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Transcript

Thermodynamics of

Air Conditioners

Charlie Herzog

Atmospheric Pressure

Chapter 20

Atmospheric is the pressure exerted by the weight of the atmosphere, or the air, around you.

Atmospheric pressure applies to air conditioners because air conditioners do not simply take in air, cool it, and then pump it back out. Air conditioners work by using pressure to change the temperature of the air that passes through the system.

Air conditioning does this by having a loop where air can flow, and a valve at the bottom of the loop that can increase the pressure of one side of the system, making it hot, and decrease the pressure of the other side, making it cold. The buildup of air behind the valve makes the molecules move more rapidly, which causes the temperature change. In the example of a window air conditioning unit, the hot side would be on the outside of the house and would put hot air out, while the cold side of the system would be on the inside of the house pumping cold air in.

Bernoulli's Principle

Bernoulli's principle states that when the speed of a fluid or gas increases, its pressure decreases.

This relates to air conditioners because in order for the temperature of the air to decrease, the pressure must decrease first. Air conditioners achieve this change in pressure by forcing lots of air through a small opening, therefore causing the speed of the air to increase.

Thermal Equilibrium

Chapter 21

Thermal equilibrium occurs when two systems come into contact and can freely exchange energy. This then results in the two objects exchanging heat until they both have the same temperature and when they do, they are then in thermal equilibrium.

This applies to air conditioners because the main purpose of air conditioners is to bring your body to a thermal equilibrium with the air around you, with the temperature being of your choosing. The hotter the house is, the colder the air conditioning has to be to be able to reach a thermal equilibrium.

Specific Heat Capacity

Specific heat capacity is the quantity of heat that is required to raise the temperature of a unit mass of a substance by one degree Celsius.

This relates to air conditioners because the coolant, or refrigerant, used in air conditioners to keep the pipes cold has a very high specific heat capacity, This is very important because in order for the coolant to keep the pipes as cold as possible for as long as possible, the coolant needs to be able to resist changes in its temperature. Objects with low specific heat capacities, such as iron and other metals, would not work because they are not able to resist changes to their temperatures, and would warm up significantly as hot air flows through them.

Newtons Law Of

Cooling

Chapter 22

Newton's law of cooling states that the colder an objects surroundings, the faster that object will cool. It also states that the rate of cooling of an object is proportional to the temperature difference between the object and its surroundings. The hotter an object is compared with its surroundings, the faster it will cool.

In the scenario of air conditioning, that object is you. The colder you set your air conditioning to, the faster you will cool down, and the same is true with heaters as well.

Convection

Convection is a means of heat transfer where the two substances exchanging heat do not come into contact.

In convection, heat always transfers from the hotter substance to the colder one. This is shown in air conditioning when the air conditioning unit sucks hot air out of the room, without actually coming into contact with the objects that it is cooling.

Conduction

Second Law of Thermodynamics

Chapter 24

The second law of thermodynamics states that heat always flows from a warmer object, and will never flow from a cold object to a hotter object.

This is important in air conditioning because it shows how the cold air is not actually cooling your body, but rather the air conditioner is taking your body's heat from you and taking it in.

First Law of Thermodynamics

First Law of Thermo

The first law of thermodynamics is essentially just the law of conservation of energy, but with thermal systems. It states that whenever heat is added to a system, it always transforms to an equal amount of some other form of energy, and no energy is lost.

For example, if your room was a thermal system, then the heat from your body that is taken into the air conditioning unit is not simply lost or destroyed, it is just transformed into other forms of energy.

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