Download Energy: what is it exactly?

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