HEAT 4.1 UNDERSTANDING THERMAL EQUILIBRIUM 1. Define: The measure of the degree of hotness of an object. (a) Temperature 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 (b) Heat Heat is transferred from hotter object (higher temperature) to colder object (lower temperature) 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 Two objects are in thermal contact when heat energy contact 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. (f) Thermal When two objects are in thermal equilibrium, there is Equilibrium no net flow of heat between them. Two objects in thermal equilibrium have the same temperature 60
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.
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 1. be easily seen the liquid used in 2. expand and contract rapidly over a wide range liquid-in-glass of temperature/ expand uniformly when heated thermometer 3. not stick to the glass wall of the capillary tube. How a liquid-in1. The bulb of the thermometer contains a fixed glass thermometer mass of mercury. The volume of the mercury works? 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 1. A temperature scale is obtained by choosing thermometer be two temperatures, called the fixed point. calibrated? 2. Definition of ice point and steam point Fixed point Definition Value Lower The temperature of 0°C point: Ice pure melting ice Point 100°C Upper The temperature of point: steam from water that steam point is boiling under standard atmospheric pressure.
When the two fixed points have been marked on the stem of the thermometer, the range between them is divided equally into 100 divisions or degrees. The thermometer now has a scale.
Explain the working principle of a thermometer
When the thermometer is placed in contact with hot water, heat is transferred from hot water to the thermometer. Thermal equilibrium between the thermometer and hot water is reached when the net rate of heat transfer is zero. The thermometer and the water are at the same temperature. At this point, the thermometer reading shows the temperature of the water. 1. 2. 3. 4. It is a good conductor of heat it has a high boiling point, 357°C it expands uniformly when heated it is opaque (does not allow light to pass through) and it can be seen easily. Mercury freezes at a temperature of - 39°C and it is...
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