3.2: Energy Transfer
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)One important example of kinetic energy is thermal energy, which comes from the tiny movement of many molecules in a system. In Chapter 1, we discussed how temperature is a measure of the average speed of atoms and molecules in a system. Here we can further describe temperature as being proportional to the average kinetic energy of the random motions of the molecules in a system. The faster the molecules move, the higher the temperature.
The transfer of thermal energy due to the temperature difference between two objects is what is known as heat. Heat is a form of energy in transit, and once transferred, it is stored as internal energy. There are four main methods of heat transfer: conduction, convection, radiation, and the absorption or release of latent heat.
When you directly touch a hot object, such as the stovetop, the energy from the hot stove top is immediately transferred to your cool hand due to a difference in the speed of the molecules, causing you to feel a burn. This is an example of conduction: energy directly transferred through a substance without the movement of material. Certain materials are better conductors of heat than others. Metal, for example, conducts heat very efficiently, while air, which acts as an insulator, is a very poor conductor of heat.
Another type of energy transfer is convection, which occurs in liquids and gases (both fluids) because molecules can move freely and currents can naturally occur in them. This happens constantly in the atmosphere. Convection refers to movement within a fluid due to the tendency of lower density fluid to rise over higher density fluid, which sinks due to the force of gravity resulting in heat transfer within the fluid.
In the figure below, a beaker is being heated from the bottom by a flame. The arrows show upward movement in the center where the fluid is heated and therefore is less dense and buoyant. The cooler fluid at the top of the beaker is more dense and sinks toward the bottom under the influence of gravity.
Finally, when it comes to the atmosphere, the most important form of energy is the energy we get from the Sun, which is called radiant energy or radiation. Radiation is another type of heat transfer that was covered in Chapter 2. Remember that radiant energy can be transferred to an object without the space in between being heated. This is the result of electromagnetic waves from the Sun, which are then absorbed by the Earth’s surface or atmosphere and converted to thermal energy. Electromagnetic waves do not need any matter to travel through and propagate at the speed of light in a vacuum which is about 300,000 km·s–1.
Heat, or the transfer of thermal energy due to the difference in temperature between two objects, is defined as \(\Delta q\) and is given in standard units of joules per kilogram (J·kg–1). One joule (J) is the standard unit of energy or work (kg·m2·s-2).