Unit 2 Tutorial Solutions

CC2404 Applied Physics and Instrumentation in Health Care Problems Unit 2: Solutions 2.1

What does it mean to say that two bodies are in thermal equilibrium? The temperatures of the two bodies are equal.

2.2

Give an example of a physical property that varies with temperature and describe how it is used to measure temperature. volume – the volume of certain liquids or gases expand when the temperature increases and contract when the temperature decreases. resistance – the resistance of most of the metals increases when the temperature increases and vice verse.

2.3

When a cold mercury thermometer is placed in a hot liquid, the column of mercury goes down slightly before going up. Explain why. Heat transfer from the hot liquid through the glass of the thermometer and causes the glass expands first and thus the mercury goes down slightly. After the heat transfer to the mercury, the mercury expands and goes up.

2.4

For every 100 J of electrical energy which enters a television set 10 J of useful light energy is produced and 5 J of useful sound energy. (a) Calculate the efficiency of the television set 10 J + 5 J = 15% 100 J

(b) Suggest what form the remaining energy might have changed into. Most of the remaining energy converts into heat. 2.5

Explain why a metal window frame feels cold when you touch it but a plastic window frame does not feel so cold. Higher heat conductivity of metals means that more energy transfers from the body to the metals.

2.6

Explain why two sheets of glass separated by a thin layer of air (double glazing) will provide better insulation for your house than a single piece of glass which has twice the thickness. Air is a poor thermal conductor.

2.7

What is the Fahrenheit temperature of a person with a 39.0 °C fever? 9 T = 39 + 32 =102 .2°C 5

2.8

To conserve energy, room temperatures are kept at 68.0 °F in the winter and 78.0 °F in the summer. What are these temperatures on the Celsius scale? 5 9

winter: T = (68 − 32 ) = 20 °C 2.9

5 9

summer: T = (78 − 32 ) = 25 .6°C

Name three methods by which thermal energy can move. Which of these methods is the only one which will carry thermal energy through a vacuum? conduction, convection and radiation. only radiation will carry thermal energy through a vacuum.

2.10 A bimetallic strip is made from aluminium and copper. When heated it bends in the direction shown in the figure below. Which metal expands more for the same rise in temperature? Draw a diagram to show how the bimetallic strip would appear if it were cooled to below room temperature. Aluminium expands more. The bimetallic strip bends upward if it were cooled to below room temperature.

2.11 See lecture notes. 2.12 When water in the vapor phase condenses, is the surrounding air warmed or cooled? Warmed. Energy is released to surrounding when water changes from vapor to liquid. The water vapor’s molecules transfer their kinetic energy to the surrounding air. 2.13 Which of the following methods is not used for core temperature measurement? (c) liquid crystal thermometer