Thermodynamics and Heat Transfer

Temperature and Thermal Energy

Temperature is how cold or hot something is relative to a standard. The standards include the Celsius scale (where 0 is defined as the freezing point of water and  100 is defined as the boiling point of water) and the Fahrenheit scale (where 32 is defined as the freezing point of water and 212 is defined as the boiling point of water).

A reason we need standards to define temperature is because our sense of hot and cold is warped. If you dip your one of your hands in a cup of cold water and one of your hands in a cup of hot water, and then dip both of your hands in a cup of warm water, one of your hands will feel as the warm water is hot and the other will feel as if the warm water is cold. Isn't that interesting?

You're probably wondering, "Well, Twisha, what's the difference between temperature and thermal energy? How am I supposed to pass high school physics?" Don't worry baby girl, I got you. Temperature is defined per-particle, while thermal energy is the sum of the total KE of all the particles. Thus, if you have double the substance, you have double the thermal energy. However, its temperature is the same regardless of the amount of the substance. Make sense?

When two objects are in thermal equilibrium, they have the same average KE per particle. Basically, when the temperatures of two objects balance out and no more heat transfer is made, the objects are in thermal equilibrium. 

If you ever thought you could psychotically freeze your sister indefinitely, I'm sorry to say that temperature does have limits. It has no upper limit, you can increase the temperature of something to as high as you want - which is not good news for your poor sister. However, it does have a lower limit. When something cools, it contracts. Does that mean that if an object reaches a certain temperature, its volume will be zero? That is exactly what scientists realized. If you cool an object by 273 degrees Celsius, its volume will be zero. However, this is impossible - you cannot have zero volume. Thus, this unreachable point is called absolute zero and the scale used to measure it is called the Kelvin scale.

The Kelvin scale starts at absolute zero (0K) and increases at the same rate as the Celsius scale. So, 273 K is 0 C. 

Heat

Heat is thermal energy transferred from one object to another due to a temperature difference. Heat always flows from high temperature to low temperature. 

Heat is moving energy, and is thus measured in joules. However, here in the US, we're not like the other girls, and we use calories. The kilocalorie (kcal) is 1000 calories, which is the heat needed to make 1 kg of water increase in temperature by 1 C. the kcal is really just called the Calorie, with a capitol C, to help differentiate, because we just couldn't come up with another name.

A way to change the temperature of an object without changing its environmental temperature is by compressing or expanding it. Compressing an object warms it, and expanding it cools it. I won't go too deep into this, but if anyone wishes to know, please feel free to email me at twisha.sharma30@gmail.com.

Alright, now that you're not a rookie anymore, let's talk about how heat transfer works.

Conduction

Conduction is heat transfer through touch. I'm sure you've all felt what conduction feels like when you touch a hot stove - ouch! In conduction, heat transfers through interactions between atoms. 

Convection

Convection is heat transfer through fluids. In fluids (gases and liquids) the warmer air/water rises, while the colder air/water sinks. This creates currents that rotate up and down, known as convenction currents. In a pot of water, hot water rises, and then cools at the top. It then sinks, forcing the process to repeat. Convention currents also produce winds through our atmosphere, like the beach winds you feel when you head to the seashore.

Radiation

Radiation is heat transfer through a vacuum. It happens with electromagnetic waves (light). 

Everything emits radiant energy, we just cant see all of it until it reaches the range of visible light. The frequency and wavelength of radiant energy depends on its temperature, so when something gets hot enough, it emits frequencies that are akin to visible light. 

The longest visible wavelengths, red light, is seen first. As the object gets hotter, yellow is seen, and then white, and then blue.

However, objects dont just radiate energy - they absorb it too. Good radiators are also good absorbers, and vice versa. If a surface absorbs more than it emits, it is a net net cooler, and if it emits more than it absorbs, it is a net emitter.

Lastly, (we will discuss light far more extensively during the waves unit) not only is there absorption, there is reflection. Good absorbers are bad reflectors, and vice versa. 

Conductors, Insulators, and Newton's Law of Cooling

Now that we've talked about that, we need to talk about conductors and insulators. Conductors are materials that conduct heat well, while insulators conduct it poorly. Both of them are extremely useful. Blankets are insulators. they keep heat inside and keep the cold out. Cooking pans are made of conductive metals that heat up quickly to make your food life easier. As you can see, they're both really important.

However, interestingly enough, insulators dont stop the flow of thermal energy - they just slow down its rate.

