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Изоляторы и диэлектрики: в чём разница?

Изоляторы и диэлектрики: в чём разница?

Channel: GetAClass - Физика в опытах и экспериментах

Что такое физический парадокс?

Что такое физический парадокс?

Channel: GetAClass - Физика в опытах и экспериментах

Момент силы

Момент силы

Channel: GetAClass - Физика в опытах и экспериментах

Зазор в магнитопроводе и токи намагничивания

Зазор в магнитопроводе и токи намагничивания

Channel: GetAClass - Физика в опытах и экспериментах

Коэффициент полезного действия (КПД)

Коэффициент полезного действия (КПД)

Channel: GetAClass - Физика в опытах и экспериментах

Быстрее ветра и против ветра ● 1

Быстрее ветра и против ветра ● 1

Channel: GetAClass - Физика в опытах и экспериментах

Зачем нужен диэлектрик внутри конденсатора

Зачем нужен диэлектрик внутри конденсатора

Channel: GetAClass - Физика в опытах и экспериментах

Как возникает солнечная дорожка?

Как возникает солнечная дорожка?

Channel: GetAClass - Physics in experiments

Что такое число Россби?

Что такое число Россби?

Channel: GetAClass - Физика в опытах и экспериментах

Куда исчезла энергия конденсатора?

Куда исчезла энергия конденсатора?

Channel: GetAClass - Physics in experiments

физика

physics

опыты по физике

школьная физика

физические эксперименты

видеоурок физика

00:00:01

[music]

00:00:14

hello dear lovers of physics and

00:00:16

physical experiments and the experiment that

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I will now show you,

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if you weren’t shown it at school, then

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it’s very in vain because it does not

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require any complex equipment,

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the result is very impressive and for this

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experiment I will like this

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plastic white one the pipe is

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also white inside and the back wall is very light

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and this lid has a hole in the middle

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and the lid itself is sealed with black paper and

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now look what I will do, I will close

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the pipe with a lid and

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surprisingly it turns out that the

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hole in the center is much blacker

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than the lid although there is inside we remember the

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white walls, well, another part of this

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experience is I separate the paper and under it

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there is velvet paper which

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reflects probably less than one percent of the

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light falling on it, and you can see that

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the hole is either as black

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as the velvet paper or maybe

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even a little bit why not and well, this

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blackness of the hole, despite the fact that inside

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the pipe is completely light, as we

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now see it again, we need to take it, I need to

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explain

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for this, let's consider a ray of light entering

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the chamber through the holes into this ray and

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scatters onto the walls in all directions and

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let each throw the region loses 10

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percent of its energy and retains 90

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percent of the hole in our experiment was

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very small

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and let its area be one

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thousandth of the entire surface of the chamber then after the

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first plant from the camera sees 0 9

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multiplied by one thousandth of the

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light that entered during the second Russian light again

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loses 10

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cents of energy and saves 09 squared

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from the original value of which

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one thousandth comes out the third time

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of the lilac and gives us 9 cubed and so on

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we have a geometric progression

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whose sum is equal to nine so in

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general in our model

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9 times will come out through the hole per one thousandth of the

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incoming light, which can be rounded to

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one percent, and this value exactly

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corresponds to the reflectance coefficient of

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black velvet, and now we can give

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the definition of an absolutely black body as

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physicists understand it - this is a body

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that does not reflect the rays falling on it,

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or otherwise absorb and the flax

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ability of which is equal to one and,

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accordingly, the reflectivity

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is equal to zero in this sense, we got a

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very good model of an absolutely

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black body and, of course, it could be

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improved even more if we

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also covered the entire inside of the pipe with velvet 09 in

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our formula it would turn into 001 we

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plus we would also make the hole smaller and

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then

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simply the tiniest fraction of the

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external light falling on it would come out of this system,

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but at the same time we must understand that although an

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absolutely black body does not reflect the

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radiation that hits it, at the same time

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it itself can emit simply

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because it has some

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temperature, well, from here, from the hole

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now, in addition to the light that just a little

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bit came out, let’s assume

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that it wouldn’t have been there, but

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radiation from the walls of this device comes out, but

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why don’t we see it,

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firstly because it very, very

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weak and secondly because its

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simon is not at all visible but in the

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infrared range, well, to

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understand this radiation it makes

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sense for us to compare it with the radiation of the sun and

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now I must make a statement that is surprising at

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first glance,

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which is that our

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sun

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represents is an excellent example of an

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absolutely black body, but the fact is that

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if some external radiation were applied to the sun,

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this radiation would not be

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reflected back or scattered, but all of

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it would be somehow absorbed by the

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solar photosphere by that outer

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layer of the sun that we see and which

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means, so to speak, glows, shines and the light

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of which reaches us, and for our

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subsequent discussions it will be important to

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know the temperature in the solar photosphere;

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well, it is stated that it varies from 6000

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degrees Kelvin at the lower limit to

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four and a half thousand at the upper

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limit, the

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average value is usually taken

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which is equal to

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5700 degrees Kelvin, but for

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calculations we can still

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round it up to 6000 Kelvin and now we

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need to compare the radiation of our almost

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completely black body with the radiation of

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the sun, and for this we need two

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laws of radiation, the

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Wien law and the Stefan-Boltzmann law, the

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Wien law relates temperature

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emitter with a wavelength at which

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the radiation in the spectrum is maximum and the

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product of these two quantities is the

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same as that of the sun, that of our device, the

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temperature of the sun is 6000 kelvins,

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here we have room temperature, let

