#### 1729

##### Active Member
I understand that it doesn't transmit nor reflect any EMR, but absorbs it perfectly.

1) Why does a black body radiate? Why is it a 'perfect emitter'?

2) Does a blackbody only radiate when it becomes hotter than its surroundings?

3) If no EMR falls on a blackbody, does it still emit radiation?

4) When it absorbs EMR, does it absorb it as heat ONLY? Therefore to stay in thermal equilibrium does it emits thermal radiation?

5) And why does a blackbody radiate when at a constant temperature?

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#### sida1049

##### Well-Known Member

The main idea you need to understand is the concept of an equilibrium: a thermal equilibrium is achieved when the rate of emission and rate of absorption are matched. If we're not an at equilibrium, then the rates will be unbalanced; e.g. if the temperature of the surroundings is higher, then the rate of absorption of radiation will exceed the rate of emission, and eventually the black-body will achieve thermal equilibrium. But there is always this exchange going on.

Heat is energy, so when we say the black-body absorbs EMR, what we're really saying is that the black-body absorbs the energy of the EMR as heat. Oh, and a black-body is a perfect emitter in the sense that it absorbs EMR of all frequency perfectly; no EMR is reflected. Whenever a unit of EMR "touches" the black-body, it has 100% chance of having all of its energy absorbed in the form of heat and 0% chance of being reflected in another direction.

If no EMR falls on a black body, does it emit radiation? What you're asking is what would happen if a black-body is in a perfect vacuum with no source of EMR or anything - a complete, infinite void. Then the black-body will emit radiation IF it has any energy left. Eventually, the black-body will be devoid of any energy, like its surrounding void, and an equilibrium will be reached where the rate of emission trivially equals the rate of absorption - zero.

I hope this helps. I've not even kidding when I say I know nothing about physics; I did HSC physics, but I didn't even remember what a black-body is (or anything about it) until I saw this thread.

The most important part to remember about what a black-body is, is that it's an idealised concept - in other words, it's false and doesn't exist. It's just there because it makes the maths easier for physicists. So when you're thinking about black-bodies, don't fall into the trap of thinking that you're describing something that exists - it helps to know that you're talking about a hypothetical.

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#### 1729

##### Active Member

The main idea you need to understand is the concept of an equilibrium: a thermal equilibrium is achieved when the rate of emission and rate of absorption are matched. If we're not an at equilibrium, then the rates will be unbalanced; e.g. if the temperature of the surroundings is higher, then the rate of absorption of radiation will exceed the rate of emission, and eventually the black-body will achieve thermal equilibrium. But there is always this exchange going on.

Heat is energy, so when we say the black-body absorbs EMR, what we're really saying is that the black-body absorbs the energy of the EMR as heat. Oh, and a black-body is a perfect emitter in the sense that it absorbs EMR of all frequency perfectly; no EMR is reflected. Whenever a unit of EMR "touches" the black-body, it has 100% chance of having all of its energy absorbed in the form of heat and 0% chance of being reflected in another direction.

If no EMR falls on a black body, does it emit radiation? What you're asking is what would happen if a black-body is in a perfect vacuum with no source of EMR or anything - a complete, infinite void. Then the black-body will emit radiation IF it has any energy left. Eventually, the black-body will be devoid of any energy, like its surrounding void, and an equilibrium will be reached where the rate of emission trivially equals the rate of absorption - zero.

I hope this helps. I've not even kidding when I say I know nothing about physics; I did HSC physics, but I didn't even remember what a black-body is (or anything about it) until I saw this thread.

The most important part to remember about what a black-body is, is that it's an idealised concept - in other words, it's false and doesn't exist. It's just there because it makes the maths easier for physicists. So when you're thinking about black-bodies, don't fall into the trap of thinking that you're describing something that exists - it helps to know that you're talking about a hypothetical.
Thank you

Is thermal equilibrium the same thing as constant temperature? On google it says: "Two physical systems are in thermal equilibrium if no heat flows between them when they are connected by a path permeable to heat."

But isnt there heat flowing between a black body and its surroundings?

In the case of a star (not a perfect blackbody), since it is always hotter than its surroundings does it mean is constantly emitting radiation so that the temperature of the surroundings approaches the temperature of the star? (just like when a hot object transfers heat to a cold object)

"Then the black-body will emit radiation IF it has any energy left."
With this, is this radiation classified as blackbody radiation or would it just be thermal radiation since there is no absorption and only emission? (absorption rate =/= emission rate)

#### sida1049

##### Well-Known Member
Thank you

Is thermal equilibrium the same thing as constant temperature? On google it says: "Two physical systems are in thermal equilibrium if no heat flows between them when they are connected by a path permeable to heat."

But isnt there heat flowing between a black body and its surroundings?
I'm no physics guy, so take everything I say with a grain of salt.

I'm pretty sure when we talk about "heat flow", we're talking about changes in heat, and is actually determined by the the rate at which something gains heat (through absorbing EMR) and the rate at which it loses it. So when two connected bodies have the same temperature, there is no "heat flow" in the sense that the temperature in system A is falling while the temperature in system B is rising, but you can bet your ass EMR is still constantly being emitted and absorbed. (This concept of equilibrium is the same as chemical equilibrium in chemistry, and other disciplines.)

In the case of a star (not a perfect blackbody), since it is always hotter than its surroundings does it mean is constantly emitting radiation so that the temperature of the surroundings approaches the temperature of the star? (just like when a hot object transfers heat to a cold object)
Yup. You can generally imagine that a black-body emits and absorbs heat like anything else, really. Except it's better at it.

"Then the black-body will emit radiation IF it has any energy left."
With this, is this radiation classified as blackbody radiation or would it just be thermal radiation since there is no absorption and only emission? (absorption rate =/= emission rate)
Black-body radiation is just thermal radiation (which is really just EMR) but emitted by a black-body. So regardless if a black-body absorbs energy or not (it generally will unless you make the assumption it exists in an sufficiently large void), it's still black-body radiation.