LAWS OF THERMODYNAMICS

First Law of Thermodynamics The first law of thermodynamics is often

called the Law of Conservation of Energy.

This law suggests that energy can be transferred from one system to another in many forms. Also, it can not be created or destroyed. Thus, the total amount of energy available in the Universe is constant.

The quantity of heat energy transferred to a system is equal to the work done by the system plus the change in the internal energy of the system.

Second Law of Thermodynamics Heat cannot be transfer from a colder to

a hotter body. As a result of this fact of thermodynamics, natural processes that involve energy transfer must have one direction, and all natural processes are irreversible.

This law also predicts that the entropy of an isolated system always increases with time.

Entropy is the measure of the disorder or randomness of energy and matter in a system.

Entropy increases as disorder increases.

Systems have a natural tendency to have disorder.

Think about your bedroom....

Third Law of Thermodynamics The third law of thermodynamics states

that if all the thermal motion of molecules (kinetic energy) could be removed, a state called absolute zero would occur. Absolute zero results in a temperature of 0 Kelvins or -273.15° Celsius.

The Universe will attain absolute zero when all energy and matter is randomly distributed across space. The current temperature of empty space in the Universe is about 2.7 Kelvins.

Absolute zero can NEVERbe reached.

Applications of the Laws Heat Engine A heat engine changes heat energy into

mechanical work. This concept is used in all engines, steam, gasoline, diesel, or jet.

All heat engines operate using the following 3 characteristics:

1) The absorption of heat energy from a high temperature source.

2) The transfer of some of the heat energy into mechanical work.

3) The release of heat energy into a cold sink or reservoir.

We can summarize: QH = W + QL

QH is the quantity of thermal energy flowing from the higher temperature.

W is the mechanical work done. QL is the quantity of thermal energy

flowing out at a lower temperature.

There is no device that can transform heat completely into work. There is always some amount of thermal energy (waste energy) flowing into a lower temperature area.

Steam Engine

A boiler heats water into steam. The steam passes through an intake valve

(open in the diagram) into a cylinder. Inside the cylinder, steam pressure

pushes a piston back. The piston is attached to a crankshaft or

flywheel by a connecting rod. Work is done as the flywheel or crankshaft turns a pulley or set of wheels.

Leftover or waste energy (steam) exits through the exhaust valve into a lower pressure condenser as the piston returns.

Steam changes into liquid water in the condenser due to cooler temperature and less pressure.

The pump circulates the water back to the boiler to start the process again.

Steam is used today to turn turbines to produce electricity in coal or oil or gas fired generators. Nuclear reactors heat water so steam can turn turbines to generate electricity.

In a car engine, the gasoline and air mixture is ignited to create the heat. The heat flows to the air in the cylinder which expands and moves the piston(s).

The leftover or wasted energy is exhausted out a tailpipe and muffler, and some of waste heat is transferred to water, then to a radiator and then to the surrounding air.

A car engine is really very inefficient, as a huge amount of energy is wasted or not used to do some work.

How a Refrigerator Works Recall that heat naturally flows from a

hot object to a cold object. However, in a refrigerator heat is taken from the inside of the cool refrigerator and expelled to the warm air in the kitchen. In order for this to happen, work must be added.

Heat from warm food is transferred to the cool liquid freon (or similar substance) in the cooling coils of the refrigerator.

As the liquid gains heat, it changes to vapour. The vapour is compressed into hot vapour in the

compressor. Work is done here by the compressor.

The hot vapour is pumped to condensing coils outside the refrigerator (usually on the back).

Heat is transferred from the hot vapour to the cooler condensing coils, which then transfers the heat to the surrounding air.

As heat leaves the freon in the condensing coils, the vapour turns back to liquid.

The high pressure liquid passes through an expansion valve, becoming less pressurized as it returns to the cooling coils back in the refrigerator.