Energy and Human Ambitions on a Finite Planet, 2021a

5 <strong>Energy</strong> <strong>and</strong> Power Units 70
<strong>Energy</strong> Form Formula Chapter(s) Applications
gravitational potential mgh 11, 16 hydroelectric, tidal
1
kinetic
2 mv2 12, 16 wind, ocean current
photon/light hν 13 solar
chemical H − TS 8, 14 fossil fuels, biomass
thermal c p mΔT 6, 16 geothermal, heat engines
electric potential qV 15 batteries, nuclear role
mass (nuclear) mc 2 15 fission <strong>and</strong> fusion
Table 5.2: <strong>Energy</strong> forms. Exchange is possible
between all forms. Chemical energy is
represented here by Gibbs free energy.
A bedrock principle of physics is conservation of energy, which we take
to never be violated in any system, ever. 6 What this means is that energy
can flow from one form to another, but it is never created or destroyed.
Box 5.2: <strong>Energy</strong>: The Money of Physics
A decent way to conceptualize energy conservation is to think of
it as the money of physics. It may change h<strong>and</strong>s, but is not created
or destroyed in the exchange. A large balance in a bank account is
like a potential energy: available to spend. Converting to another
form—like heat or kinetic energy—is like the act of spending money.
The rate of spending energy is called power.
6: The only exception is on cosmological
scales <strong>and</strong> times. But across scales even as
large as the Milky Way galaxy <strong>and</strong> over
millions of years, we are on solid footing
to consider conservation of energy to be
inviolate. It is fascinating to note that conservation
of energy stems from a symmetry
in time itself: if the laws <strong>and</strong> constants of the
Universe are the same across some span of
time, then energy is conserved during such
time—a concept we trace to Emmy Noether.
See Sec. D.2 (p. 393) for more.
Example 5.2.1 traces a few familiar energy conversions, <strong>and</strong> Figure
5.1 provides an example illustration. A more encompassing narrative
connecting cosmic sources to daily use is provided in Sec. D.2.2 (p. 395).
Example 5.2.1 Various illustrative examples:
◮ A rock perched on the edge of a cliff has gravitational potential
energy. When it is pushed off, it trades its potential energy for
kinetic energy (speed) as it races toward the ground.
◮ A pendulum continually exchanges kinetic <strong>and</strong> potential energy,
which can last some time in the absence of frictional influences.
◮ A stick of dynamite has energy stored in chemical bonds (a
form of potential energy). When ignited, the explosive material
becomes very hot in a small fraction of a second, converting
chemical energy into thermal energy.
◮ The fireball of hot material from the exploding dynamite exp<strong>and</strong>s
rapidly, pushing air <strong>and</strong> nearby objects out of the way at
high speed, thus converting thermal energy into kinetic energy.
◮ Light from the sun (photons) hits a black parking lot surface,
heating it up as light energy is converted to thermal energy.
◮ A uranium nucleus splits apart, releasing nuclear (potential)
energy, sending the particles flying off at high speed (kinetic
energy). These particles bump into surrounding particles transferring
kinetic energy into thermal energy.
◮ Thermal energy from burning a fossil fuel or from nuclear fission
P.E. = 7 J
K.E. = 0 J
P.E. = 5 J
K.E. = 2 J
P.E. = 3 J
K.E. = 4 J
P.E. = 1 J
K.E. = 6 J
Figure 5.1: Example exchange of potential
energy (P.E.) into kinetic energy (K.E.) as
an apple drops from a tree. The total energy
always adds to the same amount (here
7 J). The apple speeds up as it gains kinetic
energy (losing potential energy). When it
comes to rest on the ground, the energy will
have gone into 7Jofheat (the associated
temperature rise is too small to notice).
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.