Lesson 2 – Heat engines and Efficiency

System

§The part of the Universe whose properties you are investigating.

§It is enclosed by a boundary defined by you.

Surroundings

§The rest of the Universe outside the System boundaries.

Law of Conservation of Energy

•Energy cannot be created or destroyed; it can only be transferred from one form of energy to another. The total amount of energy in a closed system remains constant over time

Mechanical

•Potential energy due to mass and position in gravitational field

•Or kinetic energy due to mass and speed

Electrical

•Associated with static electric charge

•Or with moving charges, current and magnetism

Chemical

microscopic structure and chemical bond energies

Nuclear

•Due to binding of protons and neutrons in the nuclei of atoms

Thermal

•Due to microscopic vibrations of atoms and molecules

•These movements are measured as temperature

useful nd wasted energy

§When energy is transferred from one for to another, not all the energy is converted into the desire form. Some energy is dissipated during the transfer.

§The energy that transfers to the intended form is considered the useful energy.

§The energy transferred to other forms is called the wasted energy. We say that the wasted energy has been dissipated.

§The useful and wasted energy always add up to the total energy input, because energy can not be created or destroyed (according to the law of conservation of energy

equation for useful

Total energy input = Useful energy + Wasted energy

Energy efficiency

•It is the fraction of the total energy that is useful.

Efficiency = (useful energy out)/(total energy in)

Temperature

•Is a measure of the degree of hotness of an object. It is directly proportional to the average kinetic energy of the molecules.

§It indicates how hot or cold something is.

§The common unit for temperature is degrees Celsius (°C)

•The physical quantity that determines the rate at which heat will flow from a hot body to a colder one (i.e lower temperature)

•It is directly proportional to the average kinetic energy of the molecules

Heat

§Is a form of energy that is transferred between objects or systems due to a difference in temperature

§Heat normally flows from regions of higher temperature to regions of lower temperature. 

§Is measured in Joules (J).

The symbol for heat is usually Q.

•The quantity of thermal energy transferred during a process

Thermal equilibrium

•Exists when 2 systems at the same temperature are in contact. There is no transfer of heat energy.

Heat engines

•A machine designed to convert thermal energy into useful mechanical work