Temperature, Heat and Air Conditioning

In my previous post here, I tried to discuss how a body could have a form of energy (gravitational potential energy) by its position in a gravitational field. That post was inspired by how physics tends to explain the most mundane things and enables us to make sense and apply the explanations towards making life more comfortable. Today, I would like to discuss another trivial subject: Temperature. A discussion with my fellow African from Kenya working on her PhD in Italy sparked the interest that would result in this post. According to her, the temperature there feels like a negative 19 degrees Centigrade and she still gotta work in the lab. What follows is a unique synthesis of thoughts about what it means to be hot or cold in both macroscopic and atomic level. Hopefully, before the end of the post, we shall examine how air-conditioners and heaters take advantage of the principles of physics to enable us to enjoy more comfort at home, offices, shopping malls and motor vehicles.

^{_{Pixabay CC0: A Temperature Measuring Instrument - The Thermometer}}

If you care about comfort, you must find ways to manipulate your surroundings under different temperatures. For instance, on hot days, you would learn to put your drinks on ice and keep the drinks insulated. You would use a microwave oven to warm your meals. When it is cold outside, you will use heaters and fire to warm your home. We also try to keep our motor vehicles equipped with both a heater and an air-conditioning to ensure we can keep going under different temperature conditions. It would be so nice if we could perfectly insulate these systems (our drink, food, home, motor vehicles) from the external environment so that no matter the prevailing outside temperature, we can have those systems just at the temperature we want them.

Systems? A system here just means a space that is isolated from the rest of the Universe or in a narrower sense, the immediate environment. Excellent. So it is unfortunate that these systems cannot have a different temperature from the rest of the surrounding. This is because, whenever a system has a different temperature from the surrounding, heat would flow from the hotter region to the cooler area until an equilibrium temperature is reached. This state of affairs is referred to as the Zeroeth Law of Thermodynamics. Odd name. The law received its name from the fact that it was recognised as a law of thermodynamics after the other, other laws of thermodynamics had been recognised. It could have been called the fourth law of thermodynamics except that it was more fundamental to the first, second and third law. So it made sense just to let it be the Zeroeth Law.

To begin to understand what goes on in the Zeroeth Law we should take a few steps back and discuss Temperature and Heat

Temperature is hotness or coldness of an object according to my high school physics teacher. And he is right because we all have a shared qualitative way of describing the temperature of a room as either hot or cold. Unfortunately, this would have to be the end of this discussion unless I can find a more quantitative definition of temperature. So how do we define temperature? A simple search would reveal more than five definitions, including my teacher's definition. However, it serves my purpose to examine only two of these:

While our previous definition of temperature makes more sense to someone without a science background, the last two descriptions are more scientifically accurate and exploring the meaning of these definitions would enable us to come to a deeper understanding of the subject.

My previous post was mainly about potential energy. However, towards the end, I discussed how it is of water which is converted to hydropower. Particles in matter possess kinetic energy which varies according to the temperature of the object. There are three main types of kinetic energy, namely:

• Vibrational Kinetic Energy which is the kinetic energy of particles which makes them vibrate about a fixed point without making a net displacement.
• Translational Kinetic Energy which is the kinetic energy of a particle moving about a given space from one point to the other.
• Rotational Kinetic Energy which is the kinetic energy of a particle rotating about a fixed point.

It may not be apparent to an observer that the particles of water molecules placed on a counter are not still. The particles are in constant motion and if the temperature is increased (that is if the water is made hotter), the particles move faster by vibrating faster, rotating at a higher frequency or moving from one end of the glass to the other. If, on the other hand, the same glass of water is placed in a freezer and allowed to freeze, the particles are slowed down considerably, and the temperature is said to be cold. Therefore, a thermometer is an instrument which measures the kinetic energy of an object.

To adequately describe what heat is, consider a container of hot water placed on a counter. If the temperature of the water was raised to boiling point, then the water and the cup should be at close to 100^_oC. At this point, you may be unable to hold the cup. However, with time, the temperature of the cup and its content would reduce and eventually, you would not only be able to hold the cup but also drink the water.

But we know that temperature (a measure of hotness or coldness) is evidence of the kinetic energy of the water particles. Therefore if the water cooled down, we must assume that the particles have lost some of their kinetic energy. But the thing is that energy can neither be created nor destroyed according to the law of conservation of energy. Therefore, the energy lost by the particles must have been transferred somewhere else, and it is not difficult to see where it has been transferred to if you place your hand on the counter.

