Heat and Temperature

There are three states of matter we will be discussing in these next few chapters: solid, liquid, and gas. The molecules of a solid are fixed in a rigid structure. The molecules of a liquid are loosely bound and may mix with one another freely, but the liquid still has a definite volume, although it takes the shape of its container. The molecules of a gas interact with each other only slightly, and generally move at high speeds compared to the molecules of a liquid or solid. But in all three states of matter, the molecules are moving and therefore have energy. They have potential energy because of the bonds between them and kinetic energy because the molecules have mass and speed. Relatively speaking, the potential energy between gas molecules can be ignored, so we will focus only on their kinetic energy. The sum of the potential and kinetic energies of the molecules in a substance is called the internal energy of the substance. When a warmer substance is brought into contact with a cooler substance, some of the kinetic energy of the molecules in the warmer substance is transferred to the cooler substance. The kinetic energy of molecules that is transferred spontaneously from a warmer substance to a cooler substance is called heat energy.

Heat is the kinetic energy of molecules transferred from a warmer substance to a cooler one.

Temperature is the measure of how hot or cold a substance is, relative to some standard. It is the measure of the average kinetic energy of the molecules in a substance.

Temperature is a measure of the average kinetic energy of the molecules in a substance.

The two temperature scales used most widely in scientific applications are the Celsius scale and the Kelvin scale. The only difference between them is where each starts. On the Celsius scale, the freezing point of water is 0°C, and the boiling point of water (at standard pressure) is 100°C. The Kelvin scale has temperature units that are equal in size to the Celsius degrees, but the temperature of 0 Kelvin is absolute zero, defined as the temperature at which all molecular motion in a substance ceases. Zero Kelvin is equal to −273.15°C, so we can convert between the Kelvin scale and the Celsius scale by this equation:

K = °C + 273

(Note that we have rounded 273.15 to 273.)

Mechanical Equivalent of Heat

The unit most often used for mechanical energy is the joule. Historically, the unit for heat has been the calorie. One calorie is defined as the heat needed to raise the temperature of one gram of water by one degree Celsius. In the mid-19th century, the British engineer James Joule showed there is a relationship between energy in the form of work in joules and energy in the form of heat in calories. Joule performed experiments that revealed that doing mechanical work on a substance can make its temperature rise. For example, rubbing your hands together causes them to heat up, and stirring a drink adds heat to it. The conversion between joules and calories is

1 calorie = 4.186 joules.

Quick Quiz

What is the boiling point of water in Kelvins?

Answer:

K = 100°C + 273 = 373 K

The numbers here are not really important for the SAT Subject Test: Physics, but only the concept that mechanical work and heat are both forms of energy and can be converted from one to the other.

Thermal Expansion of a Solid

When a solid is heated, it typically expands. Different substances expand at different rates, which is why it makes sense to heat the lid of a jar when the lid is too tight. The metal lid will expand more than the glass jar when it is heated, making it easier to loosen. Solids undergo two types of expansion when heated: linear thermal expansion, which is the increase in any one dimension of the solid, and volume thermal expansion, which results in an increase in the volume of the solid. In the case of linear expansion, the change in length Δl is proportional to the original length and the change in temperature of the solid. The same is true for volume expansion. The change in volume ΔV is proportional to the original volume of the solid and its change in temperature.

Heat Transfer

There are three ways of transferring heat from one place to another:

1. Conduction. Conduction is the transfer of heat directly through a material, or by actual contact between two materials. Metals are typically good heat conductors. In fact, materials that are good electrical conductors are usually good heat conductors as well. A material that is not a good heat conductor, like wood or air, is called an insulator. If you place an iron skillet on a fire, heat is transferred by conduction to the handle of the skillet. If you grasp the iron handle with your bare hand, you will feel it transfer heat to your hand by conduction.
2. Convection. Convection is the transfer of heat by the bulk movement of a fluid (liquid or gas). If the air near the floor of a cool room is heated, it expands and becomes less dense than the air above it, causing it to rise. As it rises, it cools, becomes more dense again, and falls toward the floor. If the air near the floor is continually heated, the cycle will repeat itself. Water heated in a pan is another example of heat transfer by convection, since water near the bottom of the pan near the fire is heated, rises, cools, then falls again. If the temperature gets high enough, the water begins to boil as it cools itself by transferring heat to the air by convection.
3. Radiation. Radiation is the process by which heat is transferred by electromagnetic waves. We receive heat from the sun by radiation principally in the form of visible light, infrared, and ultraviolet waves. Microwave ovens use microwaves to transfer heat to food. If you stand near a roaring campfire, you will feel the heat radiating from the fire in the form of light and infrared rays.

Three ways of transferring heat are conduction, convection, and radiation.