Goals: The student will learn|
The student will learn
- How the Sun supplies almost all the energy we use.
- About radiation emitted by hot objects ("black body radiation").
- About the balance between incoming solar energy and outgoing heat, radiated by Earth back into space.
- About the "greenhouse effect" and gases that contribute to it.
- That convection of heat in the atmosphere is the cause of weather phenomena.
- That water vapor also carries solar heat and plays an important role in atmospheric convection.
Terms: radiation (visible, infra-red, ultra-violet), radiation balance, greenhouse effect, greenhouse gases, ozone, convection, thermal currents, buoyancy, stratosphere, troposphere, (humidity)
Starting the lesson:
Today we start our discussions of the Sun, by first looking at the effect Sun has on our environment on Earth. Almost all the energy used by people on Earth comes from the Sun. Can anyone give an example?
- Food grown by crops exposed to sunlight, or obtained from animals which eat such crops.
- Solar cells that power satellites, calculators etc.
- Solar water heaters on rooftops.
- Cars use gasoline or other liquid fuels that originate from fossil plants.
- Electricity may be generated by coal--from fossil plants, too.
- Windmills are powered by winds, whose motion is caused by solar heat as discussed later. (Ocean wave energy also comes from winds.)
- Hydro-electric power comes from water descending from mountains: it was lifted into rain clouds by the Sun's heat.
- In addition, all the water we drink is distilled by sunlight.
Were it not for the Sun, all water would be salty.
- Sunlight dries laundry strung on a line.
- In some countries, sunlight is used to produce salt from
sea-water (in ponds) and to dry tomatoes, figs, fish etc.
Do you have an example of energy not from the Sun?
(may be skipped if time is short)
- Nuclear power, from splitting up the nuclei of heavy atoms, as will be discussed in section #S-8.
Nuclei carry a net positive electric charge, because of the positive protons which they contain. A heavy nucleus of uranium or plutonium can be made to split into two parts, each positively charged, and because the fragments strongly repel each other, each gains a lot of speed and energy. As the fragments collide with matter around them, their energy becomes heat, that heat is used to create high-pressure steam, which turns turbines turning electric generators.
(Teacher draws diagram on the board: fission in reactor becomes heat in a boiler, which turns turbines to generate electricity.)
- Geothermal heat, e.g. the heat of volcanoes, geysers and hot springs, on land and on the ocean floor. In some places (e.g. Iceland) geothermal steam is used to produce electricity. This energy comes from radioactivity.
Rocks contain a very small amount of radioactive uranium and thorium (and radioactive substances created by them, "daughter products"), as well as radioactive potassium. All these emit fast particles which heat up the surrounding material. This is a slow process, governed by chance, and the probability of any atom of the above substances undergoing "radioactive decay" in a given year is less than one in a billion.
However, even a weak heating process can create high temperatures, if the heat has difficulty escaping.
You can think of an area of 1 meter2 in the ground on which you stand as the top of a deep wedge running all the way to the center of the Earth. On the average, the heat produced by radioactivity in that wedge must escape through that same area. So much volume, so little surface! That is why the Earth deep down has a temperature of thousands of degrees, able to melt lavas and produce volcanos.
- The tides, raised by the pull of the Sun and the Moon. In principle, tidal energy could be exploited, except that technically it is very difficult to run turbines on small differences of height.
- (Someone may mention starlight: yes, in principle...)
Then go into the lesson. Questions about the material:
How is the temperature of the Sun related to sunlight?
- The Sun radiates because it is hot. All hot objects radiate, though sometimes the radiation is outside the range of the human eye.
What does sunlight tell about the Sun's temperature?
- Judging by its color distribution, the emitting layer of the Sun has an absolute temperature of about 5780° (degrees centigrade from the "absolute zero" of -273.1° centigrade, at which all heat motions cease). The Sun's surface temperature will be again discussed in a later lesson.
What does this say about the surface of the Sun?
- The material on the surface of the Sun cannot be solid or liquid, because at 57800 all known substances are vaporized. In fact, some will be ionized, with at least one electrons ripped off and floating independently nearby.
