Physics

• Medium is not needed
• Heat transfer in the form of electromagnetic wave of speed m/s
• Fastest Process
• Heat Radiation lies in IR region
• Inverse square law in Heat Radiation i.e

Where I=intensity of heat radiation and R =Radius of sphere around the source

Note:

• When heat falls on a surface, a part of it is absorbed, a part of it is reflected and remaining part is transmitted
• Good reflectors are bad absorber and vice versa

### Black body:

• Body that can absorb the entire heat radiation incident on it.
• It can also radiate all the wavelengths if heated at high temperature
• No known body is perfectly blackbody.
• A surface coated with lamp black absorbs 96%to 98% of the incident radiation.
• A black body is perfect radiator.
• In ferry`s black body small opening acts as the black body radiator.

### emissive power:

• Total energy of all wavelengths radiated per second per unit area of the body.
• Its SI unit is

### emissivity

• Represented by 'e'.
• The ratio of energy radiated per unit area per unit time of a body and perfectly black body at same temperature
• value is 1 for perfectly black body and less than 1 for any other bodies.
• e=

### stefan-boltzmann law:

• J/ms, where = is Stefan's Constant
• Total amount of heat energy radiated per second per unit area of a perfectly black body is directly proportional to fourth power of its absolute temperature.
• Where E=Energy radiated per unit time per unit area
• If A is the surface area of the body then

### thermal enclosure

A body of temperature T is placed in an enclosure of Temperature T  then,

1. If , then rate of energy radiated ()
2. If , the rate of energy absorbed ()

### prevost's theory of heat exchange

• According to this theory, everybody radiates heat radiation at all temperature
• But when the body is at absolute zero temperature (-273˚C) it will not radiate any heat energy.

### Wein's displacement law

• Can be used to determine the temperature of heavenly bodies.
• At maximum intensity of radiation for blackbody.
• where mK is called Wein's constant
• =

Note:

• If Body absorb radiation = Black
• If body radiate more than surrounding = Bright
• If body radiate same as surrounding = Invisible