Unit 6: Thermal Physics

Unit 6: Thermal Physics

6.1 Thermal Expansion

Objectives Describe qualitatively the 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 100m concrete bridge would expand by about 5cm 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 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 the 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, the thermometric property of a liquid-inglass thermometer is the thermal expansion of the liquid.

Liquid-in-Glass Thermometer 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 the 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 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 Θ 2 C

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 the 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 3:00pm was 40 C. If the thermal capacity of aluminum is about 90 J/ C, how much energy did the bike absorb?