Unit 6 Temperature & Heat Chapters 21, 22 12/1/2018
Temperature The average kinetic energy within the particles of a substance or object. Hot and cold are only sensations and not measures. Thermometer Galileo credited with making the 1st thermometer. Celsius and Fahrenheit are the scales that we see in common use. 12/1/2018
Temperature 12/1/2018
Thermal Expansion Why do your windows and doors tend to stick more in the summer time? How do you open a stuck jar lid? If you place a handful of ice cubes in a glass of water, will the water level go up, down, or stay the same when they melt? Why do we allow telephone lines to sag when stringing them in the summer? 12/1/2018
Thermal Expansion What happens to materials as they are heated? Do ALL materials expand when heated? Water has special properties… Blinking Christmas lights… 12/1/2018
So, What do you think? 12/1/2018
So, What do you think? 12/1/2018
How thermometers work… What happens to the material in the thermometer as the temperature goes up? Liquids expand in a predictable manner as temperature goes up. Most thermometers rely on expansion of liquids. 12/1/2018
Celsius-Fahrenheit conv. Fahrenheit-Celsius conv. 12/1/2018
The Kelvin scale or absolute scale Based on absolute zero. What is absolute zero? KE in the matter is zero. “No molecular movement” This is a theoretical point 12/1/2018
Celsius-Kelvin conversion K = ºC + 273 0 K = -273 ºC 0 ºC = 273 K ºC ºF K 0 32 273 100 212 373 Freezing point of water Boiling point of water 12/1/2018
You Try it... Convert body temperature to Celsius…and to Kelvins. 98.6ºF = 37ºC = 310K Convert room temperature to Celsius…and to Kelvins. 70.0ºF = 21.1ºC = 294K 12/1/2018
HEAT A transfer of thermal energy between two objects or substances. Transfers in nature are always from high energy to low. High temperature to low. 12/1/2018
Objects can have potential, kinetic, or thermal energy. Objects can not “have” heat or work. These are not properties. Heat and work are energy transfers. 12/1/2018
Thermal Equilibrium Thermal equilibrium is reached when any two or more objects in thermal contact have equal temperatures. Objects in contact in nature seek to be at the same temperature 12/1/2018
What is a calorie? The calorie is widely used to discuss food energy. 1 calorie = 4.18 Joules 1 Calorie = 4184 J = 4.18 KJ 12/1/2018
Think about it… What has more thermal energy, a glass of boiling water, or the pond across the road when the water is 40ºF??? What does thermal energy depend on? Mass Temperature 12/1/2018
So, What do you think? 12/1/2018
Specific Heat Capacity (Cp) The amount of energy transferred as heat that will raise the temperature of 1g of a substance by 1 ºC. Differs for each substance. Can be used to identify a substance. 12/1/2018
Units of Specific heat 12/1/2018
Think about it… On a hot summer day, why does the pool water stay cool? Specific heat of water is higher than the air around it. 12/1/2018
Think about it… What has a higher specific heat capacity, water or sand? How can you tell from every day life??? 12/1/2018
Changes in thermal energy can be measured. Change in thermal energy = Q Q = m·Cp·ΔT ΔT = change in temperature m = mass Cp = specific heat of substance 12/1/2018
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Try it out… Ex #1: How much thermal energy is needed to heat 2.00 kg of copper from 21 ºC to 35 ºC? Ex #2: How much thermal energy is radiated off as a cup of 225 mL of hot coffee cools from 98.0 ºC to 90.5 ºC? 12/1/2018
So, What do you think? 12/1/2018
Global Warming? What does current data show? What does historical data show? What/Who is responsible for current trends? Should you sell your SUV? 12/1/2018
Methods of Energy Transfer Heat energy transfer can occur in 3 ways: Conduction Convection Radiation 12/1/2018
Conduction Transfer of energy as heat between particles as they collide. Within a substance or between two in contact with each other. Objects of differing temperatures in contact with each other will always experience conduction. 12/1/2018
Convection The transfer of energy by the movement of liquids or gases with differing temperatures. As molecules are heated, they expand and become less dense. These particles then rise. º 12/1/2018
Convection: Can only occur in liquids and gases. Solids are too tightly packed. Molecules are not free to move. 12/1/2018
Convection currents This is the cycle of heated (less dense) liquid or gas rising and being replaced by cooler (more dense) that falls to take its place. Responsible for plate tectonics Responsible for weather and ocean current patterns 12/1/2018
So, What do you think? 12/1/2018
Radiation Transfer of heat energy by electromagnetic waves. As these waves strike molecules they result in an increase in the KE of the molecules they contact. Radiation differs from conduction and convection. Does not involve the movement of matter. Radiation is energy transfer that can occur in a vacuum. 12/1/2018
Conductors and Insulators Materials that are conductors transfer energy quickly. Pans are made of metal because metal is a good conductor. Gases are poor conductors, due to particles being so far apart. 12/1/2018
Insulators Solids in general are the best conductors. Rubber and wood are solids which are poor conductors. Metals in general are better than non-metals. Insulators Poor conductors of energy transfer. We use these to prevent unwanted heat transfer. 12/1/2018
Emitters and Absorbers Some materials absorb thermal radiation better than others? Can you name some? Good Absorbers are also good emitters! What is the best color inside/outside for a cold weather sleeping bag? 12/1/2018
So, What do you think? 12/1/2018
Brownies anyone? Why are there different cooking times for brownies if you are cooking in a metal pan vs. a glass pan? Which cooks faster? Why? 12/1/2018