Home | Science | Why is the Sky Dark at Night? | * Why is the Sky Dark at Night? 02: Some Elementary Physics 03: The Heat Pump 04: The Rubber Band Experiment 05: Olbers' Paradox 06: Red Shift 07: Summary 08. Program Notes 09. The Space Applet 10. Reader Replies to "Why is the Sky Dark at Night?" Share This Page

Why is the Sky Dark at Night?
A discussion of Olbers' Paradox, the Big Bang and related issues — 20th anniversary edition.

Copright © 1996 - 2016, P. Lutus

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Some Elementary Physics

Here are some basic ideas that you will need to know to understand what follows:
• Heat energy flows from hot areas to cold areas, heating the cold areas until the temperatures are equal.
• In an enclosed space with no avenue of escape for energy, any source of heat, no matter how small, will eventually heat the entire space to its own temperature.
• If you change the volume of a gas, you also change its temperature. If you decrease the volume, the temperature rises. If you increase the volume, the temperature drops.
 Temperature Kelvins 0 Pressure KPa 0
Here is an example of the last point. If you drag your pointing device on the image at the right, you will make the barrier move against the gas volume and change its pressure and temperature1.

There are several ways to explain this effect. One way is to imagine a single gas molecule colliding with the descending barrier. In this case, the barrier's velocity is added to the velocity of the molecule as it bounces off, thus increasing its speed and its temperature. Conversely, as the barrier rises, a gas molecule that collides with it will recoil with less velocity, therefore its temperature will drop.

Another, more general, way to explain this effect is to realize that, when you compress the gas, you are making a fixed amount of energy occupy a smaller volume, "concentrating" it, so to speak. This equal amount of energy in a smaller volume raises the temperature of the gas.

1. This virtual engine disregards some second-order effects, so don't depend on its pressure / temperature readout if you're designing a rocket — or, for that matter, a toaster.

 Home | Science | Why is the Sky Dark at Night? | * Why is the Sky Dark at Night? 02: Some Elementary Physics 03: The Heat Pump 04: The Rubber Band Experiment 05: Olbers' Paradox 06: Red Shift 07: Summary 08. Program Notes 09. The Space Applet 10. Reader Replies to "Why is the Sky Dark at Night?" Share This Page