How are they units for heat, light and microwave satellite noise?
The Kelvin is properly a unit of temperature. It’s possible to have a conversion from temperature to other physical quantities, but Kelvins themselves do not measure these quantites. You mention heat, for instance. The conversion factor between heat and temperature for a substance is called its specific heat. The specific heat of one mole of water is 4.186 J/K (Joules per Kelvin), so one mole of water which gets heated by 1 Kelvin gains 4.186 Joules of heat. But it would be wrong to say that it gains 1 Kelvin of heat.
I beg your pardon.
Why are Kelvins used to measure not only temperature, but light intensity and the noise in microwave signals?
I’ve never seen light intensity quoted in [sup]o[/sup] Kelvin.
Electrical thermal noise is proportional to the product of the Boltzman Constant, the temperature in [sup]o[/sup] K and the bandwidth.
It’s not the intensity of light that’s measured in Kelvins, it’s the color of light.
Similar to the noise measurement, it’s the color a hypothetical blackbody would be at that temperature.
When Kelvins are used in radio (and therefore microwave) they refer to blackbody radiation.
An object at a given temperature (measured in kelvins) emits a certain intensity, given by the blackbody formula.
Radio engineers (and astronomers) often express the intensity of noise in the system as the temperature of an equivalent blackbody filling the beam of the antenna. This is called the system temperature, T[sub]sys[/sub].
The total intensity recieved by the telescope can be expressed as the antenna temperature, T[sub]A[/sub]. It’s the eqivalent blackbody temeperature of the source if it were spread out through the beam (or its actual temperature, if it’s a blackbody and it’s unresolved–which a microwave antenna is probably not 
but an astronomical object could be.) If you want to detect your source, then T[sub]A[/sub]>T[sub]sys**, so you can see that a smaller T[sub]sys** implies a better, less noisy antenna.
And, David Simmons, officially in SI the unit is a Kelvin, not a degree Kelvin. It’s [sup]o[/sup] F or [sup]o[/sup] C, but just plain K.
When Kelvins are used in radio (and therefore microwave) they refer to blackbody radiation.
An object at a given temperature (measured in kelvins) emits a certain intensity, given by the blackbody formula.
Radio engineers (and astronomers) often express the intensity of noise in the system as the temperature of an equivalent blackbody filling the beam of the antenna. This is called the system temperature, T[sub]sys[/sub].
The total intensity recieved by the telescope can be expressed as the antenna temperature, T[sub]A[/sub]. It’s the eqivalent blackbody temeperature of the source if it were spread out through the beam (or its actual temperature, if it’s a blackbody and it’s unresolved–which a microwave antenna is probably not but an astronomical object could be.) If you want to detect your source, then T[sub]A[/sub]>T[sub]sys[/sub], so you can see that a smaller T[sub]sys[/sub] implies a better, less noisy antenna.
And, David Simmons, officially in SI the unit is a Kelvin, not a degree Kelvin. It’s [sup]o[/sup] F or [sup]o[/sup] C, but just plain K.
When Kelvin is used to describe a light source, it refers to the spectral distribution of the emitted photons. Specifically a rating of say 7800 Kelvin is meant to imply that the spectrum emitted the light source is similar to that emitted by a black body heated to 7800 degrees Kelvin. The sun emits photons with the same energy distribution as a 7800 degree black body, so a 7800 degree lamp could be used to imitate it. A 1500 degree lamp would work better for imitating the light of a campfire.
-I see I was too slow here :eek:
Thanks, it is lighting for reef aquariums, where various creatures need special colors of light.
Thanks all. Boltzman and blackbody radiation I remember from physics.
The symbol is capitalized, but the SI unit is the kelvin (small kay). The word is capitalized when speaking of the [essentially obsolete] “degrees Kelvin.”
How does it come to pass that?
The Watt is capitalised, right, because it is named after James Watt; the Volt after Count Volta.
The kelvin is named after a guy named Kelvin.
The current convention is not to capitalize watt or volt either. (Or henry, ohm, newton, etc. Contrast with British thermal unit, where the “B” is capitalized.) Note that, as bibliophage points out, the abbreviations for many of these units are capitalized, but not the words for the units themselves.
What about Hertz?
When used as a unit of measurement hertz isn’t capitalized either. I believe the convention in the most modern incarnation of the metric system, the International System of Units (or Système International D’unités, hence the abbreviation SI), is that the names of units aren’t capitalized, whether derived from proper names (“newton”) or not (“meter”).
You mean, Peter Meter wasn’t a real person??
“How are they units for heat, light and microwave satellite noise?”
Lotsa good posts but none are quite exactly correct in a literal sense:
Answer - kelvins aren’t units for heat, light and microwave noise. A kelvin is the SI unit of temperature, by definition 1/273.16 of the triple point of water.
Nice answers, though, to the implied question of how heat, light and microwave noise may be indexed or expressed with temperatures in kelvins.
Thanks for the info. Now when I use [sup]o[/sup]K I’ll know that it’s wrong. But I think that the form I use makes it clear what I mean and I’m much too old to change.
His real name was William Thompson. He was made a Lord for his work in physics, and took the title Kevin after the River Kelvin, which flows by The University Of Glasgow where he worked.
The Watt is named after James Watt, in honour of his work with steam engines at … The University Of Glasgow.