# Thermal Energy & Heat Chapter 6 Sections 1 & 2.

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Thermal Energy & Heat Chapter 6 Sections 1 & 2

Temperature & Thermal Energy
Section 1

What is Temperature? When we think of temperature we think of how hot or cold something is. Temperature – the measure of the average value of kinetic energy of molecules in random motion.

Think of a glass of water…
Its made of molecules that are in random motion. Random = different speeds – some fast, some slow If they are moving, the molecules have KE. Temperature measures the average value of their movement.

The more kinetic energy the molecules have, the higher the temperature.
High temp = faster molecules Low temp = slower molecules Ex. Water molecules move faster in a cup of hot water than they do in cold water.

Thermal Expansion When molecules speed up, they move farther apart.
The object expands (stretches) When molecules slow down, they move close together. The object contracts (shrinks)

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.

The amount of expansion or contraction depends on:
The type of material Liquids expand more than solids The degree to which temperature changes The greater the temperature change, the more expansion or contraction

Measuring Temperature
Thermometer – a device used to measure temperature Consists of a liquid such as mercury or alcohol sealed inside a narrow tube with markings symbolizing degrees. Temperature changes cause the liquid to rise and fall as the liquid expands and contracts.

Temperature Scales There are three common scales:
Fahrenheit Celsius Kelvin Each is divided into regular intervals

Fahrenheit Scale The most common in the U.S.
Divided into degrees Fahrenheit (oF) Freezing point of water = 32oF Boiling point of water = 212oF Space between boiling and freezing is 180 equal degrees.

Celsius Scale Most commonly used in the rest of the world
Divided into degrees Celsius (oC) Freezing point of water = 0oC Boiling point of water = 100oC Space between boiling and freezing is 100 equal degrees oC are bigger than oF

Kelvin Scale The SI Unit for temperature
Divided into Kelvins (not degrees!) Boiling point of water = 373 K Freezing point of water = 273K Absolute zero = 0 K (the lowest possible temperature an object can have!) Kelvin and Celsius degrees are the same size!

Temperature Conversions
Fahrenheit  Celsius oC = (5/9)(oF – 32) Celsius  Fahrenheit oF = (9/5)(oC) + 32 Please Excuse My Dear Aunt Sally!

Temperature Conversions
Kelvin can only be calculated if you know the object’s temperature in oC. K = oC + 273 oC = K - 273

Thermal Energy Is the sum of the KE and PE of all molecules in an object. All molecules have PE that can be converted into KE They exert attractive forces to each other PE increases as molecules are farther apart.

Temperature Vs. Thermal Energy
How hot or cold something feels does not necessarily give you enough information to determine thermal energy. Thermal energy is related to quantity of the molecules!

Examples A glacier and an ice cube have the same temperature, but the glacier has much more thermal energy due to its size. Compare a glacier to a boiling pot of water. Which has a higher temperature? Which has more thermal energy?

Heat Section 2

Heat Is thermal energy that is transferred 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 two objects. Heat continues to be transferred until equilibrium is reached (both objects are at the same temperature).

Transfer of Heat Thermal energy always moves from warmer to cooler objects. Heat never flows from a cooler object to a warmer object. The warmer object loses thermal energy and becomes cooler while the cooler object gains thermal energy and becomes warmer until their temperatures are equal.

There are three ways heat can be transferred:

Conduction The transfer of thermal energy by direct contact
Ex: The bottom of a pot on a stove is heated by direct contact between it and the heat source Occurs when particles in a material collide with neighboring particles. Energy is passed from molecule to molecule during these collisions.

Occurs most easily in solids and liquids where atoms and molecules are closer together
As a result, heat is transferred more rapidly by conduction in solids and liquids than in gases.

Radiation The transfer of energy in the form of electromagnetic waves
Ex: You feel the warmth of the Sun when standing on the beach. These waves can carry energy through empty space as well as through solids, liquids, and gases.

Convection The transfer of thermal energy by the movement of molecules from one part of a material to another Ex: water boiling in a pot on the stove Occurs most easily in liquids or gases where molecules can move much more easily than in a solid Forms convection currents

Convection Current A continuous cycle of heating and rising, cooling and sinking A substance is heated which makes it less dense and causes it to rise. When it rises it cools, which causes it to become more dense and sink. The process continually repeats itself.

Natural Convection Natural convection occurs when a warmer, less dense fluid is pushed away by a cooler denser fluid. Ex: Wind movement near a lake or ocean is caused when air over land is heated and becomes less dense. Denser cool air rushes in , pushing the warm air up. The cooler air is then heated by the land and the cycle is repeated. You feel the cooler air rushing in as wind.

Forced Convection Occurs when an outside force, such as a fan, pushes a fluid, such as air or water, to make it move and transfer heat Ex: Computer use fans to keep electronic components from getting too hot.

Thermal Conductors Any material that easily transfers heat
If a material has some electrons that are not held tightly by the nucleus, they are free to move around and bump into each other, transferring thermal energy. The best thermal conductors are metals, such as gold and copper.

Thermal Insulators Any material in which heat does not flow easily
Liquids and gases are usually better insulators than solids because their particles are farther apart and cannot transfer thermal energy by conduction as easily as a solid.

Air is a good insulator. Many insulating materials contain air spaces.
Good conductors are poor insulators and vice-a-versa.

Specific Heat The amount of heat needed to raise the temperature of 1 kg of substance by 1 °C More heat is needed to change the temperature of a material with a high specific heat than one with a low specific heat.

Ex: Sand heats up and cools down faster than water.
Hotter than the water during the day Cooler than the water at night

Thermal Pollution The increase in the temperature of a body of water caused by adding warmer water Sources: Hot water produced by power plants that is released into a body of water Rain falling on hot asphalt that runs off into waterways

Reducing thermal pollution:
Effects: Contains less oxygen Can cause some organisms to become more sensitive to chemical pollutants, parasites, & disease Reducing thermal pollution: Use cooling towers to cool warm water before it is released into waterways

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