Showing posts with label Kirchoff. Show all posts
Showing posts with label Kirchoff. Show all posts


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The Kilogram is the usual scientific unit of mass. The standard Kilogram is the mass of a particular cylinder of platinum iridium alloy kept near Paris in France. All masses that are measured are compared with this.

Kirchhoff’s first Law
It states that at any junction of several circuit elements, the sum of currents entering the junction must be equal to sum of currents leaving it.

Kirchhoff’s second Law
The algebraic sum of charges in potential around any closed resistor loop must be zero.

Kelvin’s Thermodynamic Scale
The ratio of two temperatures measured on absolute scale is equal to the ratio of heat absorbed to the heat rejected by a reversible engine working between these temperatures.

Kirchoff’s law (thermal radiation)
The ratio of emissive power to the absorption power is same for all bodies at a certain temperature for a certain wavelength and it is equal to emissive power of a perfectly black body at that temperature and that wavelength.

Kilowatt hour (KWH)
It is the unit of electrical energy. 1KWH=36 x 105 Joules.

Kepler’s first law
Every planet revolves round the sun in an elliptical orbit with the sun at one of its foci. This law is known as law of orbits.

Kepler’s second law
The radius vector joining a planet to the sun sweeps equal areas in equal intervals of time.
The areal velocity of a planet round the sun is constant. This law is known as “law of areas”.

Kepler’s third law
Square of the time period of revolution of a planet round the sun is directly proportional to the cube of mean distance of the planet from the sun.

Kelvin’s statement of second law of thermodynamics
It is impossible to get a continuous supply of work by cooling a body to a temperature lower than that of coldest of its surroundings.

Kilo Calorie
The amount of energy required top increase the temperature of one kilogram of water by 1oC.

Black body and its Radiation

A perfectly black body is the one which absorbs all the radiations of all wavelengths incident on it. Since it neither reflects not transmits any radiation it appears black in color what may be the color of incident radiation.

According to Kirchoffs law, a body which is capable of absorbing radiation must also be capable of emitting all possible wavelengths. So a perfectly black body is a good absorber as well as good radiator. When it is heated to a suitable high temperature, it emits radiations of all wavelengths (continuous spectrum). As the radiations emitted by black body is rich in maximum possible wavelengths and hence such Radiations are known as full Radiation or Total Radiation.     

The wavelength of emitted Radiation by a black body depends only on its temperature and is independent of the material of the body.

There is no body acting as perfect black body. The nearest approach is lamp black or platinum black. These are capable of absorbing the visible and a part near infrared but far infrared (heat Radiation) are reflected. So perfectly black body is just an ideal concept. For all practical purposes a lamp blacked surface can be considered as perfectly black body.

Energy Distribution in Black body Radiation

The distribution of energy in black body radiation for different wavelengths and at various temperatures was determined experimentally by Lummer and Pringsheim in 1899. They used the black body as an electrically heated chamber with narrow aperture.

The temperature of heated enclosure is measured by thermocouple.

The parallel beam of Radiation is allowed to incident on a "fluorspar prism" instead of a glass prism. The reason behind not using glass prism is that it absorbs some heat radaition.

The radiation is detected by means of Bolometer. Bolometer is an instrument to detect Thermal Radiation. The Bolometer is a linear type due to Lummer and Kurlabaum and is fitted with galvanometer 'G'. The deflection produced in the Galvanometer gives the intensity of Radiation, Eƛ. This is defined such that quantity Eƛ.dEƛ is the energy, for wavelengths lying between ƛ and ƛ+dƛ emitted per second per unit surface area of black body.   

The wavelengths at different parts of the spectrum was calculated by "Prism Dispersion Formula".

The experiment results are as follows:

1) The emission from a Black body at any temperature is composed of Radiation from all wavelengths.

2) At a given temperature, the energy is not uniformly distributed. As the temperature of the black body increases, the intensity of radiation for each wavelength increases. This shows that the total amount of energy is radiated per unit area per unit time increases with rise of temperature.

i.e., T 𝛼 Eƛ

3) The total energy of radiation at any temperature is given by the area between the curve corresponding to that temperature and horizontal axis. The increase in area found in accordance with Stefans law.

4) The amount of radiant energy emitted is small at very short and very long wavelengths. At a particular temperature, the spectral radiance Eƛ is maximum at particular wavelength ƛm. Most of the energy is emitted at wavelengths not very  different from ƛm.

5) The wavelength corresponding to the maximum energy represented by the peak of the curve shifts towards shorter wavelengths as the temperature increases. This is called Wiens Displacement. According to this law ƛm x T = constant.

This shows that as the temperature is increased, the black body emits the radiation of shorter wavelengths such that the product of temperature 'T' and maximum wavelength ƛm is a constant. 
The constant is called Wiens Displacement constant and has value 0.2896 x 10⁻²mK. 

There is change in wavelength due to Doppler effect.

Laws Related to Black Body:

a) Kirchoffs Law
b) Stefan-Boltzmann law
c) Wiens Law
d) Rayleigh-Jeans Law
e) Plancks Law

Important points to be noted:

i) Wiens formula agrees in short wavelength region.
ii) Rayleigh-Jeans formula agrees for long wavelength region.
iii) Plancks formula covers the entire region.
iv) When radiation is passed through a black body is passed through a prism, acontinuous spectrum is obtained. The energy is distributed in various wavelengths varying from 0 to infinity.
v) The law that connects the intensity with the wavelength is known as law of distribution of intensity of black body radiation.
vi) According to Stefans law, u=𝝈T⁴ where 𝝈 is Stefans constant and 'u' is energy density.