Air Conditioners–

Hello, and welcome to episode three of the How Things Work podcast. My name is Jamie Aycock, and in each episode we’ll explore the inner workings of things we see and use every day. As it’s about 99 Fahrenheit (37 Celsius) outside, in this episode we’ll explore how air conditioners work.

Most air conditioners (and refrigerators and freezers) have four main components – condenser coils, evaporator coils, a compressor, and an expansion valve. Flowing through this series of tubes, compressors, and valves is a refrigerant like Freon. The basic law of physics behind the operation of air conditioners is called the ideal gas law. The ideal gas law states that the pressure of a gas multiplied by its volume is equal to the temperature times a constant times the amount of the gas. For our discussion, we can simplify that a bit and say that as we pressure a gas, the temperature will increase. Likewise, as we de-pressurize a gas the temperature will decrease. Take a look at the Wikipedia entry for more detail on the ideal gas law if you’re interested.

To give a basic description of how the parts of an air conditioner are arranged in a typical system, the compressor sits outside the building along with the condenser coils. The condenser coils are connected to an expansion valve, which is then connected to the evaporator coils, which are located within the building to be cooled. The evaporator coils connect back to the compressor, completing the loop.

The refrigerant is compressed by the compressor (big surprise there!). Remembering what we said about the ideal gas law, once the gas is compressed we know that it has heated up. This heat has to be dissipated, so the now hot refrigerant flows through the condenser coils. Usually a big fan blows across these coils outside, venting as much of the heat as possible. The now cooler (but still warm) refrigerant flows through the expansion valve, dropping in pressure as it does. As the pressure of the refrigerant drops, the temperature drops as well. The now very cold refrigerant flows through the evaporator coils. Here’s where the big payoff occurs. The refrigerant cools the evaporator coils and air is blown across these coils inside the house or building to cool the building. This cool air is routed throughout the house by the ductwork, spreading relief to everyone.

One way to look at the air conditioner system is as a machine that simply moves heat from one place to another. When we’re trying to cool things off inside, we’re taking heat from the inside of the building and dumping it outside. When we look at the air conditioning system like that, an easy question to ask would be “can we do the opposite to heat the inside of the house?� Absolutely! If your home has a heat pump installed to provide warmth during the cold months, that’s exactly what is happening. When the temperatures dip and you move the switch on your thermostat from “COOL� to “HEAT�, the air conditioner basically runs backwards. The compressed refrigerant flows through the coils inside the building, venting its warmth. The refrigerant then flows through the expansion valve, cooling it off. After cooling off, the refrigerant travels through the coils outside the house (the coils that used to be the condenser coils, now acting as the evaporator coils), warming the refrigerant up a bit (even in the cold of the winter).

That’ll wrap things up for this episode of How Things Work. A transcript of this episode can be downloaded from howthingswork.libsyn.com, and if you have any questions, comments, or topic ideas, please drop me a line at howthingswork@verizon.net. Thanks for listening.

The music you heard was How’d You Know That by Scott Brown. It’s available on Random Chance records at www.randomchancerecords.com and is made available through the IODA Promonet.

 

Citations –

http://home.howstuffworks.com/ac1.htm

http://en.wikipedia.org/wiki/Ideal_gas_law

 

 

Solar Cells –

Hello, and welcome to episode two of the How Things Work podcast. My name is Jamie Aycock, and in each episode we’ll explore the inner workings of things we see and use every day. In this episode we’ll explore how solar cells work.

Before we discuss the details of the solar cell, we need a little background. Silicon has 14 electrons arranged in three shells. The inner two shells are full, but the outer shell has only four of the eight electrons it can hold. Now, our friend silicon is preoccupied with trying to fill up his outer shell – he (or she, I didn’t really look) is always trying to find 4 more electrons. Lucky for silicon, he’s willing to share. Bring two silicon atoms together and each atom will share one of its outer shell electrons with its neighbor– each atom would now have five electrons in its outer shell.  Expand that idea out to having each silicon atom share outer shell electrons with four neighbors and each silicon atom will be full and happy.

Now, let’s cause a little havoc in our peaceful silicon neighborhood. Every once in a while, let’s replace one of the silicon atoms with phosphorus. While silicon has four electrons in its outer shell, phosphorous has five. Phosphorous tries to get with the program and bonds to four adjacent silicon atoms, but that leaves one electron “left over� and not part of a bond with an adjacent silicon atom. The electron doesn’t just run off – the negatively charged electron is held loosely in place by its attraction to the positively charged proton in the atom’s nucleus. Since this now impure silicon has “left over� electrons, and electrons are negatively charged, this is called n-type material.

In a similar fashion, we can take another slab of silicon and replace a few of the silicon atoms with boron. Boron has only three electrons in its outer shell, so when it tries to share an electron with each of its four silicon neighbors, we end up one electron short, creating what is sometimes called an electron “hole�. This material is called p-type material. If we take a chunk of p-type material and a chunk of n-type material and get them up against each other, we get (drum roll please) – a p-n junction.

Now, let’s add a little light to the equation. Light is made of little packets of energy called photons. Photons from the sun travel through space and end up landing on our little solar cell, crashing into the n-type and p-type material. It doesn’t do a whole lot to the p-type material, but since our “left over� electron in the n-type material isn’t tightly bound, the photon has enough energy to knock the extra electron loose.

