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Angle between ecliptic and Milky Way

(Continuation)--About the year 2012

396A Posssibility of Asteroid Hitting Earth (1)
396B Posssibility of Asteroid Hitting Earth (2)

If you have a relevant question of your own, you can send it to
stargaze["at" symbol]phy6.org

114. Why not use a heat shield going up?

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117. Why not use nuclear power for spaceflight?

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something

Project Orion: The True Story of the Atomic Spaceship

118. "Doesn't heat rise?"

I was helping my wife in her 3rd grade class yesterday and one of the students came up with an interesting question. We were talking about how it gets cooler as you increase your altitude (specifically in the mountains). I was guessing that the temperature drops about 3 degrees for every 1000 ft that you climb.

I was really caught off guard with a question from the "peanut gallery." It was - "Doesn't heat rise?" I said that that is correct, and conversely cold would tend to remain near the ground. He further questioned - "If heat rises then why wouldn't it get hotter as you increase your altitude? I had no explanation. Can you help?

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The answer below is tailored (I hope!) to the 3rd grade level, not an easy task.
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It is true--the atmosphere is hot at its bottom and cool higher up (at least for the first 10 miles or so). It is true even though, as we know, hot air tends to rise! It all happens because THE BOTTOM OF THE ATMOSPHERE IS WHERE AIR RECEIVES ITS HEAT.

That heat arrives when sunlight hits the ground and warms it up. Think of what would happen if no way existed for removing it! The ground would get hotter and hotter--oceans, lakes and rivers would boil away, life would become impossible. Actually, we see rather little temperature change near the ground--just day-to-night fluctuations, and slow changes with weather and seasons. Such observations suggest that on the average, heat is removed just as fast as it is received.

Where can it go? Only one place--outer space, the sky above! We know that anything that is warm shines in some sort of light ("radiates"). A lightbulb filament is hot enough to shine in visible light, but a hot teapot (say) also shines (radiates). Our eye cannot see such "infra-red" light, produced (at a much lower rate) by moderately warm objects, but a hand held close to the pot will sense the radiation, as heat streaming out. (Rattlesnakes have special sensors to detect infra-red (IR) light, helping them find warm blooded prey.)

So at first sight, this appears to be a simple situation. The Sun shines on the ground and gives it heat, and the ground returns that heat to space as invisible infra-red radiation.

But this simple process is made complicated by the so-called "greenhouse effect." Air is relatively transparent, but some gases in it absorb and re-emit infra red very efficiently--water vapor, methane and increasingly, carbon dioxide, so called "greenhouse gases." Put a few drops of India ink in a glass of water, and it darkens appreciably; with these gases, similarly, a little bit of those gases ABSORBS A LOT. By absorbing and re-emitting IR, they make the IR light bounce around, rather than letting it head straight to space.

That makes it difficult for heat to escape, and keep the ground warm; something similar happens in a gardener's greenhouse, enclosed by glass panes, which let sunlight in but absorb IR. Such random bouncing-around continues until the IR (some of it, anyway) reaches a layer so high that not enough air and water vapor are left to send it back, and then the radiation escapes to space. That layer is known as the "tropopause" and it is typically 8 miles up.

This "greenhouse effect" would keep the air warm near the ground, even if our cars and power plants did not emit carbon dioxide (although those emissions make the effect more pronounced). The air then does a second thing to help getting rid of its heat: IT RISES.

Air cools as it rises, because air pressure around it is lower, and an expanding gas cools (that is how air-conditioners work). But because this process is happening everywhere and all the time, the SURROUNDING air is already cooler than the air near the ground. As long at air is warmer than its surroundings, it keeps rising. If a chunk of air started out extra-warm, it may well STILL BE warmer than the air around it, even higher up, so it keeps rising. Ideally, it rises until it arrives near the tropopause, where it can get rid of its heat. After that, being cooler, it sinks down again and is replaced by more heated air from below.

(Why does air pressure get lower as one rises? Because air near the ground is compressed by the weight of all the other air above it. If you rise about 5 kilometers--3 miles and a little bit more--half the air is below you, only half of it is above and contributes to the compression, and so, the pressure there is only half of what it was near the ground. Go up 5 more kilometers and the pressure is about half as much again--a quarter of what it is near the ground. That is where jetliners fly, and the low pressure is the reason their cabins are sealed and pressurized--also why climbers on Mt. Everest carry oxygen bottles).

This process of cooling, rising and finally radiating heat away is very important. THAT IS THE REASON WE HAVE WEATHER! The whole weather process is driven by the heat of the Sun, and by the collection of processes by which the Earth returns heat to space.

The above is very, very simplified, especially since it ignores humidity. Actual air also contains water vapor--water which was evaporated by the Sun and dissolved in the atmosphere, just as sugar dissolves in a cup of coffee. Since the Sun has provided heat energy for the evaporation, water vapor acts a bit like extra heat given to the air; when the water is removed as rain, air gets that heat back and is warmed, which is what drives thunderstorms. More about all these in

http://www.phy6.org/stargaze/Sweather1.htm

119. Have any changes been observed on the Moon?

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"First Light" by Richard Preston

120. Why isn't our atmosphere flung off by the Earth's rotation?

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121. Can kinetic energy be reconverted to work?

I read your article about energy from the following web site:

http://www.phy6.org /stargaze/Senergy.htm

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You zoom on your bicycle down a valley and gain kinetic energy. That energy can help you rise again on the upslope on the other side. Rising against gravity is doing work.
Your car has to stop at a red traffic light. If it is an ordinary car, you press the brake pedal, which pushes brake pads against wheel disks or drums. Friction converts the kinetic energy into heating of the disks or drums, but you can hardly convert that to work.

If however your car is of the new "hybrid" type with electric motors on the wheels (like the Toyota "Prius" or the Honda hybrid), by braking you connect the motors to the car's batteries. The motors act as generators and charge the batteries, turning your kinetic energy into chemical energy of the battery, which can be reused.
The space shuttle in orbit has a tremendous kinetic energy. Upon re-entry, its orbit is nudged into the atmosphere, the shuttle turns so the underside of its wings (covered with heat tiles) faces forward, and air resistance, helped by a shock, converts the energy into heating the air. Not much can be done with that.

If however you link the shuttle to a conducting tether, as was done once (with some problems), the motion of the tether across the Earth's magnetic field lines creates a voltage, which may be tapped, e.g. for charging batteries. See http://www.phy6.org/Education/wtether.html.
Accelerators of high-energy particles used to depend on pulsed magnets. I remember visiting one such accelerator around 1960, the "Cosmotron" in Brookhaven, Long Island. Its magnet needed a large electric current, which created a great amount of magnetic energy. It made little sense to waste that energy every time the magnetic field was allowed to decay.

The solution was a large flywheel, connected to the generator providing the current. When the current decreased, the generator acted as a motor (a bit like that of the hybrid car) and spun up a flywheel weighing a few tons. The next cycle, the flywheel provided most of the energy for generating the magnet's current, slowing down again; only a little extra power was needed to make good friction losses. Thus the energy bounced back and forth between magnetic and kinetic.

Someone pointed out to me that the way the flywheel rotated was carefully chosen. In case the flywheel's bearings somehow gave way, its rotation was such that it would roll through the wall of the building and out into the field--not in the opposite direction, which would have brought it into the crowded accelerator hall.

I hope you get the idea by now.