Heat

The heat of an object is the total kinetic energy of its molecules, while the temperature of the object is the measurement of the average energy of its molecules. The Molecular Theory of Matter gives an explanation of heat and temperature. Specific heat is a way to compare heat from different materials. Heat energy can cause expansion in many materials.

Questions you may have about heat include:

• How does the Molecular Theory explain heat?

• What is specific heat?

• Why do things expand when heated?

Molecular Theory explains heat

An explanation of what heat is comes from the Molecular Theory of Matter, which states that matter is made up of tiny particles called molecules. These particles are in constant motion within the bounds of the material.

Velocity of molecules

Since the relationship between kinetic energy of an object and its velocity is:

KE = � mv2,

where:

• KE is kinetic energy or energy of a moving object

• m is the mass of the object

• v is the velocity

• ^v2 is the velocity squared or velocity times itself

This means that the more energy an object has, the faster it is traveling (or vice versa).

Thus, when you provide extra energy to an object, you cause its molecules to speed up. Those molecules, in turn, can cause other molecules to speed up. The sum effect of the speed or energy of these molecules is the phenomenon we call heat.

Sources of heat energy

Molecules can go into high-energy motion, causing heat, from various energy sources.

• Light from the sun radiating on an object can transfer energy to the object's molecules, causing them to move faster. In other words, the object heats up.

• Chemical reactions (such as something burning) cause nearby molecules to accelerate and thus produces the effect of heat.

• Electrical resistance can heat a wire, causing its molecules to move faster.

• Friction and nuclear reactions can also produce heat.

Specific heat

Since the molecules of various materials have different weights and sizes, the amount of energy required to speed up those molecules will depend on the type of material. Also, since the amount of heat an object has depends on the total energy of its molecules, that energy is proportional to the mass of the object. From these factors, we can define a unit of heat for a specific material. This is called the specific heat.

Energy to raise temperature

The specific heat of a material is the amount of heat (or energy) required to raise the temperature of 1 gram of the substance 1 degree C. This unit of measurement of this amount of heat is called a calorie. The specific heat of water is defined as 1.0 calorie/gram-degree.

Note that this calorie is different than the dieter's Calorie (with a capital "C").

BTU

In the English system, specific heat is the amount of heat required to heat 1 pound of water 1 degree F. Its unit of heat is the BTU (British Thermal Unit). Often, you can see heaters and furnaces in the U.S. rated in BTUs.

Thermal expansion

An early observation on the effect of heat is the fact that most substances expand when heated and contract when cooled. One application of this is seen in the several inch separations in sections of freeway overpasses, which are there to account for the expansion of the overpass in the summer, when the heated material expands.

Water and ice

Although water contracts when cooled, the change to ice is an exception to that rule. When water changes to ice, it expands. This is a major cause of potholes in the street in the winter. After it has expanded, ice later contracts slightly as it cools.

Gases

The Law of Charles states that the volume of a gas varies directly with its absolute temperature, if pressure is held constant:

V is proportional to T

where:

• V is the volume of the gas

• T is the absolute temperature of the gas is degrees Kelvin

This means that as you heat a gas, its volume becomes greater. This is seen in the example of the hot air balloon. It also means that if you compress or expand a gas, you can increase or decrease its temperature and heat.

The refrigerator is an application of this law, because it uses the expansion of gas to decrease temperature.

In conclusion

Heat of an object is the total kinetic energy of its molecules. The Molecular Theory of Matter shows when molecules have more energy, they move faster. Specific heat is a way to compare heat from different materials. Heat can cause expansion in many materials.