Chapter 6 Thermal Energy
Temperature & Thermal Energy Section 1 Temperature & Thermal Energy
Temperature A. Temperature is the measure of the average value of the kinetic energy of the molecules in a substance The higher the temperature, the faster the molecules are moving.
Objects contract when they are cooled. 1. Objects tend to expand with increased temperature because their molecules speed up and move farther apart Objects contract when they are cooled. a. the amount of expansion or contraction depends on the material and the amount of change in temperature b. Liquids usually expand more than solids.
examples Solids: Liquids: Gases: In the summertime power lines will sag, whereas in the winter they will be pulled more tightly. Liquids: You probably been told never to put a glass bottle of soda in the freezer. Why? It can explode and you will have glass all over you freezer. Gases: Place a balloon in a very warm room and it will expand, then place it in a very cold room and it will shrink.
2. Temperature is commonly measured using a Thermometer.
Thermometer A thermometer is a device that measures the temperature of things. The name is made up of two smaller words: "Thermo" means heat "meter" means to measure. One of the earliest inventors of a thermometer was probably Galileo. Galileo is said to have used a device called a "thermoscope" around 1600 - that's about 400 years ago!!
How does a thermometer work? When you look at a regular outside bulb thermometer, you'll see a thin red or silver line that grows longer when it is hotter. The line goes down in cold weather. This liquid is sometimes colored alcohol but can also be a metallic liquid called mercury. Both mercury and alcohol grow bigger when heated and smaller when cooled. Inside the glass tube of a thermometer, the liquid has no place to go but up when the temperature is hot and down when the temperature is cold. Numbers are placed alongside the glass tube that mark the temperature when the line is at that point.
Another type of thermometer "spring" thermometer. A coiled piece of metal that is sensitive to heat is used. One end of the spring is attached to the pointer. As the air heats, the metal expands and the pointer moves higher. As the air cools, the metal contracts and the pointer moves lower. Typically, these type of thermometers are less accurate than bulb or digital thermometers.
3. Thermometers need numbers on a scale to give it a temperature reading. a. Fahrenheit scale: Water freezes at 32F Water boils at 212F b. Celsius scale: Water freezes at 0C Water boils at 100C
C = (5/9) (F - 32) C = (5/9) (F - 32) C = (5/9) (78 - 32) c. The formula to convert temperature from F to C is: C = (5/9) (F - 32) If it is 78F in the classroom how many C is it? C = (5/9) (F - 32) C = (5/9) (78 - 32) C = (.55555556) (46) C = 25.6C
F = (9/5) (C) + 32 F = (9/5) (C) + 32 F = (9/5) (21) + 32 d. The formula to convert temperature from C to F is: F = (9/5) (C) + 32 If the classroom was 21C what would it be in F? F = (9/5) (C) + 32 F = (9/5) (21) + 32 F = (1.8) (21) + 32 F = (37.8) + 32 F = 69.8F F = 70F
f. Formula for the Kelvin scale C = K - 273 K = C + 273
e. The lowest possible temperature an object can have is 0 K known as ABSOLUTE ZERO
B. An object’s Thermal energy is the sum of the kinetic & potential energy of all the molecules in the object. 1. Potential energy is energy that can be converted into kinetic energy. Potential energy changes as molecules move closer together or farther apart. 2. Temperature and thermal energy are different concepts Thermal energy is related to the quantity of molecules.
Why is temperature and thermal energy different? Lets say you had 2 glasses of water at exactly the same temperature. You poured them into a pot. Will the temperature of the water change? NO Will the thermal energy change? YES why? BECAUSE THERE ARE MORE MOLECULES. How can we know this for sure? IT TAKES LONGER FOR MOLECULES TO HEAT UP OR COOL DOWN WHEN THERE ARE MORE OF THEM AS OPPOSED TO LESS. IT TAKES LONGER FOR A SWIMMING POOL OF WATER TO WARM UP IN THE HOT SUN COMPARED TO JUST A GLASS OF WATER.
