The Terms of Thermodynamics

• Chemical energy - is related to the relationships between molecules in chemical compounds. When chemicals mix they may give of heat (exothermic reaction) or require heat (endothermic reaction)
• Electric energy - is related to electrons moving through a conductor
• Energy- can be reduced to the concepts of heat and work and can be found in various forms: potential energy, kinetic energy, thermal or internal energy, chemical energy, and nuclear energy
• Enthalpy - is a term with energy units that combines internal energy with a pressure/volume or flow work term
• Entropy - is a property of matter that measures the degree of randomization or disorder. The natural state is for entropy to be produced by all processes
• Heat- is energy in motion from one region to an other as a result of temperature difference
• Internal energy - has to do with activity within the molecular structure and is typically observed with temperature measurement
• Kinetic energy - is the energy of motion and is proportional to the square of the velocity as well to the mass of the moving body
• Nuclear energy - is related to the energy of atomic relationships between the fundamental particles. Nuclear fission and fusion are reactions which release nuclear energy
• Potential energy - is the energy of location or position of a mass in a force field
• Property - is a measurable characteristic of a system or substance. Temperature, density, pressure etc
• Specific Heat Capacity- The specific heat (also called specific heat capacity) is the amount of heat required to change a unit mass (or unit quantity, such as mole) of a substance by one degree in temperature
• Temperature- is a term used to quantify the difference between warm and cold level of internal energy of a substance
• Work- is an energy form which can be equated to the rising of a weight as moving a mass in a force field or moving a liquid against a resisting force

The First Law of Thermodynamics forms the

• basis for quantitative analysis of chemical reactions

The Second Law of Thermodynamics is used to

• identify the directions of chemical reactions

The Third Law of Thermodynamics states that

• the entropy of any pure substance in thermodynamic equilibrium approaches zero as the temperature approaches zero (Kelvin), or conversely
• the temperature (Kelvin) of any pure substance in thermodynamic equilibrium approaches zero when the entropy approaches zero

The Third Law of Thermodynamics can mathematically be expressed as

lim S_T→0 = 0 (1)
where
S = entropy (J/K)
T = absolute temperature (K)

At a temperature of absolute zero there is no thermal energy or heat. At a temperature of zero Kelvin the atoms in a pure crystalline substance are aligned perfectly and do not move. There is no entropy of mixing since the substance is pure.
The temperature of absolute zero is the reference point for determination entropy. The absolute entropy of a substance can be calculated from measured thermodynamic properties by integrating the differential equations of state from absolute zero. For a gas this requires integrating through solid, liquid and gaseous phases.

The most common units for heat are

• BTU (Btu) - British Thermal Unit
• Calorie
• Joule

BTU - British Thermal Unit

The unit of heat in the imperial system - the BTU - is

• the amount of heat required to raise the temperature of one pound of water through 1^oF (58.5^oF - 59.5^oF) at sea level (30 inches of mercury).
• 1 Btu (British thermal unit) = 1055.06 J = 107.6 kpm = 2.931 10^-4 kWh = 0.252 kcal = 778.16 ft.lbf = 1.05510¹⁰ ergs = 252 cal = 0.293 watt-hours

An item using one kilowatt-hour of electricity generates 3412 Btu.

Calorie

A calorie is commonly defined as

• the amount of heat required to raise the temperature of one gram of water 1^oC
• the kilogram calorie, large calorie, food calorie, Calorie (capital C) or just calorie (lowercase c) is the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius
• 1 kcal = 4186.8 J = 426.9 kp.m = 1.163 10^-3 kWh = 3.088 ft.lbf = 3.9683 Btu = 1000 cal

Be aware that alternative definitions exists - in short:

• Thermochemical calorie  
• 4 °C calorie
• 15 °C calorie
• 20 °C calorie
• Mean calorie
• International Steam Table calorie (1929)
• International Steam Table calorie (1956)
• IUNS calorie (Committee on Nomenclature of the International Union of Nutritional Sciences)

The calorie is outdated and commonly replaced by the SI-unit Joule.

Joule

The unit of heat in the SI-system the Joule is

• a unit of energy equal to the work done when a force of one newton acts through a distance of one meter
• 4.184 joule of heat energy (or one calorie) is required to raise the temperature of a unit weight (1 g) of water from 0^oC to 1^oC, or from 32^oF to 33.8^oF
• 1 J (Joule) = 0.1020 kpm = 2.778 10^-7 kWh = 2.389 10^-4 kcal = 0.7376 ft.lbf = 1 kg.m²/s² = 1 watt second = 1 Nm = 1 ft.lb = 9.478 10^-4 Btu.
• Converting between heat and energy units 

 Work

When a body is moved as a result of a force being applied to it, work is done.
The amount of work is the product of the applied force and the distance:

W = F s         (1)
where
W = work done (J)
F = force acting on the object (N)
s = distance object moved in the direction of the force (m)

The unit of work is joule, J, which is defined as the amount of work done when a force of 1 Newton acts for distance of 1 m in the direction of the force.

1 J = 1 Nm

This is the same unit as energy.
The work done by a constant force and a spring force can be visualized as the area under the graph in distance force diagrams like

 work-graph

Example - Constant Force

A constant force of 20 N is acting a distance of 30 m. The work done can be calculated as

W = (20 N) (30 m)
    = 60 (J, Nm)

Example - Spring Force

A spring is extended 40 mm by a force of 20 N. The work done can be calculated as

W = 1/2 (20 N) (0.040 m)
    = 0.4 (J, Nm)