0.16 Bis2a 04.0 energy and thermodynamics v1.2  (Page 11/21)

Section summary

In studying energy, scientists use the term “system” to refer to the matter and its environment involved in energy transfers. Everything outside of the system is called the surroundings. Single cells are biological systems. Systems can be thought of as having a certain amount of order. It takes energy to make a system more ordered. The more ordered a system is, the lower its entropy. Entropy is a measure of the disorder of a system. As a system becomes more disordered, the lower its energy and the higher its entropy become.

A series of laws, called the laws of thermodynamics, describe the properties and processes of energy transfer. The first law states that the total amount of energy in the universe is constant. This means that energy can’t be created or destroyed, only transferred or transformed. The second law of thermodynamics states that every energy transfer involves some loss of energy in an unusable form, such as heat energy, resulting in a more disordered system. In other words, no energy transfer is completely efficient and tends toward disorder.

Summary review questions

Appendix i: energy units

In the International System of Units (SI), the unit of work or energy is the Joule (J). For very small amounts of energy, the erg (erg) is sometimes used. An erg is one ten millionth of a Joule:

Power is the rate at which energy is used. The unit of power is the Watt (W), named after James Watt, who perfected the steam engine:

Power is sometimes measured in horsepower (hp):

Electrical ene rgy is generally expressed in kilowatt-hours (kWh):

It is important to realize that a kilowatt-hour is a unit of energy not power. For example, an iron rated at $\begin{array}{cc}\text{2000}& \text{Watts}\end{array}$ would consume $\begin{array}{cc}\mathrm{2x3}\text{.}\mathrm{6x}{\text{10}}^6& J\end{array}$ of energy in $\begin{array}{cc}1& \text{hour}\end{array}$ .

Heat energy is often measured in calories. One calorie (cal) is defined as the heat required to raise the temperature of $\begin{array}{cc}1& \text{gram}\end{array}$ of water from 14.5 to 15.5 ºC:

An old, but still used unit of heat is the British Thermal Unit (BTU). It is defined as the heat energy required to raise the energy temperature of 1 pound of water from $\text{63}$ to $\text{64}{}^{°}F$ .

$\begin{array}{cc}1& \text{BTU}\end{array}=\begin{array}{cc}\text{1055}& \text{Joules}\end{array}$