Mechanical EngineeringME217 Energy, Environment & Society

ME 217: Energy, Environment & Society Fall 2015

Energy: what is it exactly?

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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

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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

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Basic definition of work

Work done by a force moving along a path:

W = Fd

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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

...

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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.”

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Kinetic energy

The ability of a moving object to do work

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Thermal energy

The kinetic energy associated themicroscopic motion of particles

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Potential energy

The ability of a conservative force fieldto do work

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Chemical energy

The potential energy associated withchemical bonds

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Electrical energy

Energy associated with the motion of electrons in a conductor

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Electromagnetic energy

Energy associated with photons

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Nuclear energy

Energy associated with nuclear bondsbetween subatomic particles

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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)

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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?

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La Bajada example - 1

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La Bajada example - 2

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Heat is the most common energy

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Direction of energy flow (SLT)

hot reservoir @ TH

cold reservoir @ TL

HE

QH

QL

W

All reversible heat engines operating between a hot and a cold reservoir have the same efficiency:

The efficiency of a reversible heat engine is the highest possible efficiency of a heat engine.