4.2: Kinds of Energy
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- 88916
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)We can easily recognize different kinds of energy around us, like heat, light, electricity, chemical energy, nuclear energy, sound, etc. These different forms of energy are measurable (e.g., calories, joules, volts, decibels, quanta, photons). Even mass is a form of energy; recall Albert Einstein’s \(\rm e = mc^2\) equation, the theory of special relativity (or the law of relativity).
Nuclear energy arises out of the Theory of Relativity. Explain this in a sentence or two.
The problem in thinking about thermodynamics is that the universe is so big; there are too many kinds of energy to contemplate at once! To simplify, let’s imagine only two kinds of energy in the universe: potential energy and kinetic energy. A helpful example is a dam. The water above the dam has potential energy. As the water flows over (or through) the dam, its potential energy is released as kinetic energy. In the old days, the kinetic energy of flowing water could be used to power (i.e., turn) a millstone to grind wheat and other grains into flour. These days, water is more likely to flow through a hydroelectric dam, where kinetic energy is converted (transduced) to electricity. In this simple view, heat (molecular motion), electricity (a current of electrons), sound (waves), and light (waves or moving “particles”) are different forms of kinetic energy. The energy of mass or its position in the universe is potential energy. For example, the energy in a molecule of ATP is potential energy. Physicists talk a lot about potential energy and about the flow and conversion kinetic energy.
Why are hydroelectric energy, nuclear energy, and potential energy in coal controversial sources of electricity?
Here is an equally simple way to conceptualize energy: it is either useful or useless. This concept led directly to the arithmetic formulation of the thermodynamic laws. In this utilitarian way of thinking about energy, useless energy is entropy, while useful energy can be any of the other forms of energy (potential or kinetic).
A key to understanding bioenergetics is to recognize the difference between closed and open systems in the universe. Systems such as biochemical reactions in a test tube are closed and will reach equilibrium. Closed systems are artificial and only possible in a laboratory, where one can restrict and measure the amount of energy and mass entering or escaping the system. Cells on the other hand (and in fact, every reaction or event in the rest of the universe), are open systems. Open systems readily exchange energy and mass with their surroundings.
With this brief introduction, let’s imagine ourselves to be early scientists trying to understand energy flow in the universe by asking how the laws of thermodynamics apply to living systems (bioenergetics). We’ll see that the laws can be mathematically demonstrated precisely because all kinds of energy can be measured and all units of energy (e.g., volts into calories, light quanta into volts, and joules into decibels, mass into photons…) can be interconverted.