The top layer of the Sun, from which sunlight comes, is known as the Sun's photosphere. All words that start with "photo" are related to light. Know any?
- Photography and words derived from it--photoelectric flash, photogenic, Photostat (picture) etc.
Also photosynthesis, photo-electric cell (or "photo-electric eye"; also "phototube"), photonphot (unit of light, favorite in crossword puzzles), photolysis (chemical dissociation caused by light) etc.
If the Sun constantly heats the Earth, how come the Earth does not heat up?
- The Earth also radiates back into space, emitting infra-red light (IR) which our eyes cannot see. On the average, the energy the Earth receives is balanced by the energy it loses.
How can the Earth radiate back as much as it receives from the Sun, if its
absolute temperature is only around 3000, while that of the Sun is 57800?
- Take an area A on the surface of the Earth. It receives "hot" sunlight only from a small patch of the sky, about 0.50 across, but it radiates back infra-red (IR) to half the sky. Clearly, if incoming and outgoing energy flows are to be equal, it does not have to shine anywhere as brightly as the Sun!
You probably know that with a magnifying glass you can use sunlight to create a lot of heat--enough even to start a fire. Why?
- (Someone may say "because it concentrates the Sun's light.")
Correct. Is there a limit to the temperature you can get?
- Someone may say "no" because all the light seems to converge to a tiny point.
Actually, you cannot get a greater temperature than that of the surface of the Sun. Here is why.
Suppose you use a magnifying glass to set a piece of paper on fire. Viewed from the point on which the sunlight is focused, the Sun is magnified--no longer 0.50 across but maybe 30, 50 or 100. Its heating power is then magnified by the square of the ratio between the angles--36, 100 or 400 times, because the heat the paper receives goes like the area of the bright patch which shines on it.
There exist solar furnaces with arrays of mirrors that can magnify the Sun even more, so that viewed from the focus, it covers (for instance) 1/10 of the sky. The heat generated may be intense enough to melt iron. However, as the object at the focus heats up, it also radiates away more heat!
Suppose that by some clever arrangement of lenses and mirrors we have managed to illuminate the object at the focus from all directions. Whichever way it "looks" it sees the Sun. At equilibrium, the heat it gets equals the heat it radiates back. How hot does it get?
Suppose it is hotter than the Sun. If brightness depends only on temperature (very nearly true), then it is also brighter than the Sun, and in any direction it radiates back to the Sun more than it receives. That is obviously impossible, so we conclude that the sample can never exceed the temperature of the photosphere which provides its light in the first place.
(end of optional discussion)
So far we have assumed the Earth receives visible light radiated by the Sun and radiates it back into space as infra-red light, less concentrated but in all directions.
Actually this assumption is more appropriate for the Moon or for the planet Mercury. It is not quite accurate for Earth. Why?
- Clouds reflect heat before it reaches the ground
- Air absorbs infra-red light
Correct. One certainly can understand the effect of clouds.
But with the atmosphere--isn't any energy absorbed by the atmosphere radiated out again--or else, the atmosphere would get hotter and hotter?
Yes, it is. Anything absorbed by the atmosphere is radiated away again. But some of it is radiated downwards and returns to Earth! Therefore the existence of the atmosphere impedes the outflow of heat.
Why is this process called "The Greenhouse Effect? "
- Because the same process keeps glass-covered greenhouses warm. The Sun heats the ground and greenery inside the greenhouse, but the glass absorbs the re-radiated infra-red and returns some of it to the inside.
What substances in the atmosphere contribute to the "greenhouse effect"?
What is ozone?
- Ozone is a form of oxygen, forming the molecule O3 instead of the usual O2. It forms in two places: high in the atmosphere, between 18 and 50 km (peak at 25) where it is beneficial in absorbing ultra-violet light, and near the ground, part of urban air pollution. Ozone is very reactive chemically, so near the ground it erodes paint and is unhealthy to breathe.
Is ozone a greenhouse gas?
- Yes, it is one, too. It is true it also prevents solar UV from reaching the ground, and that reduces the heating of the ground: but it is a very slight effect, because UV does not carry much energy. Of greater concern are the harmful effects of increased UV on the skin and eyes.
