell

It was invented in 1836 by john Fredric Daniel, a British chemist. It is a type of cell having a zinc anode in dilute Sulphuric acid separated by a porous barrier from a copper cathode in copper Sulphate solution. The principle behind the cell is that a liquid solute called electrolyte between metal plates begins to dissolve zinc and copper into positively charged ions as ions leave the metal, free electrons remain behind. The zinc dissolves more rapidly than the copper, which means that the zinc soon contains more electrons. If one connects two pieces of metal with a wire the electrons will migrate through the wire from zinc to copper, creating electric current.

Dark Matter

Hypothetical predicted matter by astrophysicists, which is not seen directly by telescopes. The unaccounted missing mass is termed as dark matter. It neither emits nor absorbs light. It was first postulated by John Oort in 1932.   

Davisson Germer Experiment

In 1927, Clinton J. Davisson and Lester H. Germer confirmed experimentally the wave nature of electrons by bombarding a nickel target with electrons and measuring the intensity of electrons scattered from target. Davisson and Germer determined wavelength of electrons from known atomic separation and the measured Bragg angle at which the electron intensity exhibited a maximum. They found that the wavelength calculated is well in agreement with electron wavelength calculated from debroglie relationship. Davisson and Germer experiment demonstrated diffraction of electrons and gave evidence of wave nature of electron.


Unit of time defined as interval equal to 24 hours.


 The interval of light between two successive nights; The time between sunrise and sunset.

Debroglie Wave

 Wave nature attributed to particle.

Debroglie Wave length

Ratio of Planck’s constant to momentum of particle. 

Debroglie’s Hypothesis

Hypothesis put forward by French scientist Debroglie, as per which materialistic particle possess both wave and particle characteristics. The wave and particle nature for matter are complimentary in nature.

Debye Theory of Specific Heats

A theory of the specific heat capacity of solids put forward by Peter Debye in 1912; in which it was assumed that the specific heat is a consequence of the vibrations of the atoms of the lattice of the solid. In contrast to the Einstein theory of specific heat, which assumes that each atom has the same vibration frequency, Debye postulated that there is a continuous range of frequencies that cuts off at a maximum frequency, which is characteristic of a particular solid. The theory leads to the conclusion that the specific heat capacity of solids is proportional to T3, where T is the thermodynamic temperature. This result is in very good agreement with experiment at low temperatures. The Debye temperature is characteristic of a particular solid.


It is unit of electric dipole moment.

1 Debye = 3.33 x 10-30 Coulomb-meter