If the cup contained ice blocks, the block would soon melt, and the water would become warmer. In this case, the counter would become colder. What has happened in this case is not that the glass has transferred coldness to the bar. Instead, the counter has transferred heat to the glass cup until the temperature of both the system (cup) and the surrounding (counter and the kitchen) are the same. This return to the same temperature is called Thermal Equilibrium which brings us back to the Zeroeth Law of Thermodynamics - a hot body will lose temperature to a cold body until their temperatures are the same.

There are three methods through which heat can be transferred. For brevity sake, I shall discuss the three at once. The first form of heat transfer is transfer by Conduction. This is a method of heat transfer that involves only the transfer of energy from the particles of higher kinetic energy to the particles of lower kinetic energy, without the actual movement of the particles themselves.

From the image, the handle of the skillet is heated by conduction. Another method of heat transfer is Convection which is the mechanism of heat transfer in most liquids due to how their particles are arranged. The particles with higher kinetic energy exhibit translational kinetic energy and move to collide with the particles with less kinetic energy, transferring their energy and heat to them.

The third mechanism is Radiation. As it turns out, this is the method through which we receive the energy of the sun. Heat through radiation does not require mass to be transferred because the heat is transferred in the form of electromagnetic waves.

Now we are ready to examine air-conditioners and how they work using the principles already discussed.

Like many discoveries and inventions, the first person who built the modern air conditioning system did not set out to invent something.

He was trying to solve a humidity problem. It was 1902 at Sackett-Wilhelms Lithographing and Publishing Company in Brooklyn, N.Y. A young electrical engineer, Willis Haviland Carrier, was trying to prevent the paper stock at the plant from absorbing moisture during a warm summer which made it difficult for the company to apply the layered inking technique that was used for publishing at the time. To solve this problem, Carrier figured that if he could reduce the temperature inside the building, then the ability of the air to hold water (humidity) would be diminished significantly and therefore keep the paper in the condition needed.

He solved this problem by treating the air inside the building by passing it through chilled pipes. As the air passes through the pipes, the air temperature drops, thereby reducing the moisture retention ability of the air. This became the beginning of the air-conditioning technology.^[3]

The above description of the air-conditioning process is, of course, overly simplistic. Modern air-conditioners employ a few mechanical tricks to improve the results. So, what exactly happens in a modern air-conditioner?

Modern air-conditioners use to cool the inside of a building or motor vehicle by taking advantage of the temperature and heat transfer principles. Recall that we talked about temperature being a measurement of kinetic energy? When we heat water, it absorbs heat, and the kinetic energy of the water molecules increases, and they vibrate, rotate and translate in the container. At a point, the kinetic energy of the molecules get so high that the water evaporates, that is, turn into gas. This is called a phase conversion. This means that when a liquid converts to a gas, it absorbs heat.

Luckily, we have a special group of compounds called refrigerants which are easily forced to condense and evaporate over and over again within a set of closed coils. These compounds can change phase at low temperatures. Air-conditioners are equipped with fans which extract warm interior air and pass them over low-pressure evaporator chilled coils containing the refrigerants. When this warm air passes over the coils, it heats the refrigerant inside the coil such that it changes state from liquid to gas while in the process, the air becomes chilled. Another fan would take the chilled air and blow it back into the interior.

Great. Things are going well but, for the air-conditioner to continue working efficiently, the refrigerant must be returned to a liquid state. To do this, we only need to increase the pressure of the gaseous refrigerant, and this is done by the compressor. The process of compressing the refrigerant generates heat which is sent outside by a fan and a second set of coils called the condenser coils. As the refrigerant cools, it changes back to a liquid, and the evaporation process retakes place. But how does the air-conditioner know when to stop? Well, it has a built-in thermostat which monitors the temperature and turns off the air-conditioner when the interior reaches a set temperature and turns it back on when the interior gets warm again.

You are wonderful for sticking with me this long and I'd like to thank you for reading.

1. Dictionary | Temperature
2. Wikipedia | Heat Transfer
3. ASME.org | Global Cooling: The History of Air-Conditioning
4. HowStuffWork | Air Conditioners
5. The Physics Classroom | What is Heat?

---

^{If you write STEM (Science, Technology, Engineering, and Mathematics) related posts, consider joining #steemSTEM on steemit chat or discord here. If you are from Nigeria, you may want to include the #stemng tag in your post. You can visit this blog by @stemng for more details. You can also check this blog post by @steemstem here and this guidelines here for help on how to be a member of @steemstem. Please also check this blog post from @steemstem on proper use of images devoid of copyright issues here.}

---