Space physics--some key principles

(under construction)

by Wm. Robert Johnston
last updated 2 February 2005


A molecule is the smallest component of a substance that preserves its properties. An atom is the smallest component of an element preserving its properties. Molecules are combinations of atoms, joined by electron interactions. A neutral atom includes protons and (usually) neutrons in a compact nucleus, surrounded by electrons.

Chemical reactions rearrange atoms in molecules, nuclear reactions rearrange particles in nuclei.

Four states of matter:


Electromagnetic radiation

Radiation includes more than just high-energy nuclear radiation; it also includes the visible light which is now allowing you to read this sentence.

Electromagnetic radiation--any type of energy transmitted by electric and magnetic field waves. They travel at the speed of light and can cause charged particles (like electrons in atoms) to oscillate and move.

EM radiation is classified by the amount of energy in individual waves:

The very small portion of this electromagnetic spectrum that our eyes are sensitive to is called visible light. Different colors of light represent different energies. From low to high energy, visible light includes red, orange, yellow, green, blue, and violet.

<-------------lower energy higher energy------------> radio waves microwaves infrared visible ultraviolet x-rays gamma rays

From lower to higher energy, here are examples of where these occur:

These categories are based on different ways that these waves interact with matter (which also gives different applications). White paper reflects all colors of visible light. Blue paper reflects blue and absorbs the rest. Visible light does not go through wood and brick walls, but radio waves and gamma rays do, to a degree. Visible light goes through glass windows, but some ultraviolet does not.

Infrared goggles take this invisible infrared glow and convert it to visible light for the viewer to see. (Low light goggles, on the other hand, enhance small amounts of visible light.)

Gamma rays, x-rays, and the highest energy ultraviolet waves have enough energy in the individual wave packets (photons) to knock electrons from atoms, ionizing the atoms. These types of radiation are thus called ionizing radiation. The individual wave packets for any other type of electromagnetic radiation cannot ionize matter.

Blackbody radiation

Blackbody radiation is EM radiation given off by all matter, as a function of the matter's temperature. Hotter objects give off more total EM radiation, and also give off a greater fraction of that radiation as higher energy radiation.

Gas and plasma behavior

Orbital mechanics

Orbits of planets and moons and comets, the arc of a tossed ball, the straight down plunge of a dropped book--all represent freefall motion under the influence of gravity.

Consider an object thrown horizontally sideways: it curves down until it hits the ground, going further if it is thrown faster. Thrown sufficiently fast, it will take so long to curve down that the Earth's surface will curve away that fast. The object will then miss the surface, continuing to curve around the Earth in an orbit.

While in freefall, an observer experiences weightlessness. The observer is falling, so there is a gravitational force on them, but there is no apparent gravity in their frame of reference.

The force of gravity decreases with distance. Extended objects in freefall are attracted differently at different points, due to varying distances from the attracting object. At the center of mass of the object one would observe weightlessness, but would observe a stretching force towards and away from the attracting object. These are tidal "forces".

We might say pseudoforces, since this is only a force to the observer in that moving frame of reference. The same is true for centripetal "force", the apparent outward gravity experiences by an observer in a spinning frame of reference.

Thus, a space shuttle orbiter orbiting the Earth is in freefall, constantly falling towards the center of the Earth, but always missing. Astronauts aboard in this state of freefall feel weightless. NASA now calls this "microgravity" instead of "zero gravity" to acknowledge the imperceptible but still measurable tidal forces across the orbiter. But keep in mind the Earth is exerting a force on the orbiter and the astronauts, forces about 88% of what they experience while on the Earth's surface.

Temperature and heat

Heat--internal energy of an object associated with internal kinetic energy of the particles making up the object. Heat is a form of energy; forms of energy include

Temperature scales:
Fahrenheit (° F)Celsius (° C)Kelvin (K)
-459-2730absolute zero
320273water freezes
5814287average Earth temperature
9938311average body temperature
212100373water boils
1020056005900surface of Sun

Temperature measures heat or specifically the kinetic energy of the molecules, atoms, etc., making up an object. A cold enough object has no internal atomic motion; this temperature is absolute zero.

© 2005 by Wm. Robert Johnston.
Last modified 2 February 2005.
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