- 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
- 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
- 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.
- Total energy of all wavelengths radiated per second per unit area of the body.
- Its SI unit is
- 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.
- 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
A body of temperature T is placed in an enclosure of Temperature T then,
- If , then rate of energy radiated ()
- 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
- If Body absorb radiation = Black
- If body radiate more than surrounding = Bright
- If body radiate same as surrounding = Invisible
Application of Radiation:
- White clothes are preferred in summer and dark colored clothes in winter. (Reason=White reflect and dark absorb radiations)
- Polished reflectors are used in electric heaters to reflect maximum heat in the room.
- Hot water pipes and radiators used in room are painted black because they can radiate maximum amount of heat to the room.
Instruments for detection and measurement of heat radiation: