1. Chapter 9 Thermal Energy

2. Thermal Energy “the total energy of all its atoms and molecules as they wiggle & jiggle, twist & turn, vibrate or race back and forth” Temperature is the measure of the average kinetic energy in a substance Measure of the total energy (potential & kinetic) in a substance Varies with the number of molecules

3. 50 ml of warm water and 100 ml of warm water at the same temperature. Compare the thermal energies.

4. Thermometer Measures temperature by expansion or contraction of a liquid, usually colored alcohol (transfer of thermal energy) Temperature has no upper limit Temperature has a lower limit  Absolute zero: -273ºC

5. Heat The thermal energy transferred from one substance to another due to a temperature difference between the two substances Direction of thermal energy flow is always from a warmer substance to a cooler one Matter doesn’t contain heat, it contains thermal energy Measured in joules

6. Touch the inside of a 220°C hot oven and burn yourself. But when the 1800°C white hot sparks form a 4th-of-July-type sparkler hit your skin, you're okay. Why?

7. Cold An object is cold because it lacks thermal energy; cold is the result of lowered thermal energy

8. Calorie Another unit used to measure heat Amount of heat needed to change 1 gram of water by 1ºC 1 cal = 4.2 J

9. Laws of Thermodynamics 1 st law – whenever heat flows into or out of a system, the gain or loss of thermal energy equals the amount of heat transferred 2 nd law – heat never flows from a cold substance to a hot substance 3 rd law – no system can reach absolute zero

10. Different substances have different capacities for storing thermal energy Apple pie filling vs crust Soup vs toast

11. Different materials require different amounts of thermal energy to raise temperature Boiling water vs milk Because different materials absorb energy in different ways – “jiggling” atoms (kinetic energy) or storing energy for later use (potential energy)

12. Specific heat capacity The amount of heat required to change the temperature of a unit mass of the substance by 1 degree What a substance does with the thermal energy that it acquires (stores the energy as potential or uses it as kinetic energy – raises the temperature) Examples: apple pie filling vs crust; soup vs toast; boiling water vs milk

13. If we had an equal mass of water and sand, which would require more heat energy to change the temperature 1ºC? Which has a higher specific heat capacity, water or sand?

14. Blocks of five different metals – aluminum, iron, copper, zinc, and lead all of same mass and same cross- sectional area, but different heights because they have different densities. First the blocks are put in a pan of boiling water to heat them all to the same temperature. Then they are transferred to a block of paraffin. The diagram shows the relative depths to which they melt the paraffin. This demonstration shows that different materials absorb or give out different amounts of heat, even though the materials have the same mass and undergo the same temperature change.

15. Key points to remember: The higher the specific heat the more thermal energy it takes to raise the temperature of the substance Thermal energy in a substance is the total energy of all its atoms and molecules (both potential and kinetic) Temperature is measuring average kinetic energy

16. Specific heat of some common substances (J/g ˚ C) Aluminum Copper Ethyl alcohol Iron Lead Olive oil Silver Table salt Water.92.39 2.13.46.13 2.02.23.88 4.184

17. If water has such a high specific heat capacity, what does that mean about water when heating and cooling?

18. Why is it warmer by the ocean in the winter and cooler by the ocean in the summer?

19. Law of heat exchange: No thermal energy is lost when substances of unequal temperatures are mixed. In any heat- transfer system, the heat lost by hot substances equals the heat gained by cold substances

20. Thermal Expansion Most substances expand when heated (because molecules in substance “jiggle faster” and move farther apart) and contract when cooled Different substances expand at different rates  Bimetallic strips Liquids expand more than solids with increases in temperature

21. When you can’t loosen a metal lid on a glass jar, how can you use the concept of thermal expansion to rescue the situation?

22. A Concorde supersonic airplane is 20 cm longer when in flight then when parked on the ground. Explain.

23. When the temperature of a metal ring increases, does the hole become larger, smaller, or stay the same?

25. When the temperature of the piece of metal is increased and the metal expands, will the gap between the ends become narrower, or wider, or remain unchanged?

27. Expansion of water - heating Water expands when heated except in the temperature range between 0ºC and 4ºC Figure 9.19 Ice has a crystalline structure with open structured crystals which takes up a greater volume than molecules in the liquid phase 0ºC - 4ºC collapsing ice crystals causes a decrease in volume as the molecules begin moving faster and filling in the empty spaces After 4ºC expansion overrides contraction because most ice crystals have collapsed

28. Expansion of water - cooling Figure 9.22 as water cools, it sinks until the entire pond is at 4ºC. Then, as water at the surface is cooled further, it floats on top and can freeze. Once ice is formed, temperatures lower than 4ºC can extend down into the pond Very deep bodies of water are not ice-covered even in the coldest of winters Because of water’s high specific heat and poor ability to conduct heat, the bottom of deep bodies of water in cold regions remain at a constant 4ºC year round