However, ozone also absorbs infra-red light. As far as the heat balance is concerned, this effect is more important. Overall, therefore, the presence of ozone helps keep the Earth warm.
What other processes affect the heating and cooling of the Earth, besides
absorption and emission of light and of radiations like IR and UV?
Someone may say "heat generated by human activities":
- Big cities are indeed slightly warmer, but overall the heat
released by human activities is too small to have any great
Someone may say "weather":
- Well--yes. But what are the two things that characterize weather besides the temperature of the air?
So let's talk about wind, and then about rain--or to use more technical terms
- The flow of air
- The evaporation and precipitation of water
Why does warm air rise?
- Warm air is less dense than cold air, and therefore floats up (the way oil that is less dense floats on top of water).
How does a hot-air balloon work?
- A burner heats the air that fills the balloon: it is lighter than the air around it and rises.
What does rising air have to do with the warming of the Earth by the Sun?
- It is one way the ground removes heat: it warms up the air near it, which rises. Later, higher up in the atmosphere, the air radiates its heat out to space, cools down and gets denser, then sinks down again, replaced by warm air that is still rising. This is known as the convection of heat.
How is heat convected near a cold window? How does air flow there?
- It flows downwards. Air touching the window cools, gets heavier and drops to the floor. It then spreads into the house where (if the house is heated) it is warmed up again. Meanwhile other air is replacing it near the window and is getting cooled in its turn. You can get a "cold draft" from the window even if it is shut tight!
(Note: Fiberglass is a good heat insulator, because it prevents the air inside it from flowing. As long as the air does not move, it does not carry away much heat. Wool blankets and sweaters work the same way. "Double windows" with two panes set close to each other hamper the up/down flow of air and also reduce heat loss.)
Convection in the atmosphere stops around 10-15 kilometers. What is the layer above that called, and what do you know about it?
- It is called the stratosphere and is pretty stable and cold (though its higher layers get heated by ultra-violet, absorbed by ozone.) The region below it--where weather is found--is the troposphere, and the boundary between the two is the tropopause.
(If you have seen an isolated thunderstorm from a distance, you might have noted that its top is flattened and spread out. It is flattened against the bottom of the stratosphere, which blocks the convection of the storm from rising any further.)
What role does water play in moving the Sun's heat outwards?
- Part of the energy of sunlight goes to heating the ground, but another part evaporates water, from the oceans, lakes, rivers and plants.
As warm humid air rises, it carries energy in two forms--some as heat, some as humidity.
What causes rain?
- As warm air rises, it expands (air pressure is smaller high up) and cools. Cold air cannot hold as much water and some is forced out, creating clouds and rain.
It is well known in most of the world that mountains get more rain than the plain below them.
[The musical play "My Fair Lady" has the line "The rain in Spain stays mainly in the plain". In the play this serves as an exercise for learning to pronounce the sound "ai", but it need not be taken literally!]
Why do you think it is so?
- When the wind blows humid air towards mountains, the slope of the land forces the air to rise, making it expand and cool. If it starts holding an appreciable amount of water, as it rises and cools, some of that water must be given up as rain.
This can happen when wet air is blown from the sea towards a range of mountains. Frequently, if the mountains just form a ridge of limited width, when the land goes down on the other side of the ridge, the land is dry, or even a desert. Why?
Past the top of the ridge, the land slopes down again, and air is pushed by the wind along it. It then gets a little denser and a little warmer, and immediately it stops giving up any more water.
One can see this in California. The islands of Hawaii, too. tend to have a wet side (where seasonal winds come from) and a "dry side" facing away from the wind.
Madagascar is like this, and many other places.
On a summer morning you get up and find the grass outside covered with dew--with drops of water. How did they get wet?
- The ground cooled down during the night, and cooled the air. The cooled-down air had to give up some water, and deposited it on the grass.
When humid air gives up water as rain, does that speed up its cooling or slow it down?
- Slow it down. When sunlight evaporated that water, it invested energy in the process. But energy in nature is always conserved! When the water is forced out, that energy is returned to the air, adding to its heat.