1. Chapter 6: Thermal Energy

2. Temperature and Heat How do you describe temperature?
We use words such as hot or cold.
All matter is made of tiny particles – atoms and molecules.
In all materials – solids, liquids, & gases – particles are moving in a constant motion.
Faster moving particles have more kinetic energy. Figure 1.

3. Why do some objects feel hot and others feel cold?
Temperature of an object is related to the average kinetic energy of the atoms or molecules.
Faster moving particles = more kinetic energy = higher the temperature
SI unit of temperature is measured in kelvins (K)

4. Thermal energy
Why does cold butter melt sitting on a table in your kitchen?
Air in the room is at a higher temperature than the butter. Molecules in the air have more kinetic energy than butter molecules. Collisions between molecules transfer energy from faster-moving molecules in air to slower-moving molecules in the butter. Molecules in butter start moving faster and the temperature increases.
Thermal energy- is the sum of the kinetic and potential energy of all the molecules in an object.
Figure 2

5. Thermal energy and temperature
Thermal energy and temperature are related
Thermal energy of an object increases when the average kinetic energy of its molecules increases
So the thermal energy of an object increases as its temperature increases

6. Thermal energy and mass
You have a glass and beaker of water that are the same temperature. The beaker contains twice as much water as the glass. The water in both containers is the same temperature. Which has more thermal energy?
The beaker because it has twice as much thermal energy
If temperature doesn’t change, the thermal energy in an object increases if the mass of the object increases.

7. Heat Can you know if someone has been sitting in your chair?
Heat- is thermal energy that flows from something at a higher temperature to something at a lower temperature
Heat always flows from warmer to cooler temperatures
Heat is measured in joules
Figure 3

8. Specific Heat
As a substance absorbs heat, its temperature change depends on the nature of the substance, as well as the amount of heat that is added.
Specific Heat- the amount of heat that is needed to raise the temperature of 1 kg of some material by 1 C or 1 K.
Specific heat is measured in joules per kilogram Kelvin . J/ (kg K)
Water is a coolant because it has the highest specific heat. It can absorb heat without a large change in temperature. Figure 4

9. Thermal energy of an object changes when heat flows into or out of it
Change is related to 1) mass of the object 2) its specific heat 3) change in temp.
Change in thermal energy = mass X change in temp. X specific heat
Change in temp = T final – T initial
Q = m X (T final – T initial) X C

10. When temp changes the amount of thermal energy changes
Heat flows into object its temp usually increases. Change in temp is positive
When temp increases, heat flows into an object, and Q is positive
Heat flows out of object its temp usually decreases
Temp of object decreases, the change in temp is a negative number, and Q is also negative
Specific heat is measured by a device called a calorimeter. Figure 5

11. Sec 2: Transferring Thermal Energy
The transfer of thermal energy through matter by direct contact of particles is called conduction.
Ex: Holding a snowball with your hands, or holding a glass of hot chocolate
Heat can be transferred by conduction from one material to another or through one material. Ex: What happens to a metal ladle that is used to stir soup? Fig:6

12. Conduction can occur in solids, liquids and gases
Conduction can occur in solids, liquids and gases. Which conducts the best?
Solids because their particles are usually much closer together so they collide with each other better.
Silver, Copper, and Aluminum are good conductors
Wood, plastic, and glass are poor conductors.

13. Any material that can flow is a fluid. (liquid or gas)
Convection is the transfer of energy in a fluid by the movement of the heated particles.
In convection the more energetic fluid particles move from one location to another, and carry their energy with them.
Fluid expands as its temperature increases
When fluid expands, its volume increases, but its mass doesn’t change. Its density decreases. The density of warmer fluid is less than the cooler fluid.

14. How does convection occur?
Fig:7 Lamps contain oil and alcohol. When oil is cool it sits at the bottom. When two liquids are heated the oil becomes less dense than the alcohol causing it to rise to the top of the lamp. As it rises, it loses heat by conduction to the cooler liquid around it. Oil becomes denser than alcohol and starts to sink. This rising and sinking action is convection current.
Warmer to cooler

15. Earth’s atmosphere is made up with various gases and is a fluid
Earth’s atmosphere is made up with various gases and is a fluid. The atmosphere is warmer at the equator.
The earth’s surface is warmer than at higher altitudes. These temperature differences create convection currents that carry heat to cooler regions. Fig:8

16. Radiation is the transfer of energy by electromagnetic waves.
These waves can travel through space even when no matter is present. This is how the sun can heat the earth.
Energy that is transferred by radiation is called radiant energy.
When you stand by a fire, much of the warmth you feel is transferred from the fire to your hands by radiation

17. When radiation strikes a material some energy is absorbed, reflected, and some may be transmitted through the material.
Fig:9
The amount of energy absorbed, reflected, or transmitted depends on the material.
The transfer of energy by radiation is most important in gases.
This is because the molecules in gases are spread out more so radiation can pass through them more easily.

