202 votes

If I stood next to a piece of metal heated to a million degrees, but in a perfect vacuum, would I feel hot?

I'm more so asking about thermal radiation emitted by it. Here's a quantitative estimate. Suppose that the hot plate remained intact long enough to do the experiment. For a rough estimate, we can ...
129 votes

Why do metals only glow red, yellow and white and not through the full range of the spectrum?

The physics of why the heated metal glows like a black body has already been thoroughly covered in the previous answers. However, in order to completely bridge the gap with the physiology of color ...
  • 2,231
112 votes

Why don't we use infrared light to heat food?

We do use (near) infrared radiation to heat food – whenever we toast food or grill (UK)/broil (US) by beaming infrared downwards on to food! The point is that the infrared is strongly absorbed by the ...
  • 29.2k
96 votes

Why doesn't the brightness of a bulb change with time?

Two reasons: An incandescent bulb glows not (directly) because it has electricity going through it, but because it is hot. Even when the power going through the bulb decreases, it takes some time for ...
  • 16.8k
91 votes

Explain it to me like I'm a physics grad: Greenhouse Effect

Executive summary: Carbon dioxide in the atmosphere absorbs some of the energy radiated by the Earth; when this energy is re-emitted, part of that is directed back to Earth. More carbon dioxide $\...
  • 117k
90 votes

If I stood next to a piece of metal heated to a million degrees, but in a perfect vacuum, would I feel hot?

The other answers provide good explanation why your friend is wrong in this case. I just want to point how you could both easily reach the same conclusion without knowing much of the physics involved: ...
  • 2,898
88 votes

Why do we need insulation material between two walls?

You can think of thermal conductivity as a measure of how readily heat will flow through the material while it is stationary. The low thermal conductivity of air means that it takes a long time for ...
  • 55.8k
82 votes

Force of photons from the Sun hitting a football field = weight of 1 dime?

Photons are massless so their weight is 0. However, photons do have momentum so they can exert force. This force is due to their momentum and would occur even in the absence of gravity, so it is not a ...
  • 71.3k
78 votes

Could I survive at (or near) absolute zero with a very, very, very thick sweater?

A super-thick sweater probably isn't the way to go - you may be better off wrapping yourself in aluminum foil. The body loses heat through a handful of mechanisms: During conduction, your body ...
  • 55.8k
76 votes

Does anything in an incandescent bulb actually reach its color temperature (say 2700 K)?

Does any component in an incandescent lightbulb actually reach temperatures in the thousands of degrees? Yes, the filament. This is why the filaments are made of tungsten, which has a melting point ...
  • 1,892
75 votes

Why are stars white?

It has more to do with physiology of the eye rather than the spectrum of light produced by stars. Stars emit light over the full range of visible wavelengths. Hot stars emit more blue/violet light, ...
  • 114k
70 votes

How does the Earth's center produce heat?

Heating because of high pressure is mostly an issue in gases, where gravitational adiabatic compression can bring up the temperature a lot (e.g. in stellar cores). It is not really the source of ...
59 votes

Why does the Earth cool at night time?

It emits infrared radiation to space. This happens constantly, not only during the night, but during the day the net energy flux is positive because the amount energy coming from the Sun is much ...
  • 15.6k
59 votes

Why are solar panels kept tilted?

First, not every solar panel in India is oriented towards the south or tilted at 45°. One of the world's largest photovoltaic power stations is installed in Kamuthi (9.3°N, Southern India), with pv ...
  • 1,759
58 votes

Why most distribution curves are bell shaped? Is there any physical law that leads the curves to take that shape?

First, distributions are not always bell-shaped. A very important set of distributions decrease from a maximum at $x=0$, such as the exponential distribution (delay times until a random event such as ...
57 votes

Emissitivity of copper - Why are copper heatsinks all shiny?

Radiative heat transfer is not dominant at the temperatures at which computer/electronic heatsinks operate, so the emissivity of the heatsink fin surfaces is not important for their operation. ...
54 votes

How much additional mass does the Earth gain each day from solar radiation?

There's an answer to your question, but it's not all that meaningful. The sun strikes the Earth with $1.5\cdot10^{22}J$ of energy every day. Using $m=\frac{E}{c^2}$ we find this has a mass ...
  • 43.3k
53 votes

How to identify if a photon comes from the sun?

Photons are identical particles characterized by energy and a direction of propagation. If you see just a photon, without any other information, from these two properties, you cannot distinguish a ...
49 votes

What would a blackbody sound like?

This problem can be solved with noise-shaping. Since the shape of the spectrum is known, it can be used as a base for the power spectral density: $$ P(f,T)=\frac{ 2 h f^3}{c^2} \frac{1}{e^\frac{h f}{...
  • 1,118
48 votes

Why do we need insulation material between two walls?

If the air gap between the walls is wider than approximately 0.5 inches, the warm wall will heat the air, causing it to rise. The cold wall will cool the air, causing it to fall. This will set up a ...
  • 11.5k
47 votes

Why is the Sun approximated as a black body at ~ 5800 K?

Yes - the integral of the spectrum you refer to gives the total power per unit area emitted from the surface of the sun. If you multiply that by a factor of $\left(\frac{\text{Solar Radius}}{1\text{ ...
  • 55.8k
47 votes

Why doesn't hot charcoal glow blue?

Any black body emits radiation of all frequencies. So a hot charcoal definitely emits some blue light. It doesn't appear blue, because most of its light is red, orange, or yellow. To shift the ...
  • 12.1k
45 votes

Why doesn't the brightness of a bulb change with time?

We already have great answers, but I'll add an anecdote to this: I once played around with several light sources and a scientific camera. The overall experiment was not overly scientific, and I only ...
43 votes

Cooling a satellite

Typically, satellites use radiative cooling to maintain thermal equilibrium at a desired temperature. How they do this depends greatly on the specifics of the satellite's orbit around Earth. For ...
  • 24.1k
43 votes

How many X-rays does a light bulb emit?

The formula you want is called Planck's Law. Copying Wikipedia: The spectral radiance of a body, $B_{\nu}$, describes the amount of energy it gives off as radiation of different frequencies. It ...
  • 1,076
42 votes

Why does thermal radiation only occur at infrared and visible frequencies?

You are correct. Thermal radiation can have any frequency at all; it depends on the temperature of the radiating body. However, most bodies in the universe have temperatures that make them emit most ...
  • 10k
40 votes

Why is the color of lightning white or blue rather than nothing?

TL;DR: Air in lightning gets hot. Hot things (like the Sun) emit light in a broad spectrum; including visible. You are right there will be emissions outside the visible - but your eye doesn't pick ...
  • 117k
40 votes

If you were invisible, would you also be cold?

Not necessarily. Glass is transparent and it can be heated with convection, conduction and some radiation. It will maintain any temperature around it.
  • 3,783
39 votes

Why don't Wien displacement law curves cross?

Here are two reasons they can't cross, one from statistical mechanics and one from pure thermo. Microscopically, each frequency of light is produced by an independent mode. So asking why the curves ...
  • 96k
39 votes

Why do metals only glow red, yellow and white and not through the full range of the spectrum?

When metals (or any materials) get very hot, they emit "black body radiation". It's a funny name, because even "non black" bodies emit this thermal radiation. There may be some (small) deviations from ...
  • 117k

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