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’s take 300 kelvins, 20 times less,

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which means the wavelength should be 20 times

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more, but if for the sun we again

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take an estimated 500 nanometers, then for

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this emitter it turns out 10,000 per

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meter or 10 microns, this is

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infrared radiation, however, you and I

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also emit at the same wavelength,

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now we need to talk about the

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radiation power, here we already need

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Stefan’s law - Boltzmann who says that

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this power is proportional to the fourth

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power of temperature, which means we have a

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temperature ratio of twenty to the fourth

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power is 16 another 40 and 160000 means

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this thing emits 160 thousand times weaker than

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the sun, and if the sun’s radiation from the

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surface is six kilowatts per

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square centimeter then here we have,

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accordingly, 160 thousand times less and

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this turns out to be something in the region of 40 miles and a

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watt per square centimeter, now this is

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our black body radiating at you there with

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such intensity, but now we need to

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ask why we need to introduce

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ideas about this at all ideal object

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as an absolutely black body, this

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idea is introduced in order to derive the

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very laws of thermodynamics of radiation

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that we have just

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used to derive some of these

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laws, there are enough models of classical

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thermodynamics,

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but among them there are also such laws for the

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derivation of which Max Planck had to

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introduce the idea of quantization of

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radiation and this was the very first step in

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the construction of quantum theory, but of course we

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will not go in this direction now, but

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we will prove 2 simple, one might even say

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elementary, theorems related to the

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thermodynamics of radiation, and the very

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first theorem

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was proved by Gustav Surgeons and it

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states that radiation of an absolutely black

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body does not depend on what

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material the walls of this body are made of, it is

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determined only by its temperature, and

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to prove the theorem, we will consider

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such a

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mirror capsule, a cylinder, and on

00:09:23

opposite walls we will place two

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absolutely black bodies, and let’s assume that

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they are made of different materials,

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but at the same time they are at the

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same temperature,

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well then everything that is emitted by the first

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body will be absorbed by the second body

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and everything that is emitted by the second body will be

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absorbed first of all, well, it’s up to us to

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prove that these fluxes are equal and for

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this we will conduct a proof by

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contradiction, let’s assume that these flows are not

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equal and the first flow is greater than 2,

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well then we can say that these two flows are

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replaced in total by this arrow,

00:10:04

directed from the first body to the second,

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this is a transfer of energy, but since the energy is

00:10:09

transferred in that direction, it means the first

00:10:12

body cools down

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and the second body heats up, but initially they

00:10:18

were at the same

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temperature and it turns out that two bodies

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at the same temperature energy is transferred from one to

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the other and this

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contradicts the zeroth law

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of thermodynamics and then this body cools down

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from it energy continues to be transferred

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from the colder to the

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hotter and this contradicts the second

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law of thermodynamics which cannot be,

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therefore, these two initial fluxes

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had to be equal and we thereby

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proved the theorem.

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Now the second theorem is about how a

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body that is not absolutely

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black radiates and its absorption coefficient and from the

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word absorption is less than one, well,

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accordingly, from an absolutely black body

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we have there is not

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three flow of energy and this flow is

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now partly absorbed from us and

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partly reflected back and in order to

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maintain the balance of energies we need

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this green arrow to look like a

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Perry bend back, let’s move the

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same arrow here, this is reflected

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directly

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and this is the thermal radiation of our bodies

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that are indicated in red are smaller,

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and accordingly, the less absorption,

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the less

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radiation, and therefore the mirror walls,

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which ideally

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absorb nothing, they do not emit anything,

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and if they absorb very little, as if

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in reality, and so in a thermos then and

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thermal radiation is also very small, well,

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in fact, they say the thermos is based on this

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principle, and so that

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thermal radiation is present in our experience,

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well, not the radiation of the sun that

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comes to us from the sky and not the radiation of our

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installation, which we still cannot see with the eye,

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I’ll take it and heat it

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with a gas burner metal plate

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so that this plate becomes red

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hot,

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but now its temperature is

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apparently 1000 degrees Kelvin and

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maybe even more because from

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red it gradually turns to

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orange.

00:12:56

Well, of course, it should be noted that this was not

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black body radiation, although

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the spectrum here is in general, it’s close, but when

00:13:10

you look inside the furnaces through the window, and

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even more so inside the horse of the metallurgical

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furnace where you have red-hot

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molten metal, then what

00:13:21

comes out of this window, of course, can also be

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considered the radiation of an absolutely black

00:13:28

body red-hot, and now

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it’s time to move on to our

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the last question and it will be like this:

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how did people find out the temperature of the solar

00:13:41

surface, after all, none of us were there

00:13:44

and spacecraft did

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not reach this surface either, and if

00:13:49

they had reached they would not have been able to tell us anything,

00:13:52

your thoughts on this matter, write in the

00:13:56

comments to this video on youtube

00:14:01

[music]

00:14:12

[music]

Description:

Абсолютно чёрное тело поглощает всё излучение, которое на него падает. При этом его собственное излучение определяется только его температурой. Как это ни удивительно, наше Солнце с хорошим приближением является абсолютно чёрным телом. Ключевые слова: закон Кирхгофа, поглощательная способность, коэффициент поглощения, равновесное тепловое излучение, альбедо, закон Вина, закон Стефана-Больцмана, perfect black body, Stefan–Boltzmann law, absorption. Наш канал с дополнительными материалами https://t.me/getaclass_channel Новосибирский Государственный Университет Физический факультет НГУ https://www.nsu.ru/n/

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