I have a plethora of questions regarding thermodynamics and all of them came to my mind while I was thinking about how the greenhouse effect actually works. (hence the title)
The sun emits radiation, which after 8 minutes hits the surfaces on planet earth. Here this radiation is reflected OR absorbed and turned into thermal energy.
Now from my understanding, thermal energy is kinetic energy on the particle level. For this form of energy to be transferred or absorbed, the particles would have to hit less energetic particles and thus energize them, which then would make them “warm” right?
Assuming that my explanation is mostly correct, why does space around us not heat up? By that logic, the particles in an imperfect vacuum would have to be highly energetic because of constant unshielded exposure to the light of millions of stars.
Would that also mean that sunlight consists of extremely fast particles that energize surfaces when they hit them? (If not, how does sunlight energize particles.
Why exactly does the thermal energy not just escape into space? Earth seems to be more energetic than space around us. Normally two asymmetric thermodynamic systems would balance out.
Sorry if I’m way off. I’m merely an ignorant student.
In: Physics
You’ve asked a few different questions all wrapped up in one here. Let’s go step by step.
First off, energized particles smacking into other energized particles is not the only way energy transfer can occur. You may have heard that heat can transfer 3 ways: conduction, convection, and radiation. Both conduction and convection requires particles to smack into each other. Radiative heat transfer does not. Particles can absorb electromagnetic radiation (just a fancy word for light) to gain energy, and particles can lose energy by emitting electromagnetic radiation.
> Would that also mean that sunlight consists of extremely fast particles that energize surfaces when they hit them?
So as explained above, no. Sunlight is electromagnetic radiation. It is absorbed by particles which causes them to heat up.
> Assuming that my explanation is mostly correct, why does space around us not heat up?
There’s nothing in space to heat up. Space is empty (well mostly, but for the sake of heat transfer, it’s empty). There is no conductive or convective heat transfer between Earth and space. Only radiative heat transfer. Energy is absorbed by the earth from the sun, and is radiated back out into space as infrared. The system is more or less an equilibrium.
Climate change occurs when increasing amounts of greenhouse gases decrease the amount of infrared radiation emitted by the earth that escapes into space.
When a photon strikes a molecule there are basically two options:
* It is absorbed (and a lower energy photon is remitted shortly after)
* The photon passes through
Which of these happens depends on:
* The frequency/wavelength of the photon
* The specific electron configuration of the molecule
When the photon passes through the molecule we consider it to be transparent to that wave length of light. When it is absorbed and remitted, we say that it is opaque to that kind of light. But a substance that is transparent to one kind of light might be opaque to another. For example, glass is transparent to visible light but opaque to UV light.
So, most of the light from the sun is visible, which our atmosphere is mostly transparent to. It hits the earth and is absorbed and remitted as infrared light (which we feel as heat). Our atmosphere is opaque to infrared light, and does not allow it to pass back out into space, trapping the heat on the Earth.
That is the greenhouse effect.
Much of the thermal energy does indeed still radiate out into space. However, photons that would otherwise be headed towards space may instead be absorbed by greenhouse molecules which then re-radiate the energy in a random direction. Sometimes this direction is space, but sometimes it’s back towards Earth.
Also, the systems don’t balance out the way you propose as even though space is very cold, there’s nearly no matter for conduction or convection to draw heat away from the planet. Thus heat loss is via radiation. But when you have something hindering the radiation of heat into space, the result is that the planet increases in temperature.
The phenomena you are missing is called black body radiation. The particles that transport electromagnetic radiation is called photons and does not have any mass. So they can be created or destroyed as needed. When you have particles that have thermal energy they sometimes spontaniously create such a photon and give it energy. So hot things emit light. The frequency and intensity of this light depends on the temperature. Colder things only emit a small amount of infrared radiation but hotter things emit more radiation and starts to glow red. The hotter an object is the color changes from red to yellow to white and then mostly ultraviolet. The surface of the Sun is in the last category so the Earth gets radiated with lots of ultraviolet light from the Sun which does heat up the Earth as it absorbs the energy. However even though it is relatively cold on Earth the surface still emits black body radiation, but in the infrared spectrum. Because the Sun is so far away and appears so small in the sky we only get a tiny bit of its radiation. However the Earth emits a great bit of radiation in all directions. The temperature have equalized such that the Earth emits just as much radiation as we absorb from the Sun and thereby maintaining a constant temperature.
