Unit 6 Thermal Physics 6 1 Thermal Expansion
- Slides: 59
Unit 6: Thermal Physics
6. 1 Thermal Expansion
Objectives • Describe qualitatively thermal expansion of solids, liquids, and gases at constant pressure • Identify and explain some of the everyday applications and consequences of thermal expansion
Thermal Expansion Most substances expand when heated Hot air balloon Thermal Expansion: the expansion of a substance due to increasing its temperature. Expansion depends on the type of substance and its state
Stop and predict Neil has 2 balloons, one filled with water and the other filled with air. He makes sure that they are both the same temperature, 20°C. He then heats them both 50°C. Which balloon do you think expanded more? Why?
Thermal Expansion Gases expand more than liquids for the same temperature rise.
Thermal Expansion - Gas A burner under the balloon causes the balloon to fill with hot air which then lifts the balloon. This is an example of thermal expansion of gases.
Thermal Expansion - Solids Mr. Mac’s sister had this major problem of screwing on soda bottles way too tight when he was a kid. When he couldn’t open the bottle, he would place the cap under hot water for a few seconds. He could then open the cap with ease. Explain why.
Thermal Expansion - Solids What do you think happens to the freeway bridges in the summer in Arizona?
Thermal Expansion - Solids Expansion Gaps: A 100 m concrete bridge would expand by about 5 cm if its temperature increased 50°C. Without expansion gaps, the bridge would buckle.
Thermal Expansion - Solids Bimetallic Strip: strip of 2 different metals stuck together. When the temperature of the strip rises, one metal expands more than the other so the strip bends. Used in fire alarms and thermostats
Thermal Expansion - Summary In general, the volume of a solid increases by no more than about 0. 01% for a temperature increase of 1°C. Most liquids expand slightly more. Gases at constant pressure expand about 30 times more than solids or 0. 30% for a temperature increase of 1°C.
6. 2 & 6. 3 Thermometers
Objectives • Appreciate how a physical property that varies with temperature may be used for the measurement of temperature, and state examples of such properties • Recognize the need for and identify fixed points • Describe and explain the structure and action of liquidin-glass thermometers
Temperature is a measure of hotness. When a weather forecast tells you that tomorrow’s outdoor temperature could be 10°C lower than today, you can expect a much cooler day. Fixed points are used to define a scale of temperature. These points are ‘degrees of hotness’ that can be reproduced precisely. They are usually the melting and boiling points of pure substances.
Celsius Scale The Celsius scale of temperature (°C) is defined by two fixed points which are: Ice point at 0°C, the temperature at which pure ice melts Steam point at 100°C, the temperature at which pure water boils at standard at atmospheric pressure. The length between the two calibrations can then be marked with 100 equal intervals, each corresponding to a temperature change of 1°C. The temperature according to thermometer is then read from the position of the liquid meniscus.
Hypothesize… What do you think is the mechanism a thermometer uses to measure temperature? AKA How does a thermometer actually work? Write your hypothesis down! It might help to draw a picture
Types of Thermometers Thermometric Property: Every thermometer makes use of a physical property that varies with temperature. For example, thermometric property of a liquid-inglass thermometer is thermal expansion of the liquid.
Liquid-in-Glass Thermometer consists of a thin, glass bulb joined to a capillary tube with a narrow bore which is sealed at its other end. The liquid fills the bulb and the adjoining section of the capillary tube.
Liquid-in-Glass Thermometer When the bulb becomes warmer: The liquid in it expands more than the bulb so some of the liquid is forced into the capillary tube. The thread of liquid in the capillary tube increases in length. The liquid used contains mercury or colored alcohol. Alcohol has a lower freezing point than mercury so it is better for low-temperature measurements.
6. 4 Thermal Capacity
Learning Objectives • Relate a rise in the temperature of a body to an increase in its internal energy (6. 4) • Show an understanding of what is meant by thermal capacity of a body (6. 4)
Thermal Capacity In the summer, have you ever noticed that some things are hotter than others, even when they have been in the sun the same amount of time? Why do you think that is?
