馃攳
Class 11 Physics | Heat Transfer | #16 Stefan's Law | For JEE & NEET - YouTube
Channel: Physics Galaxy
[1]
the 1st, most useful analysis of thermal radiation
was, analyzed, and concluded experimentally
[9]
in form of, stephen鈥檚 law. we can simply
state, this law is stated as, based on several
[19]
experiments stephen concluded this relation,
that total amount. of thermal radiation, power,
[29]
that is the amount of radiation energy per
unit time, from a body. is directly proportional
[46]
to. the fourth power. of its absolute temperature.
and always remember when we鈥檙e talking about
[67]
the radiation power from a body, say when
a body is heated to a temperature t, it radiates
[80]
heat that is electromagnetic radiation to
its surrounding. always remember this, that
[82]
this radiation is emitted from the surface
of body. so always keep in mind whenever we鈥檙e
[87]
talking about radiation, it is always, from,
surface of, body. when we talk about the radiation
[98]
energy which is coming out, so we can simply
write, radiation power. per meter square or
[112]
per unit of its surface area we can write,
p-r is directly proportional to, t to power
[126]
4, as given by stephen鈥檚 law. and this radiation
power can be written as, sigma t to power
[134]
4, where sigma is a proportionality constant,
which we, call as, stephen鈥檚 constant. this
[135]
stephen鈥檚 constant, is having, a specific
value, it is also universal constant given
[142]
as, 5 point 6 7 into, 10 to power minus 8,
watt per kelvin to power 4 meter square. and
[150]
here when we talk about this radiation power
which is emitted from the body surface per
[155]
unit area, its unit can be simply given as,
joule per second per unit area, that is per
[161]
meter square, or it is watt per meter square.
say by using stephen鈥檚 law we calculate
[170]
the total amount of energy which is radiated
by a body, which is at temperature t. and
[176]
say its, total surface area, is ay. we can
state as, just we鈥檝e discussed, the radiation
[178]
power per unit surface area which is emitted
from the surface of body, is sigma t to power
[193]
4, watt per meter square. and, as we鈥檝e
already discussed that, this power is emitted
[200]
only from the outer surface, so we can simply
state, for a body. of, surface, area, ay.
[218]
total, energy emitted. per unit time, or we
can simply state, radiation power. this direct
[237]
radiation power total can be written as, sigma
ay t to power 4, which is measured in joule
[244]
per second. this p-r was the radiation power
per unit surface area, and if we multiply
[253]
it with the total surface area, this鈥檒l
give us the total amount of energy which is
[256]
emitted from the body per unit time. this
can also be written as, the rate at which
[262]
the energy is being flown out from the body
surface in form of thermal radiation. so it
[268]
is written as rate of flow of heat, d-q by
dt. after the whole analysis it was also concluded
[274]
that, this relation was derived and verified,
for black bodies. and for general bodies,
[282]
if we wish to find out the total amount of
energy radiated, we analyze it by using the
[287]
emissivity of body. always remember that,
this direct analysis of, stephen鈥檚 law is,
[294]
only applicable for black bodies.
similarly if we talk about a general body,
[302]
we can write, for a general body. having,
emissivity e, we can define its emissivity
[318]
is equal to, radiation, from body, and its
ratio to, radiation, from a black body, which
[333]
is similar to this body and at same temperature.
so it can be written as, radiation power,
[340]
from a general body divided by the radiation
power from a black body. so we can simply
[348]
state, that radiation power, emitted, from
a general body, at temperature t can be given
[367]
as, p r g, this can be written as, emissivity
times, radiation power from a black body that
[375]
we just discussed. so, radiation power from
a general body can be written as, e sigma
[382]
ay, t to power 4, which is measured in joules
per second, if ay is the area of body. and
[388]
this radiation power can also be written as,
rate of, thermal radiation energy which is
[394]
emitted from the body. so in various cases
we鈥檙e going to use this expression directly
[400]
as, stephen鈥檚 law
Most Recent Videos:
You can go back to the homepage right here: Homepage