Thermal energy is the total energy of all the atoms and molecules in a substance and is related to how fast those molecules are moving. Temperature is a measure of the average kinetic energy of molecules. Heat is the transfer of thermal energy between substances due to a temperature difference, which will continue until thermal equilibrium is reached where the substances have the same temperature. Specific heat capacity is a measure of how much thermal energy a substance needs to gain or lose in order to change its temperature by one degree.

1. Thermal Energy Ch. 9

2. Thermal energyis the total energy of all the atoms and molecules in a substance. The faster the molecules move, the more kinetic, and thus, thermal energy the substance has.

3. Temperature is the average kinetic energy per molecule in a substance. A substance with a high temperature also has molecules that are moving faster.

4. Concept Check Which has a higher temperature: a small cup of 140⁰F coffee or a large hot tub of 104⁰F water? Which has more thermal energy?

5. Absolute zero (0 kelvin, or 0 K) is nature’s lowest possible temperature, when molecules have no kinetic energy, and stop moving, a theoretical value, not yet observed. 0 K = -273°C Water freezes at 0°C, and 273 K. Water boils at 100°C and 373 K.

6. Heat is the movement of thermal energy from one substance to another because of a difference in their temperatures. When the two substances in contact with each other reach the same temperature, they are said to be in thermal equilibrium.

7. Concept Check When you touch a glass of ice water, does cold travel from the glass to your hand or does thermal energy travel as heat from your hand to the glass?

8. Heat units are energy units, measured in joules or calories. A calorie (cal) is the amount of heat needed to change the temperature of 1 gram, or mL, of water by 1 degree Celsius. 1 cal = 4.18 joules The energy in food is measured by the unit Calorie and is 1000 calories, or kcal.

9. :The Laws of Thermodynamics 1. When thermal energy transfers as heat, there is no net gain or loss. (i.e., Conservation of Energy) 2. The direction of heat flow is from warmer substances to colder substances. 3. No substance can reach absolute zero.

10. Specific heat capacity is a measure of thermal inertia, how much thermal energy a substance needs to gain or lose to change its temperature by one degree.

11. Concept Check Which has a higher specific heat capacity: water or sand? (In other words, which takes longer to get warm in sunlight or cool at night)

12. Thermal expansion is the spreading out of molecules as they heat up and move faster. Different substances have different rates of thermal expansion. Water is unusual because as it freezes the ice crystals spread out making ice less dense than liquid water. Water is most dense (compact) at 4°C.