# Thermodynamics Introduction

## Zeroth Law of Thermodynamics: Temperature

Critical to understanding thermodynamics is the concept of temperature. In general, we often refer and compare to temperature in an abstract "hot", "cold", "warmer", "colder", etc. sense, however the true definition of temperature is not accurately represented here.

More formally, the temperature of an object is directly proportional to the average kinetic energy of the particles which make up the object. Objects which feel "hotter" have a larger average kinetic energy inside them.

Following this understanding, heat is more accurately used to represent an energy transfer from an object with high temperature to one with a lower temperature. (We say something is hot because in fact heat is being transfered to our hand.)

## Temperature Scales

In general, there are three scales used to represent temperature: Kelvin, Celsius, and Fahrenheit. For all MCAT purposes Fahrenheit is never used in any scientific formulas. Celsius and Kelvin on the other hand, are both used in MCAT equations depending on the function.

Both Kelvin and Celsius use the same unit, a degree, and they're are equivalent in magnitude, however their reference frame is different. Celsius is in fact 273 degrees higher than Kelvin.

In the Celsius scale, the freezing point of water is set to 0 while the boiling point is set to 100. The Kelvin scale, which tries to incorporate internal energy sets the point at which an object has absolutely no thermal energy, absolute zero, to 0. As a consequence of this, and because it also uses degrees, water's freezing point is actually 273, and the boiling point 373.

Raising an object's temperature by one degree Celsius is the same as raising it by one degree Kelvin, and neither is equivalent to one degree Fahrenheit.

## Heat Transfer

When two objects with different temperatures are placed next to each other heat transfers between them. This can be thought of as any other forward and reverse process which eventually reaches equilibrium. The hotter object transfers more heat to the colder object than the reciprocal process and eventually the two equilibrate to one temperature in between the original two. (If both objects were exactly the same size and density then the new temperature would be the average of the two.)

There are three ways in which heat can be transfered: conduction, convection, and radiation.

### Conduction

Conduction is the process of particles in one of the objects directly transferring energy to particles in the other through molecular collisions. Metals are good conductors due to their lose electrons capable of freely and rapidly moving and exchanging kinetic energy with neighbouring particles. Gases on the other hand, are poor conductors because of the large empty space between particles, making molecular collisions rare.

### Convection

Convection is the transfer of heat through physical motion. Because the particles in solids don't physically move beyond simple vibrations, only liquids and gases can experience this process. Hot portions of liquids and gases will flow to cooler portions transferring energy throughout the process.