Ions

An ion is a particle that is similar to an atom but has more or less electrons than protons in the nucleus. In most cases, ions are present in compounds that have a metal and a non-metal (like NaCl, common table salt), but can also be present in hot gases or plasmas, like the Sun.


If an ion has more electrons than protons, i.e. it is negatively charged, we call that an anion. In the above example (NaCl), the chlorine ion has one more electron than proton (18 electrons vs. 17 protons) and therefore, has a charge of -1 e (-1.602e-19 coulombs). If the ion has less electrons than protons, it is a cation. The sodium ion in NaCl has one less electron than proton, so has a charge of +1 e.


Why are ions are important? They are found in the spectra of stars, in nebulae, and as stated above many compounds that we use everyday. In the Sun or any star, the gas is so heated, that it can strip electrons from the atoms, ionizing the gas, converting it into a plasma.


There are some elements that do not ionize easily. We call these elements inert and they are found on the far right of a periodic table. These are the noble gases: Helium, Neon, Argon, Krypton, Xenon, and Radon. This does not mean that they cannot be ionized. Under normal temperatures and pressures, the outer electrons are not readily removed from their orbits. However, under extreme temperatures and pressures, electrons can be stripped from the outer shells. In fact, the alpha particle is the bare nucleus of a helium atom with a charge of +2 e. We find helium ions in the core of the Sun, as it is the final product of the proton-proton chain. Electrons do not easily combine with the bare nucleus to form a stable helium atom.


Next time, we will learn about ion engines and how they may be a future propulsion system for solar system travel.

Interference

Waves of any sort can be combined by a process called interference. The way we can look at it is that interference is the final wave when two or more waves are superimposed on each other. If you need a refresher on waves, click here.

There are two types of interference: constructive and destructive interference. Constructive interference is when two waves are in phase and when combined, the final wave has a higher amplitude (the combined position of the crest and the trough of the wave - can be thought of as the total height of the wave) than the starting waves. Destructive interference is when the waves are combined in such a way that the amplitude of the final wave is smaller than the initial waves or even non-existent.

To look at constructive interference simply, let's take a look at just two waves. If the waves are in phase, the crests of both waves (and in turn, the troughs) will line up, and the final amplitude of the wave will be just the sum of the amplitudes of the original waves.

Vice versa, if the waves are out of phase by 180°, the crest of one wave will align with the trough of the other. The amplitudes cancel out. This is destructive interference.

In reality, most interference involves more than two waves that are necessarily in phase or 180° out of phase. The waves themselves might not have equal amplitudes either, so the combined wave may look a little more funky than what we see above.

 

You can actually experiment with interference in your own home. Surround-sound stereo systems work on the principle of interference for around waves. If you stand equidistant from either speaker, you hear the constructive interference of both speakers. Turn one speaker off, and the amplitude of the sound is lower. There are actually spots in your surround-sound system where there will be destructive interference, but these locations depend on the wavelength of the sound waves.

Reflection

The reflection of light is just the bouncing of a light wave or a photon (however you want to look at it) from a surface. The reflection of light off a flat surface is pretty simple to understand.


Light comes in at an angle and is reflected off the surface at the same angle. Using physics terms, the angle of incidence is equal to the angle of reflection and are measured from the normal to the surface. Recall that the normal to a surface is an imaginary line that is perpendicular to that surface, in this case, a perpendicular to the point of reflection.

In cases of spherical mirrors, the reflection gets a little more complicated. If you have light coming into the mirror in parallel beams or rays from the same source, the light will not be reflected to the same point. This is called spherical aberration.

But the objective mirror of a reflector is a curved mirror! And they don't suffer from spherical aberration. How does this work?


Instead of using spherical mirrors, reflectors use parabolic mirrors. A parabola is a shape that has a focal point. This way, when a mirror has a parabolic shape, parallel light beams will be reflected to the same point in front of the mirror. A reflector then will use a secondary mirror to reflect the light off to the side (Newtonian reflector) or back down the end (Cassegrain reflector).

Parabolic mirrors are not as extremely curved as the one shown above, but I've exaggerated the curvature so that it can be seen how all incident light parallel to each other (i.e. coming from the same source) will reflect off the mirror and reach a common focal point on the reflected path.

See my post on telescopes for more about the different types of telescopes.

 

Friction and Driving Fast

This is not an astronomy post, but one about physics. In general, it's about driving safely for conditions on the road.


Every winter and during rain storms, I see people driving like the roads are completely dry. The worst offenders are SUV drivers. The reality is that driving fast in wet conditions on the road is not only stupid, but extremely dangerous.

In this picture, Fg is the force due to gravity on the car. This is the same as the weight of the car. N is the normal force to the surface of the road. You can think of the normal force as the force that pushes up on the car by the road. In this case, the normal force equals the weight of the car. As the car travels at a constant velocity, there is no force acting along the road itself, so v is constant.


Let's change things up and say the car is accelerating. There is a frictional force, Ff acting opposite to the acceleration. In this case, to accelerate the car, the car has to overcome a static frictional force to begin to speed up. Once it overcomes the static frictional force, the frictional force becomes kinetic and is generally smaller than the static frictional force.

Now, suppose you want to slow down. You apply the brakes to the car and your velocity decreases. The acceleration of the car is pointed in the opposite direction of travel. (Vector physics). The acceleration of the car is dependent on two things: gravity and the coefficient of friction between the road and the tires. Now, if you assume that all tires are the same (and I know they are not), this is independent of the mass of your vehicle! For dry conditions, μ is less than one, but larger than for dry conditions. As the pavement becomes wetter and slicker due to rain, snow, or ice, μ decreases. So if you are travelling at 50 mph in wet conditions, it takes you longer (both in time and distance) than if the roads are dry.

So, please, slow down when the roads are wet, or please make sure there is more space between you and any vehicles in front of (or behind) you.