Energy and Human Ambitions on a Finite Planet, 2021a

5 <strong>Energy</strong> <strong>and</strong> Power Units 71
can be used to make steam that drives a turbine (kinetic energy)
that in turn generates electrical energy (voltage, current).
Any of the forms of energy (e.g., in Table 5.2) can convert into the other,
directly or indirectly. In each conversion, 100% of the energy is accounted
for. In the general case, the energy branches into multiple paths, so we
do not get 100% efficiency into the channel we want. For instance, the
pendulum example above will eventually bleed its energy into stirring
the air (kinetic energy) <strong>and</strong> friction (heat) at the pivot point. The stirring
air eventually turns to heat via internal (viscous) friction of the air.
One useful clarification is that thermal energy is really just r<strong>and</strong>om
motions—kinetic energy—of individual atoms <strong>and</strong> molecules. So in
the case of nuclear fission in Example 5.2.1, the initial kinetic energy
of the nuclear fragments is already thermal in nature, but at a higher
temperature (faster speeds) than the surrounding material. By bumping
into surrounding atoms, the excess speed is diffused into the medium,
raising its temperature while “cooling” the fragments themselves as
they are slowed down.
If accounting for all the possible paths 7 of energy, we are confident
that they always add up. Nothing is lost. 8 <strong>Energy</strong> is never created or
destroyed in any process we study. It just sloshes from one form to
another, often branching into multiple parallel avenues. The sum total
will always add up to the starting amount. Sec. D.2.3 (p. 396) provides a
supplement for those interested in better underst<strong>and</strong>ing where energy
ultimately goes, <strong>and</strong> why “losing energy to heat” is not actually a loss
but just another reservoir for energy.
The differences between kinetic <strong>and</strong> thermal
energy is about coherence, inthatwe
characterize the kinetic energy of a raindrop
by its bulk motion or velocity. Meanwhile,
water molecules within the drop are zipping
about in r<strong>and</strong>om directions <strong>and</strong> at very high
speeds exceeding 1,000 meters per second.
7: . . . sometimes called channels
8: Actually, the principle is so well established
that new particles (like the neutrino)
have been discovered by otherwise unaccounted
energy in nuclear processes.
5.3 Power (W)
Before getting to the various common units for energy, we should absorb
the very important concept <strong>and</strong> units of power.
Definition 5.3.1 Power is simply defined as energy per time: how much
energy is expended in how much time. The SI unit is therefore J/s, which we
rename Watts (W).
One Watt is simply one Joule per second.
While energy is the capacity to do work, it says nothing about how
quickly that work might be accomplished. Power addresses the rate at
which energy is expended. Figure 5.2 provides a sense of typical power
levels of familiar animals <strong>and</strong> appliances.
Example 5.3.1 Lifting a 10 kg box, whose weight is therefore about
100 N, through a vertical distance of 2 m requires about 200 J of energy. Weight is mg. In this case, m is 10 kg. If we’re
If performed in one second, the task requires 200 W (200 Joules in one being sticklers, g 9.8 m/s 2 , but for convenience
we can typically use g ≈ 10 m/s
second). If stretching the same task out over four seconds, only 50 W
2
without significant loss of precision.
is required.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.