Bike rider - energy input By eating food, the total internal energy of our bodies increases and this energy eventually goes into work and heat flow from the body according to the first law.

Heat convection Lava carries matter (rich in minerals) in liquid form at very high temperatures. This is the best example of heat convection. As the lava moves to the bottom of the terrain its temperature reduces. Finally, after a long time it cools and solidifies.

Gasoline engine The intake stroke opens the valve, a small quantity of gasoline is squirted into the chamber. The compression stroke then pushes the valve upwards, the gasoline and air mixture gets compressed, which leads to high pressure in the chamber. The power stroke produces spark from a spark plug, which initiates an explosion in the chamber that causes a rapid expansion of the compressed mixture, forcing the piston downwards. The exhaust stroke pushes the piston to the bottom of the path after explosion and all the waste will exhaust out of the chamber, Thus the cycle continues.

Scientists have studied the subject of heat as if their lives depended on it: Their lives-all lives–did depend on heat. Heat energy is the life sustaining energy. The tremendous amount of heat from the Sun –– 17 million billion kilowatt hours worth everyday powered all the plants on earth, their leaves - tiny solar panels - converting sun shine into biomass and physical movement through the process of 'Photosynthesis'. Plants, in turn sustained humans. The sun was like the furnace of a gigantic steam engine, producing heat that powered the earth and everything on it. We inhale oxygen, which is absorbed in the blood through the lungs. Oxygen is supplied to the cells through the blood stream. In the cells this oxygen combines with glucose to release CO₂, H₂O and heat energy. This heat released is responsible for the functioning of human bodies. The same mechanism works more or less in all life forms.

The four laws of thermodynamics define temperature, energy and entropy that characterize thermodynamic systems. Zeroth law of thermodynamics says that if two systems are in thermal equilibrium with a third system, they must be in thermal equilibrium with each other. The first law of thermodynamics, also known as the law of conservation of mass-energy, says that the total energy of a closed system remains same. The second law of thermodynamics is related to the concept known as entropy, which measures the amount of disorder in a system. The entropy of any closed system not in thermal equilibrium almost always increases. The decrease in entropy associated with all the processes of life on earth is more than compensated by the increase in entropy inside the sun which feeds energy to us. The third law of thermodynamics says that you can never cool anything to zero degrees Kelvin. An object at zero Kelvin would be in the lowest possible energy state that it could achieve and no energy could be extracted from it to do work.

Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion and exchange of thermal energy and heat between physical systems. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and transfer of energy by phase changes. Heat conduction, also called diffusion, is the direct exchange of kinetic energy of particles through the boundary between two systems. Heat transfer always occurs from a region of high temperature to another region of lower temperature, as required by the second law of thermodynamics. Heat convection occurs when bulk flow of a fluid carries heat along with the flow of matter in the fluid. Radiation is the transfer of energy through space by means of electromagnetic waves in much the same way as electromagnetic light waves transfer light.

Heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. A heat "source" generates thermal energy that brings the working substance usually a gas or a liquid to a high temperature state and some of the thermal energy is converted into work. Most forms of energy can be easily converted to heat by processes like exothermic reactions such as combustion or absorption of energetic particles. Examples of everyday heat engines include the steam engine, the diesel engine and the gasoline engine in an automobile.