Chapter 16: Thermal Energy and Heat

1. Chapter 16: Thermal Energy and Heat Section 1: Thermal Energy and Matter

2. Temperature • We think of temperature in terms of hot and cold, but what exactly is temperature? • Temperatureis a term that describes the average kinetic energy of the atoms or molecules that make up a substance. • As the atoms move faster, we perceive an object to be hotter. • If atoms are moving more slowly, we perceive an object to be cooler.

3. Motion of the Molecules • Molecules that are moving very fast (we think of these things as warm or hot) bump into the molecules of objects that are moving more slowly. • When a fast atom bumps into a slow atom, some of the energy is transferred to the slower moving atom (remember the Law of Conservation of Momentum?).

4. Thermal Energy & Temperature • The sum of all of the kinetic and potential energy of all the molecules in an object is known as thermal energy. • As the temperature of an object increases, the kinetic energy of the molecules in the object increase. As a result of this, the total thermal energy of the object increased due to the increase in temperature.

5. Thermal Energy and Mass • If two objects with different masses are at the same temperature, the object with the larger mass will have more thermal energy. • This is the case because the larger object will be composed of more molecules and the sum of the thermal energy will be greater.

6. Heat • Heat is thermal energy that flows from something at a higher temperature to something at a lower temperature. • Heat always goes from HOT  COLD • So if your mom says ”Close the door, you‘re letting the cold in!”, is this a correct statement? • NO, in fact you are letting the heat out!

7. Specific Heat • Specific heatis the amount of heat required to heat a 1 kg object one degree Celsius. • Specific heat is measured in J / kg.C. • Water is a common substance that has a very high specific heat. It takes 4,184 joules of energy to raise one kg of water one degree C. Water can absorb a great deal of heat without changing temperature.

8. Specific Heat values • The lower the specific heat value of a substance, the less energy it takes to raise it’s temperature. • Metals in general have a low specific heat as they are good conductors of heat. • Water has a relatively high specific heat as it takes a lot of energy to change its temperature. Some specific heat values (J/kg.C) Mercury 140 Steel 450 Water (at 15°C) 4184

9. Calculating Changes in Thermal Energy • A formula?.........but, of course Change in thermal energy = mass x specific heat x change in temperature ***OR*** Q = m C Dt • if Q is positive, the temperature of the object increases • If Q is negative, the temperature of the object decreases Q = change in thermal energy (J), m = mass (kg), C = Specific Heat (J/g . C), ∆T = (Tf - Ti)

10. Measuring Specific Heat • Specific Heat can be measured using a device called a calorimeter. • There is a picture and a description of a calorimeter in your book on page 478.