18. Humans and animals do things to control heat flow.
Fig:10
A material that doesn’t allow heat to flow through it easily is called an insulator.
Fig:11
Why do we put insulation in houses?
Insulation helps keep warm air inside a house. Insulation is made up with fluffy material, such as fiberglass, that contains pockets of trapped air. It also helps furnaces and air conditioners to work more properly.
About 55% of energy used in U.S. is for heating/cooling
Thermos keeps things hot by how its built. Fig:12

19. Sec3: Using Heat
Heating systems are needed throughout the United States.
All heating systems require some source of energy.
Old heating systems used wood or coal to burn in a stove. Heat is transferred from the stove to surrounding air by conduction, convection, and radiation.
Transfer of heat from the room with stove to other rooms is slow.

20. Forced Air Systems Most common heating system used today. Fig:13
Fuel is burned in a furnace and heats a volume of air. A fan blows the warm air through a series of large pipes called ducts. Ducts lead to openings called vents in each room. Cool air returns to the furnace through additional vents where it is reheated once again.

21. Radiator Systems
Radiators were used before forced-air systems in homes.
A radiator is a closed metal container that contains hot water or steam. The thermal energy contained in the hot water or steam is transferred to the air surrounding the radiator by conduction. This warm air then moves throughout the room by convection.
Fuel is burned in a central furnace and heats a tank of water. A system of pipes carry hot water to the radiator. When water cools it exists through pipes which take it back to water tank to reheat. If steam is created it flows to the radiator and when cools it condenses back to water and flows back to the tank.

22. Electric radiators are not connected to a central furnace. Fig:14
Radiators contain metal coils that are heated when an electric current pass through them. Hot coils transfer thermal energy to the room, mainly by radiation.
Electrical heating system- Uses electrically heated coils placed in ceilings and floors to heat surrounding air by conduction and convection.

23. Solar Heating Solar energy- The energy from the sun
Two types of solar energy
1) Passive- solar energy heats rooms inside a building, but no mechanical devices are used to move heat from one area to another. Materials absorb radiant energy during the day and keep the building warm at night.
Fig:15

24. 2) Active- Active solar heating systems use devices called solar collectors that absorb radiant energy from the sun.
Collectors are usually installed on roof or south side of a building where exposure to the sun is the greatest.
Radiant energy from the sun heats air or water in the solar collectors. This hot air or water is the circulated through the house.
Fig:16

25. A car’s engine converts chemical energy in gasoline to thermal energy by a process called combustion.
During combustion, a fuel such as gasoline combines with oxygen and produces heat and light. Thermal energy is then converted to mechanical energy.
An engine that converts thermal energy into mechanical energy is called a heat engine.

26. Internal combustion engines
Heat engines, those used in cars, is called an internal combustion engine because fuel is burned inside the engine in chambers or cylinders.
4, 6, 8 cylinders. More cylinders produce more power. Each cylinder contains a piston, which moves up and down.
Fig:17- Each up and down movement of the piston is called a stroke.
Fig:18 shows the four-stroke cycle in an automobile engine.
Engines are hot because only part of the thermal energy produced by burning fuel is converted to mechanical energy. Only about 26%.

27. How can a refrigerator stay cold?
A refrigerator moves thermal energy out of the refrigerator into the warmer room.
A heat mover is a device that removes thermal energy from one location and transfers it to another location at a different temperature.
A refrigerator contains coolant that is pumped through pipes on the inside and outside of the refrigerator. Fig:19
In air conditioners, warm air from the room is forced to pass over tubes containing coolant. The warm air is cooled and forced back into the room.

28. How does your body stay cool in hot weather?
Your body uses evaporation to keep its internal temperature constant. As you exercise, your body generates sweat from tiny glands. As sweat evaporates it carries away heat making you cooler. Fig:20
Humid days feel hotter than drier days?
On humid days, more water vapor is in the air around you. Your sweat doesn’t evaporate as quickly. Your body loses heat more slowly.