At the p-n junction things are starting to get interesting. On the n-type side of the p-n junction there’s a bunch of electrons looking for somewhere to be. On the p-type side there’s a bunch of holes for these electrons to reside in. Right at the boundary where the p-type and n-type material meet, electrons from the n-type side start crossing over into the p-type region and get comfortable in their new homes. As this continues, the holes nearest the p-n junction get filled, making it harder for other electrons to cross over into the p-type material. Eventually it gets so hard for the electrons to cross from the n-type material to the p-type that things just settle down and reach equilibrium.

Something pretty cool has happened as the electrons near the p-n junction were doing their little dance. When we started out, both the p-type and n-type materials were electrically neutral – for every negative electron there was a positive proton to balance things out. But now, a bunch of electrons have crossed over from the n-type to the p-type region. What does this give us? Well, now the n-type material has a positive net charge and the p-type has a negative charge. Does this remind you of anything? It’s beginning to sound sort of like a regular battery.

How do we get something useful out of this? A good analogy to give us an understanding of how to use this would be to take two tanks of water, one at higher pressure than the other. If we connect these two tanks with a pipe, water would flow from one tank to the other, until the pressure in the two tanks even out. If we put something along the pipe, like a paddlewheel, we could harness this energy. The same idea applies with the solar cell. Given a chance, electrons will flow through a wire from the p-type region to the n-type region until the electrical charges balance out. But, since the light keeps knocking electrons free in the n-type material, the cycle will continue. As long as there is light, the process continues, allowing us to use this flow of electrons to charge a battery, spin a small motor – whatever we want.

That’ll wrap things up for this episode of How Things Work. A transcript of this episode can be downloaded from howthingswork.libsyn.com, and if you have any questions, comments, or topic ideas, please drop me a line at howthingswork@verizon.net. Thanks for listening.

The music you heard was How’d You Know That by Scott Brown. It’s available on Random Chance records at www.randomchancerecords.com and is made available through the IODA Promonet.



Citations –
http://www.wisegeek.com/how-do-solar-panels-work.htm
http://www.thesolarplan.com/articles/how-do-solar-panels-work.html
http://www.nooutage.com/howsolar.htm
http://www.howstuffworks.com/solar-cell2.htm
http://www.soton.ac.uk/~solar/intro/tech6.htm
http://www.sailgb.com/sshop/tech_info.asp?ID=164
http://www.gepower.com/prod_serv/products/solar/en/how_solar_work.htm
http://library.thinkquest.org/27754/apphowpv.html
http://en.wikipedia.org/wiki/Solar_cell

We explore how solar cells work.

Microwaves ovens –

Hello, and welcome to the How Things Work podcast. My name is Jamie Aycock, and in each episode we’ll explore the inner workings of things we see and use every day. In this episode we’ll explore how microwave ovens work.

Microwave ovens were first developed by the Raytheon Corporation in the mid 1940’s. Their invention came about, like so many great inventions, by accident. A radar researcher named Percy Spencer noticed that a candy bar placed near a radar antenna had melted. He repeated this with popcorn (which popped) and an egg (which exploded). It was clear that the radar set was causing these products to heat up.

Just what was causing the radar waves to heat these items? Water, as most people know, has the chemical formula H20 – that is to say that there are two hydrogen atoms and one oxygen atom per molecule of water. These three atoms are arranged sort of like a silhouette of Mickey Mouse’s head – one large circle (the oxygen atom) in the middle with two smaller circles (the hydrogen atoms) stuck on top like Mickey’s ears.

A water molecule has a dipole moment – a dipole moment simply means that one part of the molecule (the hydrogen) has a positive electrical charge and one part of the molecule (the oxygen) has a negative electrical charge. Imagine a bar magnet set on a needle so it can spin freely. If you were to approach the positive end of this magnet with the positive end of a second magnet, since like forces repel and opposite forces attract, the magnet would spin around so that magnet on the needle came to rest with its negative end towards the positive end of the second magnet.

The very same principle is used in a microwave oven. A part of a microwave called a magnetron produces an electromagnetic wave at about 2.45GHz. An electromagnetic wave at 2.45GHz changes from positive to negative and back 2.45 billion times each second. As the positive part of the electromagnetic wave nears a water molecule in food, the water molecule turns so that its negative end (the oxygen end) is closer to the positive portion of the wave. Likewise, the positive end of the water molecule is attracted to the negative part of the wave. As the wave moves, the molecule will move, trying to maintain this arrangement.

The end result is that all of the water molecules are constantly moving around, trying to keep their positively charged end near the negative part of the wave and vice versa. As the electromagnetic wave moves, so do the water molecules. All of these water molecules moving around generate heat due to the friction of them bouncing into and rubbing against each other. This heat cooks our food.


That’ll wrap things up for this episode of How Things Work. A transcript of this episode can be downloaded from howthingswork.libsyn.com, and if you have any questions, comments, or topic ideas, please drop me a line at howthingswork@verizon.net. Thanks for listening.

The music you heard was How’d You Know That by Scott Brown. It’s available on Random Chance records at www.randomchancerecords.com and is made available through the IODA Promonet.



Citations –

http://en.wikipedia.org/wiki/Microwave_oven
http://www.zyra.org.uk/microw.htm
http://www.madehow.com/Volume-1/Microwave-Oven.html
http://www.lsbu.ac.uk/water/microwave.html
http://www.lsbu.ac.uk/water/microwav2.html
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/diph2o.html
http://www.rps.psu.edu/probing/microwave.html
http://www.colorado.edu/physics/2000/applets/h2o.html
http://www.colorado.edu/physics/2000/applets/h2ob.html

We explore how microwave ovens work.

Welcome to How Things Work, a podcast dedicated to exploring the inner workings of things we see and use every day. If you have comments or topic suggestions, please feel free to email me at howthingswork@verizon.net. Thanks!