Tiêu đề 6. Transfer of thermal energy.pdf ✅

FOR ONLINE USE ONLY DO NOT DUPLICATE FOR ONLINE USE ONLY DO NOT DUPLICATE Physics for Secondary Schools 189 Transfer of thermal energy Chapter Six Introduction Heat is a form of energy that cannot be seen but its effects can be felt. Humans get heat from different sources, including the sun, electricity, fire, and gas. On hot days people wear light clothes to allow their bodies to cool. On cold days, they wear heavy clothes or stay indoors to keep themselves warm. In this chapter, you will learn about heat transfer by conduction, convection and radiation. You will identify good and bad conductors of heat, ways of minimizing heat loss and apply the knowledge of heat transfer in daily life. The competencies developed from this chapter will enable you to apply heat transfer concepts in daily life situations, including the use of appropriate cooking tools to minimize energy losses, heat reflectors to maintain temperature and heat absorbers for proper heating of homes. Methods of transfer of thermal energy Energy can be transferred by interactions of a system with its surroundings. The form of energy that is transferred by such interactions is called heat. Heat transfer is the transfer of thermal energy due to temperature differences. Whenever a temperature difference exists in a medium or between media, heat transfer must occur. Heat transfer mechanisms can be classified into different types; conduction, convection and radiation, depending on the medium. When a temperature difference (or gradient) exists in a solid object, the transfer mechanism is conduction. Convection refers to heat transfer by the movement of molecules within the fluid under the influence of gravity. In the absence of a medium between two bodies at different temperatures, heat is transferred by radiation. Heat transfer by conduction Atoms in a solid material at a given temperature vibrate about their mean equilibrium positions. The higher the temperature the higher the vibration, and hence the higher the thermal energy in form of vibrational kinetic energy. When a hot object is brought into direct contact with a cold solid object, neighbouring atoms in contact interact. The more energetic atoms of the warm object pass energy to the less energetic PHYSICS FORM III.indd 189 09/03/2022 18:15:39
FOR ONLINE USE ONLY DO NOT DUPLICATE FOR ONLINE USE ONLY DO NOT DUPLICATE 190 Physics for Secondary Schools Student’s Book Form Three Activity 6.1 atoms of the cold object by collision. As a result, atoms of the cold object begin to vibrate vigorously. These vibrations cause atoms to collide with the nearby atoms, causing electrons in such atoms to vibrate more vigorously. Such interaction is transmitted throughout the cold object, resulting in energy transfer from a warmer end to the relatively cooler end. In the presence of a temperature gradient, energy transfer by conduction occurs in the direction of decreasing temperature. After sufficient time, the entire object will attain a uniform temperature. Metals are better conductors of heat than insulators because metals have more free electrons which move through the metal easily. Examples of metals include aluminium, copper, silver, iron and steel. In all these metals, free electrons gain the kinetic energy from collisions with hot atoms and pass on the energy when they collide with cold atoms. Different metals conduct heat with different capacities, for example copper is a better conductor than aluminium and aluminium is a better conductor than iron. Aim: To investigate the transfer of heat through metal solids Materials: candle, matchbox, metal bar (copper or aluminium), wax, stopwatch Procedure 1. Light a candle. 2. Hold one end of a metal bar and heat the other end over the lighting candle, as shown in Figure 6.1. Note your observation. Figure 6.1 3. Remove the candle and let the metal bar cool down to room temperature. 4. Place three waxes A, B and C having the same dimensions at an interval of 5 cm starting from the centre of the metal bar, as shown in Figure 6.2. Figure 6.2 5. Heat one end of the bar with wax on it, and immediately start a stopwatch. 6. Record the time taken by wax A, B and C to start melting. Questions (a) What do you feel when holding a metal bar in step 2? (b) What happens to the wax? Explain. (c) Which part of the rod is hotter than the other? Explain. PHYSICS FORM III.indd 190 09/03/2022 18:15:41