Thermal Physics / Temperature PDF

Title Thermal Physics / Temperature 1  Outline DEFENTION OF TEMPERATURE 17.1 Chapter 17.4 (17) 17.2 THERMAL TEMPERATURE EXPANSION RANGE...

Title Thermal Physics / Temperature 1  Outline DEFENTION OF TEMPERATURE 17.1 Chapter 17.4 (17) 17.2 THERMAL TEMPERATURE EXPANSION RANGES 2 Learning outcomes After studying this chapter, you will be able to: ❑ Understand the definition of temperature. ❑ Compare the temperature scales. ❑ Convert between the various temperature scales. ❑ Discuss the thermal expansion principle. ❑ Describe the linear expansion. 3  Definition of Temperature Temperature ▪ Temperature is a concept we all understand from experience. ▪ We hear weather forecasters tell us that the temperature will be 72 °F today. ▪ We hear doctors tell us that our body temperature is 37 °C. ▪ When we touch an object, we can tell whether it is hot or cold. ▪ If we put a hot object in contact with a cold object, the hot object will cool off and the cold object will warm up. o If we measure the temperatures of the two objects after some time has passed, they will be equal. o The two objects are then in thermal equilibrium. 4  Definition of Temperature Temperature Scales Several systems have been proposed and used to quantify temperature; the most widely used are the Fahrenheit, Celsius, and Kelvin scales. Fahrenheit temperature scale ❖ This scale defines the freezing point of water as 32 °F and the boiling point of water as 212 °F. Celsius temperature scale ❖ This scale defines the freezing point of water at 0 °C and the boiling point of water at 100 °C. Kelvin temperature scale ❖ This scale is based on the existence of absolute zero, the minimum possible temperature. ❖ This scale defines the freezing point of water at 273.15 K ≈273 K and the boiling point of water at 373 K. ❖ the kelvin is the standard SI unit for temperature. 5  Temperature ranges Representative Temperatures Here are some representative temperatures. 6  Temperature ranges Temperature Scales ▪ The formulas for converting between the various temperature scales are: ▪ Fahrenheit to Celsius 𝟓 𝑻𝒄 = (𝑻𝑭 − 𝟑𝟐) 𝟗 ▪ Celsius to Fahrenheit 𝟗 𝑻𝑭 = 𝑻𝒄 + 𝟑𝟐 𝟓 ▪ Celsius to Kelvin 𝑻𝑲 = 𝑻𝑪 + 𝟐𝟕𝟑 ▪ Kelvin to Celsius 𝑻𝒄 = 𝑻𝑲 − 𝟐𝟕𝟑 7 Concept Check ▪ Which of the following temperatures is the coldest? A. 10 °C B. 10 °F C. 10 K 8 Concept Check ▪ Which of the following temperatures is the coldest? A. 10 °C B. 10 °F C. 10 K 9 Example 17.1 ▪ Room temperature is often taken to be 72.0 °F. What is room temperature in the Celsius and Kelvin scales? 10 Example 17.1 ▪ Room temperature is often taken to be 72.0 °F. What is room temperature in the Celsius and Kelvin scales? SOLUTION: ▪ Convert Fahrenheit to Celsius: 5 𝑇𝑐 = 72 − 32 = 22.2 °C 9 ▪ Convert Celsius to Kelvin: 𝑇𝐾 = 22.2 + 273.15 ≅ 295 𝐾 11 Sample Problem 17.1 On a cold winter day, you notice that the temperature reading in Fahrenheit degrees is 10.0 degrees higher than that in Celsius degrees. a) Find the temperature in Celsius degrees? 12 Sample Problem 17.1 SOLUTION: a)The temperature that meets the 10-degrees-higher condition can be obtained from: 9 𝑇𝑐 + 10 = 𝑇𝐹 𝑇𝐹 = 𝑇𝑐 + 32 5 9 4 𝑇𝑐 + 10 = 𝑇𝑐 + 32 → −22 = 𝑇𝑐 → 𝑇𝑐 = −27.5°C 5 5 13  Thermal expansion Most of us are familiar with thermal expansion. ▪ You may have seen that bridge spans contain gaps in the roadway to allow for the expansion of sections of the bridge in warm weather. ▪ Why do materials expand due to a change in temperature? Its known that all matter is made of atoms. The atoms vibrate, and the amplitude of their vibrations is a function of the temperature of the matter. As a general rule, a higher temperature means larger vibrations and thus greater atomic spacing. Therefore, solids and liquids expand with increasing temperature. ▪ Thermal expansion can occur as linear expansion, area expansion, or volume expansion 14  Thermal expansion Linear Expansion Let’s consider a rod of length L. We raise the temperature of the rod by: The length of the rod increases by: The increase in length is given by: The quantity α is the linear expansion coefficient. 15  Thermal expansion Linear Expansion ▪ The linear expansion coefficient is a constant for a given material within normal temperature ranges. ▪ Some typical linear expansion coefficients are listed in the given Table. 16 Sample problem 17.2 The center span of the Mackinac Bridge has a length of 1158 m. The bridge is built of steel. Assume that the lowest possible temperature is – 50 °C and the highest possible temperature is 50 °C. How much room must be made available for thermal expansion of the center span of the Mackinac Bridge? 17  Sample problem 17.2 The center span of the Mackinac Bridge has a length of 1158 m. The bridge is built of steel. Assume that the lowest possible temperature is – 50 °C and the highest possible temperature is 50 °C. How much room must be made available for thermal expansion of the center span of the Mackinac Bridge? SOLUTION: ▪ The linear expansion coefficient of steel is α = 13·10–6 °C–1. ▪ The linear expansion of the center span of the bridge that must be allowed for is given by: 18  Equation summary (1) (2) 5 TC= (TF - 32 oF) 9 (4) (3) TK= (TC + 273.15 oC) TC= (TK - 273.15 K) (5) 19 The END OF CHAPTER (17) 20