Section 2 Heat
Heat A. Heat is the thermal energy that is transferred from one object to another when the objects are at different temperatures. 1. Thermal energy always moves from warmer to cooler objects. How do we know this? Easy experiment: take something warm like hot soup and put an ice cube in it. What happens to the ice cube? Kitchen tip: You want to defrost a frozen steak without a microwave cooking the edges even when you have it on defrost: place the frozen meat on a cast iron pan. Cast iron is heavy, therefore it has lots of molecules. The heat from the molecules will transfer into the meat and defrost you steak in about an hours time or less.
2. The transfer of heat by direct contact between the particles of a substance is called CONDUCTION. Conduction occurs most easily in solids, where molecules are close together. If you placed a pot of water and a metal spoon on a hot burner. The spoon will get hotter much quicker. Common sense says don’t pick up the spoon unless you are using a hot pad. So don’t try this at home… if you have no common sense!
3. Heat transfer by RADIATION occurs when electromagnetic waves carry energy through space or matter. Why does the road get hot in the summer? BECAUSE THE SUN’S RAYS TRAVEL ABOUT 92 MILLION MILES THROUGH SPACE DO SO. People go to tanning beds and become tan. Why? BECAUSE THE RADIATION COOKS THEIR SKIN.
4. CONVECTION describes the transfer of thermal energy by the movement of molecules from one part (warmer) of the material to another (cooler) part.
a. Convection occurs naturally as hot gas or liquid moves from one place to another Wind is caused by convection in the air Rising warmer air forms a convection cycle with falling cooling air.
Where we can see Convection taking place. Wind: air heats up land in one area more than another. Like you saw on the other page. In the mantle, In the Sun, Even in a Bowl of soup.
B. CONDUCTORS are materials that transfer heat readily. Metals such as copper and gold are the best heat conductors C. INSULATORS are materials that does not transfer heat readily. Liquids and gases are usually better insulators than solids.
D. Objects absorb heat at different rates depending on what materials they are made of. 1. SPECIFIC HEAT - amount of heat needed to raise the temperature of 1 kg of a substance by 1C 2. More heat is needed to change the temperature of a material with a high specific heat (such as water) than one with a low specific heat such as sand.
Land & Sea breezes Day time when the sun is shining the ground heat up much faster than the water does. 70F 80F Night time the land cools much quicker than the water does 60F 70F
E. Thermal Pollution caused by adding warmer water to a body of water. 1. Thermal pollution can kill fish and other aquatic organisms due to a reduction in oxygen in warmer water. 2. Thermal pollution can be reduced by cooling water from factories, power plants, and runoff before it is released into a body of water.
Engines & Refrigerators Section 3 Engines & Refrigerators
A. Engines are devices that convert thermal energy into mechanical (Kinetic) energy. 1. In an EXTERNAL COMBUSTION ENGINE such as a steam engine, the fuel is burned outside the engine to produce thermal energy. 2. In an INTERNAL COMBUSTION ENGINE, fuel burns in a combustion chamber inside the engine.
3. Most cars have a four-stroke engine with 4 or more combustion chambers, or cylinders. A. Each cylinder contains a piston that can move up and down. B. a mixture of fuel and air is injected into the cylinder and ignited with a spark, that pushes the piston down. This up-and-down motion of the pistons turns a rod called a crankshaft, which turns the wheels of the car. how a four-stroke engine works
4. Other types of internal combustion engines include: diesel engines, which use high pressure instead of a spark for ignition two-stroke gasoline engines, commonly used in lawn mowers Chainsaw
B. Refrigerators absorb heat from food and materials inside the refrigerator and transfer it to the surrounding air. 1. A liquid coolant is changed into a cold gas that absorbs heat from inside of the refrigerator. 2. A compressor compresses the coolant gas, making it warmer than room temperature. 3. The coolant gas transfers heat to the room, then back into a coolant liquid, and the cycle is repeated. 4. An air conditioner works much like a refrigerator to cool a house 5. A heat pump can be used for cooling and heating a house by reversing itself based on outside temperatures.