Chapter 10 Heat

Temperature, internal energy and thermal equilibrium Temperature is “a measure of the average kinetic energy of the particles in a substance.” (p. 331). Higher temperature means the particles are moving at higher velocities. Internal energy is “the energy of a substance due to the random motions of its component particles and equal to the total energy of those particles.” (p. 359) More particles at the same temperature means more internal energy. Thermal equilibrium is “the state in which two bodies in physical contact with each other have identical temperatures.”

Temperature, internal energy and thermal equilibrium Temperature relates to any sample of an object, while internal energy depends on the size of the sample. For example, a cup of boiling water is the same temperature as a gallon of boiling water, but the gallon has more internal energy because it has more water. When two objects of different temperatures come in contact, and stay in contact, the hotter object cools and the colder object becomes warmer. Eventually, they will reach the same temperature, which will be between the original temperatures of the two objects. When measuring the temperature of a substance with a thermometer, you must wait until the thermometer is in thermal equilibrium with the object (i.e., the temperature of the thermometer is the same as the temperature of the substance).

Thermal Expansion In general, when the temperature of a substance increases, so does its volume. This is “thermal expansion.” Gases typically expand the most; then liquids; then solids. Mercury in a thermometer expands as temperature rises. There are exceptions. For example, water expands when it freezes.

Measuring Temperature Fahrenheit scale is used in the U.S. Water freezes at 32ºF and boils at 212ºF. The rest of the world uses Celsius. Water freezes at 0ºC and boils at 100ºC. Kelvin is an absolute scale, which means it has only positive values. The lowest possible temperature is 0 K. Water freezes at 273.15 K and boils at 373.15 K. Liquid helium is less than 1 K. Laboratory experiments have reached 0.000001 K.

Temperature Conversions To convert ºC to ºF use: F = (9/5)C + 32.0 To convert ºF to ºC use: C = (5F – 160) / 9 To convert ºC to K use: K = C + 273.15 To convert K to ºC use: C = K – 273.15 Problems 1 -5 on p. 363