The issue with the greenhouse effect is that carbon dioxide absorbs infrared radiation but not ultraviolet or visible radiation. So the light comming inn from the Sun goes straigh through the carbon dioxide in our atmosphere and heats up the surface of the Earth. However the radiation from the surface of the Earth is infrared and will therefore be absorbed by the cabon dioxide in our atmosphere. And thus the Earth receives more energy from the Sun then we emit and the temparatures increase.
You’re right! The particles in the imperfect vacuum of space are highly energetic, and have been for billions of years. Each individual atom in space is moving far faster than any individual has atom/molecule on Earth.
Unfortunately, there is no thermal gradient from Earth to space because there isn’t enough stuff in space for the heat to move to. The average density of space is on the order of a few dozen *atoms* per cubic centimeter, and that doesn’t cause large amounts of conductive cooling of the Earth.
Yes, the sunlight warms the planet. As the planet rotates, the sunlight goes away. The warm atoms emit infrared light, actually all atoms can generate some light (what’s called black body radiation) but the greenhouse effect involves infrared.
Some of this infrared light makes it out of the atmosphere and into space, leaving the Earth forever. Some of this light is absorbed by gasses in the Earth’s atmosphere, warming those gas atoms. Those warmed gas atoms radiate some infrared light, half (approximately) going toward the Earth and half going away. Etc, etc, … .
The key to the greenhouse effect is the ratio between the light that “gets free of Earth” and the light that “goes back towards the Earth”. This ratio depends on the makeup of the gas in the atmosphere, because some molecules are much better at “catching” infrared light that others. The good catchers are called greenhouse gasses because they contribute the this rewarming effect.
Actually, this also happens on the sunny side of the Earth, but it’s not as easy to see as the night-time case.
Light impacting the surface does indeed create kinetic energy. But the material affected doesn’t just sit there being warm. That material conducts heat to everything it touches and radiates some of it in the form of deep infrared light.
The atmosphere is not perfectly transparent and absorbs different wavelengths of light to varying degrees. Small molecules like oxygen (O2) and nitrogen allow most long wavelengths pass. But larger molecules, such as carbon dioxide, methane and water vapour catch and absorb more of the light’s energy, especially at low wave lengths.
The result works much like the glass in a greenhouse (hence the name). The atmosphere intercepts very little visible light and a little bit of the infrared. The remaining light eventually hits the surface. Some is reflected while some is absorbed and then radiated away as heat. Some of that heat/infrared is able to escape Earth again. How much escapes and how much gets caught by the atmosphere depends on the abundance of greenhouse gases. More CO2 means more heat gets caught while incoming and more gets caught on its way out.
To make matters worse, that world wide increase in average temperature also means more water evaporating from the oceans, making the air warmer, more humid and even better at trapping radiated heat. Warmer oceans also means the release of more methane from undersea clathrate deposits. Small changes in the levels of one greenhouse gas increases the levels of other greenhouse gases. It’s a positive feedback loop.
Let me start at the beginning. The sun is very hot. Objects at all temperatures emit electromagnetic radiation also known as light. This is called black boy radiation. The peak energy or color of this light depends on the temperature of the object. The sun is hot enough to emit what we call visible light. That light travels through space to reach Earth.
Our atmosphere is made of many gases that absorb specific wavelengths/energies/colors of light. These gases mostly allow visible light through (like from the sun) but absorb lower energy light we call infrared. Some still gets out but you can think of these gases as allowing heat in but not out, like a greenhouse.
The Earth is also relatively warm. It emits light but at much lower energies/colors in the infrared range. The earth is warm because of the light that hits it from the sun. All the energy that reaches the earth is eventually radiated back into space in all directions. Otherwise, Earth would heat up a lot and very quickly.
The light leaving earth is infrared which is partially blocked and prevented from leaving by the gases in our atmosphere. Earth will heat up as it retains heat and emits more and more light until enough emitted light energy leaves through the atmosphere as what comes in from the sun. This is the stable temperature.
To your questions:
1) yes
2a) space does not heat up because heat is something that objects aka matter have. the definition of space is basically a place that (mostly) lacks matter. It therefore cannot be hot.
2b) The few particles in space do not heat up to infinity for the same reason that the earth does not, they emit black body radiation. eventually they reach a stable “temperature”
3) the photons that makeup sunlight can be considered particles and they do indeed heat up a surface when it is hit and absorbs them.
4a) Thermal energy does escape into space as radiation as explained in my paragraphs above.
4b) Earth and space do not balance out for the same reason that your warm house and the cold air outside in winter do not balance out. Earth (like your house) is constantly being heated and there is insulation to prevent this heat from leaving until a particular temperature is reached. Also, the cold receiving end is so much larger than the warm end that you wouldn’t notice any change there.
I hope that answered everything.
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