Laws of Thermodynamics

#physics#thermodynamics

Info

What is heat? :: Heat is the energy transferred between objects because of a temperature difference.

Objects are in thermal contact if heat can flow between them.

When the transfer of heat between objects in thermal contact ceases, they are in thermal equilibrium.

Zeroth Law#card §

The zeroth law states that if two thermodynamic systems are in thermal equilibrium with each other, and also separately in thermal equilibrium with a third system, then the three systems are in thermal equilibrium with each other.

First Law#card §

Energy cannot be created or destroyed. In other words, the total amount of energy in a system is conserved (constant), however, energy can be transformed from one from to another.

$U$ stands for the internal energy of a substance, which is the sum of all the energy in the substance, including:

• Potential energy in the bonds
• Rotational energy
• Vibrational energy
• Translational (kinetic) energy of the particles in the substance.

$Q$ stands for the thermal energy of a substance in Joules.

$W$ stands for the work done, and work is done when a force acts on an object and moves the object through a distance.

• Work by the system: “System pushes back on plunger and it goes up” (positive work)
• Work on the system: “I push down on plunger to compress system” (negative work)

Negative work increases the energy of a system This is because when work is done on a system, the system isn’t really doing anything and it gets work done for it, ∴ it is negative

$\Delta U=Q-W$

Work BY the system is positive work. ^1677225721447 Work ON the system is negative work.

Specific Heat Capacity#card §

The way the temperature increases when an object gains thermal energy depends on the material from which the object is made.

• The amount of thermal energy (heat energy) required to increase the temperature of an object by 1°C is termed the thermal capacity of the object (with the unit being $J°C^{-1}$, Joules per degree Celsius/Kelvin)

To make comparisons easier we can compare the amount of thermal energy required to increase 1 kg of the material by $1°C$. This is termed the specific heat capacity of the material.

The symbol for specific heat capacity $c$ with the unit $Jk{g}^{-1}°C^{-1}$ or $Jk{g}^{-1}°K^{-1}$. This can also be written as $c=\frac{J}{Kg\times K}$ This can be rearranged to $c\times kg\times K=J$ and if we replace $J$ with $\Delta Q$, $kg$ with $m$ for mass and K with $\Delta T$, we get $\Delta Q=mc\Delta T$

Example#card §

How much heat energy is required to increase the temperate of 5kg of water by 80°C? $\Delta Q=mc\Delta T$ $=5\times 4200\times 80$ $=1680000$ $=1.68\times 10^{6}J$

Latent Heat §

Latent (meaning “Hidden”) Heat refers to the heat curve of changes in phases. This means that during the phase change, the energy that would normally manifest itself in a change in temperate ($\Delta T$) is instead put towards the process of rearranging the atoms to change the phase of the material.

Specific Latent Heat §

The amount of energy required to change 1 kg of a substance from one state to another is called the Specific latent Heat. The unit is $Jk{g}^{-1}$

• $L_{vap}=$ specific latent heat of vaporisation
• $L_{fus}=$ specific latent heat of fusion

To calculate the amount of energy required to change the state of mass m

$$Q=mL$$