Energy: What Is It Exactly? - University Of New Mexico

ME 217: Energy, Environment & Society Fall 2015 Energy: what is it exactly? Mechanical Engineering ME217 Energy, Environment & Society

Energy for physicists “It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and when we add it all together it gives always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.” Richard Feynman Mechanical Engineering ME217 Energy, Environment & Society energy basics 2/19

Energy for engineers “In physics, energy (Ancient Greek: ἐνέργεια energeia "activity, operation") is an indirectly observed quantity that is often understood as the ability of a physical system to do work on other physical systems.” Wikipedia Mechanical Engineering ME217 Energy, Environment & Society energy basics 3/19

Basic definition of work Work done by a force moving along a path: W Fd Mechanical Engineering ME217 Energy, Environment & Society energy basics 4/19

Various forms of energy From the basic definition, we can derive several other forms of energy: Kinetic energy Thermal energy Potential energy (e.g. in gravitational field) Chemical energy Electrical energy Electromagnetic energy Nuclear energy . Mechanical Engineering ME217 Energy, Environment & Society energy basics 5/19

Conservation of energy (FLT) “The total amount of energy in an isolated system remains constant over time.” “The total energy is said to be conserved over time.” Mechanical Engineering ME217 Energy, Environment & Society energy basics 6/19

Kinetic energy The ability of a moving object to do work Mechanical Engineering ME217 Energy, Environment & Society energy basics 7/19

Thermal energy The kinetic energy associated the microscopic motion of particles Mechanical Engineering ME217 Energy, Environment & Society energy basics 8/19

Potential energy The ability of a conservative force field to do work Mechanical Engineering ME217 Energy, Environment & Society energy basics 9/19

Chemical energy The potential energy associated with chemical bonds Mechanical Engineering ME217 Energy, Environment & Society energy basics 10/19

Electrical energy Energy associated with the motion of electrons in a conductor Mechanical Engineering ME217 Energy, Environment & Society energy basics 11/19

Electromagnetic energy Energy associated with photons Mechanical Engineering ME217 Energy, Environment & Society energy basics 12/19

Nuclear energy Energy associated with nuclear bonds between subatomic particles Mechanical Engineering ME217 Energy, Environment & Society energy basics 13/19

What about power? Power can be defined as the rate at which energy is converted from one form to another, for example: Chemical to mechanical (auto engine) Chemical to heat (boiler) Kinetic to electric (wind turbine) Mechanical Engineering ME217 Energy, Environment & Society energy basics 14/19

Example Car driving up la Bajada hill: 3.3% gradient Elevation gain 500' Speed: 75 mph 1500 kg car What is the power required to overcome gravity? What is the total potential energy gain? Mechanical Engineering ME217 Energy, Environment & Society energy basics 15/19

La Bajada example - 1 Mechanical Engineering ME217 Energy, Environment & Society energy basics 16/19

La Bajada example - 2 Mechanical Engineering ME217 Energy, Environment & Society energy basics 17/19

Heat is the most common energy Mechanical Engineering ME217 Energy, Environment & Society energy basics 18/19

Direction of energy flow (SLT) hot reservoir @ TH QH HE All reversible heat engines operating between a hot and a cold reservoir have the same efficiency: W QL cold reservoir @ TL Mechanical Engineering ME217 Energy, Environment & Society The efficiency of a reversible heat engine is the highest possible efficiency of a heat engine. energy basics 19/19

The kinetic energy associated the microscopic motion of particles. Mechanical Engineering ME217 Energy, Environment & Society energy basics 9/19 Potential energy The ability of a conservative force field to do work. Mechanical Engineering ME217 Energy, Environment & Society energy basics 10/19