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Temperature and Thermal Energy Chapter 15

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1 Temperature and Thermal Energy Chapter 15

2 What is temperature? Temperature : What is temperature? The measure of the average value of the kinetic energy of the molecules in random motion. The temperature of a substance depends on how fast its molecules are moving. The more kinetic energy the molecules have, the higher the temperature. Thermal Expansion: When the temperature of an object is increased, its molecules speed up and tend to move further apart. This causes the object to expand. Section 1

3 Measuring Temperature
Measuring Temperature: thermometers use the expansion and contraction of materials to measure temperature. Temperature Scales: Fahrenheit: freezing 32°, boiling 212° There are 180 equal degrees on the F scale Celsius: freezing 0°, boiling 100° There are 100 degrees on the C scale Degrees on the Celsius scale are bigger than those on the Fahrenheit scale. *Imagine that clothing requirements change every 10 degrees on the Celsius scale from 0° to 30°. At 0°C, you will need a winter coat. At 10°C, you will need only a jacket. At 20°C, a long sleeved shirt is enough, and at 30°C, shorts and a t-shirt will be most comfortable. Section 1

4 Measuring Temperature
Converting Fahrenheit and Celsius °F to °C: °C= (5/9)(°F-32) °C to °F: °F=(9/5)(°C) +32 Kelvin Scale 0 K is the lowest temperature an object can have on this scale. 0 K is known as absolute zero. K = °C + 273 *Absolute zero molecules still have vibrational energy known as zero point energy. Section 1

5 Thermal Energy Thermal Energy- The sum of the kinetic and potential energy of all the molecules in an object. Potential Energy of Molecules: The molecules in a material have potential energy. As the molecules get closer together or further apart, their potential energy changes. Increasing Thermal Energy: more molecules, more thermal energy. Section 1

6 Heat and Thermal Energy 15.2
Heat is the transfer of thermal energy from one object to another when the objects are at different temperatures. The amount of thermal energy depends on the difference in temperature between the two objects. Thermal energy gets transferred between two objects until both are the same temperature. Section 2

7 Transfer of Thermal Energy
When thermal energy is transferred, it always moves from warmer to cooler objects. The warmer object loses thermal energy and becomes cooler as the cooler object gains thermal energy and becomes warmer. The transfer of thermal energy can occur in three ways: Conduction Radiation Convection Section 2

8 Conduction Conduction is the transfer of thermal energy by direct contact. Conduction occurs when the particles in a material collide with neighboring particles. Example: an ice cube melts in your hand because of conduction. The faster moving molecules in your warm hand bump against the slower moving molecules in the cold ice. Conduction occurs more easily in solids and liquids where atoms and molecules are close together.

9 Radiation Thermal energy transfer by radiation occurs when energy is transferred by electromagnetic waves. The transfer of thermal energy by radiation can occur in empty space, as well as in solids, liquids, and gases. Example: thermal energy is transferred from the Sun to Earth by radiation. Section 2

10 Convection Convection is the transfer of thermal energy by the movement of atoms or molecules from one part of a material to another. Example: as a pot of water is heated, thermal energy is transferred by convection. Section 2

11 Thermal Conductors A conductor is any material that easily transfers thermal energy. Some materials are good conductors because of the types of atoms or chemical compounds they contain. Metals such as gold and copper are examples of good conductors. Metals are made up of loosely held electrons that bump into other atoms and help transfer thermal energy. Section 2

12 Thermal Insulators An insulator is a material in which thermal energy does not flow easily. Example: most pans have handles that are made from insulators. Materials that are good insulators are poor conductors. Example: insulation in houses and buildings helps reduce the transfer of thermal energy between the air inside and the air outside. Section 2

13 Heat Absorption Specific Heat: The specific heat of a material is the amount of thermal energy needed to raise the temperature of 1 kg of the material by 1°C. More thermal energy is needed to change the temperature of a material with a high specific heat than one with a low specific heat. Example: sand on a beach has a lower specific heat than water. Its temperature changes more than the water temperature as both water and sand transfer thermal energy to their surroundings. Section 2

14 Thermal Pollution Thermal pollution is the increase in temperature of a body of water caused by adding warmer water. Increasing the water temperature can cause fish and other aquatic organisms to use more oxygen. Cooling towers are used by some power plants and factories to cool the warm water they produce. Section 2

15 Heat Engines 15.3 A heat engine is a device that converts thermal energy into mechanical energy. Heat engines are used in cars, trucks, etc. Mechanical energy is the sum of the kinetic and potential energy in an object. The Law of Conservation of Energy Energy cannot be created or destroyed. Energy only can be transformed from one form to another. No device, including a heat engine, can produce energy or destroy energy. Section 3

16 Internal Combustion Engines
Internal combustion engine: heat engine in which fuel is burned in a combustion chamber inside the engine. Many machines including cars, boats, lawn mowers, and airplanes use internal combustion engines. Most modern cars have fuel-injected internal combustion engines that have a four-stroke combustion cycle. Thermal energy is converted into mechanical energy as gasoline is burned under pressure inside cylinders. Section 3

17 Four-Stroke Cycle Intake Stroke: Fuel-air mixture fills the cylinder through the intake valve Compression Stroke: Fuel-air mixture is compressed by piston. Power stroke: A spark plug ignites the fuel-air mixture. Exhaust Stroke: Burned gases are pushed out of the cylinder Section 3

18 Refrigerators, Air Conditioners and Heat Pumps
A refrigerator uses a coolant to move thermal energy from inside to outside the refrigerator. An air conditioner uses a coolant to move thermal energy to the outside air. A heat pump absorbs thermal energy from the outside air and transfers it to the air inside. Some buildings use heat pumps for heating and cooling. Section 3

19 Refrigerators 1. The coolant is a liquid as it enters the expansion valve. As it passes through, the sudden drop in pressure makes it expand, cool, and turn into a gas (just like a liquid aerosol turns into a cool gas when you spray it out of a can). 2. As the coolant flows around the chiller cabinet (usually around a pipe buried in the back wall), it absorbs and removes heat from the food inside. 3. The compressor squeezes the coolant, raising its temperature and pressure. It's now a hot, high-pressure gas. 4. The coolant flows through thin pipes on the back of the fridge, giving out its heat and cooling back into a liquid as it does so. 5. The coolant flows back into the expansion valve and the cycle repeats itself. So heat is constantly picked up from inside the refrigerator and put down again outside it. Section 3

20 Review: The Methods of Thermal Energy Transfer
Conduction: If you have ever burned yourself on a hot pan because you touched it, you have experienced this first-hand. Conduction is heat transfer through matter. Convection: Convection is heat transfer by the movement of mass from one place to another. It can take place only in liquids and gases. Heat gained by conduction or radiation from the sun is moved about the planet by convection. Radiation is the only way heat is transferred that can move through space. All other forms of heat transfer require motion of molecules like air or water to move heat. The majority of our energy arrives in the form of radiation from the Sun.

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