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HEAT 4.1 UNDERSTANDING THERMAL EQUILIBRIUM 1. Define: (a) Temperature The measure of the degree of hotness of an object. Measured in SI unit Kelvin, K A hot object is at a higher temperature than a cold object. Form of energy, measured in Joules, J Heat is transferred from hotter object (higher temperature) to colder object (lower temperature) (b) Heat When an object is heated, it will absorb heat energy and the temperature will increase. When an object is cooled, it will release heat energy and the temperature will decrease. (c) Thermal contact Two objects are in thermal contact when heat energy can be transferred between them. (d) Heat transfer When two objects with different degrees of hotness come into thermal contact, heat energy is transferred between the two objects. (e) Mechanism of Thermal Equilibrium Energy is transferred at a faster rate from the hotter object to the colder object. Energy is also transferred from the colder object to the hotter one, but at a slower rate. There is a net flow of energy from the hotter object to the colder object. The hotter object cools down while the colder object warms up . After some time, energy is transferred at the same rate between the two objects. There is no net heat transfer between the objects. The two objects are said to be in thermal equilibrium. (f) Thermal Equilibrium When two objects are in thermal equilibrium , there is no net flow of heat between them. Two objects in thermal equilibrium have the same temperature 60

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Example of thermal equilibrium A wet towel is placed on the forehead of a person who has high fever. Initially the temperature of the cloth is lower than the body temperature of the person. Heat energy is transferred from the forehead to the towel until thermal equilibrium is reached. The towel is rinsed in tap water and the procedure is repeated. In this way heat energy is removed from the person. Cooling drinks A hot drink can be cooled by adding a few ice cubes to the drink. Heat from the hot drink is transferred to the colder ice until thermal equilibrium between the ice and water is reached. The final temperature of the drink equal the final temperature of ices. Liquid-in-glass Thermometer The characteristic of the liquid used in liquid-in-glass thermometer 1. be easily seen 2. expand and contract rapidly over a wide range of temperature/ expand uniformly when heated 3. not stick to the glass wall of the capillary tube. How a liquid-in- glass thermometer works? 1. The bulb of the thermometer contains a fixed mass of mercury. The volume of the mercury increases when it absorbs heat. 2. The mercury expands and rises in the capillary tube. The length of the mercury column in the capillary tube indicates the magnitude of the temperature. How can a
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