FUN FACT: Newton's Law of Cooling (does this guy have anything else to do) is that the rate of cooling is equal to the change in temperature. Basically, this means that, if a object is a lot hotter than its surroundings, it'll cool faster. If it's only a little hotter, it'll cool slower. If it's a lot colder, it'll heat up faster, and if it's a little colder, it'll heat up slower. Either way, it will match the temperature of its surroundings at some point. That's genius Newton. Really genius. No one else could have ever thought of that.

Is it just me or is physics just a lot of common sense used to do things that dont make any sense?

Phases and Phase Change

Alright guys, we need to put our big boy pants on and start talking phases. Matter has four common states: solid, liquid, gas, and plasma. We all know what solids, liquids, and gases are, so let me quickly brush over plasma for those of you that are confused. 

Basically, plasma is an electrically charged gas. When gases get really hot, their electrons fly off and start wandering around, and the once gaseous area you were trying to measure has turned into a soup of charged atoms and electrons. Good for you, you just made plasma. Plasma is the most common state of matter and is found in the Sun.

Okay, we know about the phases of matter - but what are the processes that matter goes through when it changes phase?

Well, let's start with our well-known bestie: evaporation. Evaporation is the process of a liquid changing into a gas. Boiling is different from evaporation because boiling happens beneath the surface instead of on top of it.

Most of us probably know this by now - but did you know that evaporation actually cools down surfaces? When it comes to your body, the phenomenon is called evaporative cooling. That's how sweat works, actually. Sweat takes energy to evaporate, and that energy comes in the form of heat. Since heat is used to evaporate the water, it cools down the surface it leaves behind. How cool!

Let's breeze through the rest of these really quick.

The exact opposite of evaporation is condensation - the process by which gases turn into liquids.

When solids turn straight into gases, its called sublimation.

Melting is the process of a solid turning into a liquid, and freezing is the process of a liquid turning into a solid.

In order for a substance to even undertake these processes and switch phases, energy needs to be given or taken away. The amount of energy that needs to be given or taken away for a certain amount of a substance to go from a solid to a liquid (and vice versa) is called heat of fusion. In the same way, heat of vaporization is the amount of energy that needs to be given or taken away for a certain amount of a substance to go from liquid to gas (and vice versa).

The Laws of Thermodynamics

The laws of thermodynamics are, stated basically:

Entropy, Specific Heat Capacity, and Thermal Expansion

Entropy is how energy tends to disorder. In the natural world, higher qualities of energy usually want to transform into lower qualities. 

As we know, some substances heat and cool faster than other substances. The amount of heat needed to change the temperature of a substance by 1 C is the specific heat capacity. Water has a very high specific heat capacity, which is useful for global climates. It helps Europe stay milder than other regions. Islands and peninsulas also have much smaller temperature extremes than interior continents. 

Thermal expansion is the expansion of substances when exposed to higher temperatures. For example, during the summer, pavements expand and often buckle against each other, warping the sidewalk. I'm sure you've seen it somewhere before. And since different substances have different specific heat capacities, different substances also expand at different rates.

Why Does Ice Float?

It is a commonly known fact that ice floats. However, it is also commonly known that solids are denser than liquids. So, why does ice float, granted that ice is the solid form of water and water is obviously the liquid form. Now, in order to understand this, you have to know a tiny bit of chemistry. There are four different kinds of bonds between atoms, but right now, you only need to know about two: covalent and hydrogen bonds.

Covalent bonds are strong bonds between atoms that form when atoms share electrons. The bond between H2O (water) is a covalent bond. Hydrogen bonds are weak bonds between atoms that form when the positive charge of hydrogen molecules bonds with the negative charge of other water molecules. 

When water is a liquid, its hydrogen bonds are constantly made and broken as the atoms move around each other. However, when water freezes, its hydrogen bonds stabilize and form crystalline shapes. This formation of molecules is spaced further apart than the liquid water molecules are. Thus, ice is less dense than water, and it floats. Intriguing, right? *strokes nonexistant mustache*

This is vital to life processes - if ice sank, ponds would freeze bottom-up, and they would freeze entirely, killing all the fish inside. Because ice floats and ponds freeze from the top, the layer of ice provides an insulator to the rest of the pond, keeping it from freezing all the way. That way, fish can stay alive during cold winters. Well, to be fair, there are bad things about ice too - I mean, why else did the Titanic sink?

Either way, I hope you learned a lot. If you want to know more, feel free to email me, and I'll see you in the next page!