Chapter 15 – Section 1 Temperature and Thermal Energy Pages 434 - 437

What is Temperature? Temperature is a measure of the average value of the kinetic energy of the molecules in random motion. Molecules have more kinetic energy when they are moving faster.

What is Temperature? The higher the temperature, the faster the molecules are moving. 3

Thermal Expansion Almost all substances expand when they are heated and contract when they are cooled. The amount of expansion & contraction depends on the type of material and change in temperature. For example, liquids usually expand more than solids. 4

Example of Thermal Expansion 5

Measuring Temperature Thermometers are used to measure the expansion and contraction of materials to measure temperature. When the temperature of a liquid increases, it expands so the height of the liquid in the tube increases. 6

Temperature Scales Three different temperature scales that are used to measure temperature are degrees: Fahrenheit (°F) Celsius (°C) Kelvin (K) 7

Converting Temperature Scales Fahrenheit to Celsius: °C = 0.5555 * (°F – 32) Celsius to Fahrenheit: °F = (1.8 * °C) + 32 Celsius to Kelvin = °C + 273 Kelvin to Celsius = K - 273 8

Practice Problem #1 On a hot summer day, a Fahrenheit thermometer shows the temperature to be 86° F. What is the temperature on the Celsius scale? Formula: Substitute: Solve: 9

Practice Problem #2 A temperature of 57°C was recorded in 1913 at Death Valley in California. What is this temperature on the Fahrenheit scale? Formula: Substitute: Solve: 10

Practice Problem Table Use the temperature conversion formulas to complete the table below: -18 255 32 273 13 100 338 79 212 11

Thermal Energy So, the temperature of an object is related to the average kinetic energy of the molecules. Molecules also have potential energy, which can be converted to kinetic energy. The sum of the kinetic and potential energy of all the molecules in an object is the thermal energy. 12

Chapter 15 – Section 2 Heat Pages 438 - 443 13

Heat and Thermal Energy Heat is the transfer of thermal energy from one object to another when the objects are at different temperatures. The amount of heat that is transferred when two objects are brought into contact depends on the difference in temperature between the objects. 14

Transfer of Thermal Energy When heat is transferred, thermal energy always moves from warmer to cooler objects. This process of heat transfer can occur in three ways—by conduction, radiation, or convection. 15

Conduction Transfer of heat by direct contact is called conduction. Conduction occurs when the particles in a material collide with neighboring particles. Examples: A frying pan on a burner. Feet on hot sand or a cold tile floor. An ice cube melting in your hand. 16

Radiation Heat is transferred from the Sun to Earth by radiation. Heat transfer by radiation occurs when energy is transferred by electromagnetic waves. There is no direct contact with radiation. Other examples besides the Sun: A fireplace A lamp 17

Convection In a gas or liquid, molecules can move much more easily than they can in a solid. The transfer of thermal energy by the movement of molecules from one part of a fluid to another is called convection. Hot fluids become less dense and rise and cooler fluids are more dense and sink. This creates density currents. 18

Natural Convection Natural convection occurs when a warmer, less dense fluid is pushed away by a cooler, denser fluid. Wind movement near a lake or ocean can result from natural convection. Hot fluids become less dense and rise and cooler fluids are more dense and sink. Thunderstorms and hurricanes are powered by convection. 19

Diagram of a Convection Cells of a Hurricane 20

Thermal Conductors A thermal conductor is any material that easily transfers heat. Metals are typically good thermal conductors because they have loosely held electrons that bump into other materials and transfer heat. Gold and copper are the best conductors of heat. 21

Thermal Insulators A thermal insulator is a material that does not transfer heat easily. Liquids and gases are usually better insulators than solids are. Fluffy insulation is put in-between walls to reduce the amount of heat transfer between the inside and outside of your house. 22

Heat Absorption Why is pavement hotter than grass? The change in temperature of an object as it absorbs heat depends on the material it is made of. The specific heat of a substance is the amount of heat needed to raise the temperature of 1 kg of that substance by 1°C. The specific heat of grass is higher than pavement so it takes more heat to increase the temperature of the grass than it does the pavement. 23