Thermal Capacity: the energy that must be supplied to raise an object’s temperature by 1°C. If an object is supplied with energy E and its temperature increases from Θ 1 to Θ 2 Equation Quantity Symbol Unit c = E/(Θ 2 – Θ 1) Thermal Capacity C J/°C Energy E J Initial Temperature Θ 1 °C Final Temperature °C Θ 2
Thermal Capacity For example, if an object is supplied with 5, 000 J of energy and its temperature increases by 2°C, its thermal capacity is 2, 500 J/°C. (5000 J / 2°C)
Example #1 A cup of water has an initial temperature of 15°C. You leave it out on the counter for about an hour and then measure its temperature again. It is then 21°C. You know the water was supplied with 2, 500 J of energy. What is thermal capacity of the water?
Example #2 You accidentally left your aluminum bike in the sun one afternoon. The initial temperature of the bike in the morning was 22°C and the temperature at 3: 00 pm was 40°C. If thermal capacity of aluminum is about 90 J/°C , how much energy did the bike absorb?
6. 5 Change of State
Objectives • Describe melting and boiling in terms of energy input without a change in temperature (6. 5) • State the meaning of melting point and boiling point (6. 5) • Describe condensation and solidification in terms of molecules (6. 5)
Melting and Boiling Points When pure ice is heated and melts, its temperature remains at 0°C until all the ice has melted. When water is heated and it boils at atmospheric pressure, its temperature remains at 100°C. For any substance undergoing a change of state, its temperature stays the same.
Melting and Boiling Points We refer to this temperature as the melting point or the boiling point of the substance according to the type of change. Change Initial and Final State Temperature Melting Solid to Liquid Melting Point Freezing (also called solidification) Liquid to Solid Boiling Liquid to Vapor Condensation Vapor to Liquid Boiling Point
Energy and Heat Suppose a beaker of ice below 0°C is heated steadily so that the ice melts and then the water formed from the melted ice boils. Draw a simple graph showing how the temperature changes with time.
Energy and Heat The Temperature: 1. Increases until it reaches 0°C when the ice starts to melt at 0°C. 2. Remains constant at 0°C until all the ice has melted. 3. Increases from 0 to 100°C until the water in the beaker starts to boil at 100°C. 4. Remains constant at 100°C until all the water turns to steam.
Think-Pair-Share Why do you think the graph plateaus at 0°C and at 100°C? 1 s will share out.
Think-Pair-Share 1. When you get a cold soda on a hot day, what happens to the outside of the cup? 2. Why do you think that happens? 2 s will share out.
Condensation State change from a gas into a liquid. Gas molecules near a cool surface lose kinetic energy, collect together and change into a liquid.
Solidification Also known as freezing. State change from a liquid into a solid. Liquid molecules cool down even more, lose more kinetic energy, and collect together in a rigid structure.
6. 7 Heat Transfer
Think-Pair-Share 1. Explain what happens when you leave a metal spoon in a pot of boiling ramen noodles. Why does the spoon feel like that? 3 s share out
Thermal Conduction Rods of different metals and non-metals could be used in the test. The results show that: Metals conduct better than non-metals Copper is a better conductor than steel
Conductors in Metals contain lots of conduction (or ‘free’) electrons. These electrons have broken free from the atoms, leaving each atom as a positively charged ion. Free electrons move about at random inside the metal and bond the positive ions together. They collide with each other and with the positive ions.
Conductors in Metals When a metal rod is heated at one end, the free electrons at the hot end gain kinetic energy and move faster. These electrons diffuse and collide with other free electrons and ions in the cooler parts of the metal As a result, they transfer kinetic energy to these electrons and ions. Energy is therefore transferred from the hot end of the rod to the colder end.
