11.1 Temperature and Thermal Energy

Section Key Terms

Temperature

What is temperature? It’s one of those concepts so ingrained in our everyday lives that, although we know what it means intuitively, it can be hard to define. It is tempting to say that temperature measures heat, but this is not strictly true. Heat is the transfer of energy due to a temperature difference. Temperature is defined in terms of the instrument we use to tell us how hot or cold an object is, based on a mechanism and scale invented by people. Temperature is literally defined as what we measure on a thermometer.

Heat is often confused with temperature. For example, we may say that the heat was unbearable, when we actually mean that the temperature was high. This is because we are sensitive to the flow of energy by heat, rather than the temperature. Since heat, like work, transfers energy, it has the SI unit of joule (J).

Atoms and molecules are constantly in motion, bouncing off one another in random directions. Recall that kinetic energy is the energy of motion, and that it increases in proportion to velocity squared. Without going into mathematical detail, we can say that thermal energy—the energy associated with heat—is the average kinetic energy of the particles (molecules or atoms) in a substance. Faster moving molecules have greater kinetic energies, and so the substance has greater thermal energy, and thus a higher temperature. The total internal energy of a system is the sum of the kinetic and potential energies of its atoms and molecules. Thermal energy is one of the subcategories of internal energy, as is chemical energy.

To measure temperature, some scale must be used as a standard of measurement. The three most commonly used temperature scales are the Fahrenheit, Celsius, and Kelvin scales. Both the Fahrenheit scale and Celsius scale are relative temperature scales, meaning that they are made around a reference point. For example, the Celsius scale uses the freezing point of water as its reference point; all measurements are either lower than the freezing point of water by a given number of degrees (and have a negative sign), or higher than the freezing point of water by a given number of degrees (and have a positive sign). The boiling point of water is 100 $\text{°C}$ for the Celsius scale, and its unit is the degree Celsius $\text{(}\text{°C}$ ).

On the Fahrenheit scale, the freezing point of water is at 32 $\text{°F}$, and the boiling point is at 212 $\text{°F}$ . The unit of temperature on this scale is the degree Fahrenheit $\text{(}\text{°F}$ ). Note that the difference in degrees between the freezing and boiling points is greater for the Fahrenheit scale than for the Celsius scale. Therefore, a temperature difference of one degree Celsius is greater than a temperature difference of one degree Fahrenheit. Since 100 Celsius degrees span the same range as 180 Fahrenheit degrees, one degree on the Celsius scale is 1.8 times larger than one degree on the Fahrenheit scale (because $\frac{180}{100}=\frac{9}{5}=1.8$ ). This relationship can be used to convert between temperatures in Fahrenheit and Celsius (see Figure 11.2).

Figure 11.2 Relationships between the Fahrenheit, Celsius, and Kelvin temperature scales, rounded to the nearest degree. The relative sizes of the scales are also shown.

The Kelvin scale is the temperature scale that is commonly used in science because it is an absolute temperature scale. This means that the theoretically lowest-possible temperature is assigned the value of zero. Zero degrees on the Kelvin scale is known as absolute zero; it is theoretically the point at which there is no molecular motion to produce thermal energy. On the original Kelvin scale first created by Lord Kelvin, all temperatures have positive values, making it useful for scientific work. The official temperature unit on this scale is the kelvin, which is abbreviated as K. The freezing point of water is 273.15 K, and the boiling point of water is 373.15 K.

Although absolute zero is possible in theory, it cannot be reached in practice. The lowest temperature ever created and measured during a laboratory experiment was $1.0\times {10}^{-10}$ K, at Helsinki University of Technology in Finland. In comparison, the coldest recorded temperature for a place on Earth’s surface was 183 K (–89 °C ), at Vostok, Antarctica, and the coldest known place (outside the lab) in the universe is the Boomerang Nebula, with a temperature of 1 K. Luckily, most of us humans will never have to experience such extremes.

The average normal body temperature is 98.6 $\text{°F}$ (37.0 $\text{°C}$ ), but people have been known to survive with body temperatures ranging from 75 $\text{°F}$ to 111 $\text{°F}$ (24 $\text{°C}$ to 44 $\text{°C}$ ).

Converting Between Celsius, Kelvin, and Fahrenheit Scales

While the Fahrenheit scale is still the most commonly used scale in the United States, the majority of the world uses Celsius, and scientists prefer Kelvin. It’s often necessary to convert between these scales. For instance, if the TV meteorologist gave the local weather report in kelvins, there would likely be some confused viewers! Table 11.1 gives the equations for conversion between the three temperature scales.

Worked Example

Converting between Temperature Scales: Room Temperature

Room temperature is generally defined to be 25 $\text{°C}\text{.}$ (a) What is room temperature in $\text{°F}\text{?}$ (b) What is it in K?

Teacher Support

Teacher Support

In an alcohol-in-glass thermometer, alcohol molecules absorb energy by heat, and as the intermolecular distances increase, the bulk alcohol expands. Ask students the temperature range for which this thermometer shows an accurate reading. Why is this the case? Ask them if it is possible to design a thermometer with any other substance. Why or why not?

Strategy

To answer these questions, all we need to do is choose the correct conversion equations and plug in the known values.

Solution for (a)

1. Choose the right equation. To convert from $\text{°C}$ to $\text{°F}$, use the equation
   $$T_{\text{°F}}=\frac{9}{5}{T}_{\text{°C}}+32\text{.}$$

   11.1
2. Plug the known value into the equation and solve.
   $$T_{\text{°F}}=\frac{9}{5}25\text{°C}+32=77\text{°F}$$

   11.2

Solution for (b)

1. Choose the right equation. To convert from $\text{°C}$ to K, use the equation
   $$T_{\text{K}}=T_{\text{°C}}+273.15\text{.}$$

   11.3
2. Plug the known value into the equation and solve.
   $$T_{\text{K}}\text{= 25 °C + 273}\text{.15 = 298K}$$

   11.4

Discussion

Living in the United States, you are likely to have more of a sense of what the temperature feels like if it’s described as 77 $\text{°F}$ than as 25 $\text{°C}$ (or 298 K, for that matter).