6. 8 Convection
Objectives Recognize convection as an important method of thermal transfer in fluids (6. 8) • Relate convection in fluids to density changes and describe experiments to illustrate convection (6. 8)
Convection happens whenever a fluid (i. e. a gas or liquid) is heated. The fluid expands where it is heated and becomes less dense. As a result, the heated fluid rises – just as objects in water float if they are less dense than water. As the fluid rises, it mixes with colder fluid and becomes cooler. The convection currents in a fluid transfer energy from the hotter parts to the cooler parts.
Using Convection Hot water at home: using a hot water tank, hot water from the boiler rises and flows into the tank where it rises to the top. When you use a hot water tap at home, you get hot water from the top of the tank.
Using Convection Hot air Balloon: A burner in the balloon heats the air making it less dense than the surrounding air. So the balloon rises.
Using Convection Sea Breezes: On a sunny day, the ground heats up faster than the sea. So the air above the ground warms up and rises. Cooler air from the sea flows in as a sea breeze to take the place of the warm air.
How Convection Works Convection takes place: Only in liquids and gases Due to circulation (convection) currents within the substance Circulation currents are caused because the fluids rise when heated (less dense) and fall when cooled (more dense). Why do you think these fluids are less dense when heated? The particles gain KE because they are heated, so fewer particles take up the same volume, because they are moving more quickly.
Thermal Expansion of a Gas A gas at constant pressure expands when it is heated. We can use this property to measure temperature. We can measure the volume of a gas a constant pressure at different temperatures. The gas is air trapped in the tube by a liquid thread. As the water bath is warmed, the gas trapped in the glass tube becomes hotter and expands, pushing the liquid up the tube. The length of the gas column increases. This is a measure of the gas volume.
Thermal Conduction If you wanted to conduct an experiment to see how well different materials conduct heat you could do the following: Compare rods of different materials as conductors: Rods need to be the same width and length Each rod would be coated with a thin layer of wax near one end The uncoated ends are then heated together The wax melts the fastest on the rod that conducts best
Unit 6. 9 Infrared Radiation
Infrared Cameras They can “see” animals and people in the dark Every object around us emits infrared radiation. The hotter an object is, the more infrared radiation it emits. If an object is very hot, it emits light too.
EM Spectrum Light and infrared (IR) radiation are part of the electromagnetic spectrum.
Detecting IR Radiation To detect IR radiation, we can use a thermometer with a blackened bulb. A prism splits a narrow beam of light from a lamp into the colors of the spectrum. The thermometer reading rises when placed just beyond the red part of the spectrum. Some of the IR radiation from the lamp bulb goes there.
Detecting IR Radiation It is called infra-red because it is next to the red part of the visible spectrum. Our eyes cannot detect IR radiation, but thermometer can. The radiation emitted by an object due to its temperature is sometimes called thermal radiation. This is because it can include light as well as IR radiation if the object is hot enough. IR Radiation doesn’t require a medium to travel through.
Which Surfaces are the best absorbers of IR Radiation? A dark surface absorbs infrared radiation better than a light surface A matte surface absorbs infrared radiation better than a shiny surface because it has a lot of cavities. Dark, matte surfaces absorb infrared radiation better than light, shiny surfaces.
Heat Transfer by Design Lots of things can go wrong if heat transfer isn’t controlled. A few ways that we keep things cool are the following: Vehicle radiator: transfers heat energy from the engine to the surroundings. It uses the conduction, convection, and radiation. Heat Sink: stops an electrical component from overheating. It increases the surface area of the component which increases heat losses due to convection and radiation.
Heat Transfer by Design The Vacuum Flask: keeps a hot drink hot and a cold drink cold. The liquid is in the double-walled glass or plastic container. The vacuum minimizes conduction and convection. Glass is a poor conductor so there is little heat conduction through the class. The glass surfaces are silvery to reduce radiation from the outer wall.
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