Hwa Chong Institution Sec 3 (SMTP) Name: ________________________________________ Class: __________ Date: ___________
Sec 3 Physics (SMTP) Topic 10: Transfer of thermal energy
THERMAL EQUILIBRIUM & THE ZEROTH LAW OF THERMODYNAMICS Thermal Equilibrium If you want to know the temperature of a cup of hot coffee, you stick a thermometer in the coffee. As the two interact, the thermometer becomes hotter and the coffee cools off a little. After the thermometer settles down to a steady value, you read the temperature. The system has reached an equilibrium condition; further interaction between the thermometer and the coffee cause no more changes. This is a state of thermal equilibrium. If the systems are separated by an insulator, such as wood, plastic or fiber glass, they influence each other more slowly. An ideal insulator is a material that permits no interactions at all between the two systems. It prevents the systems from attaining thermal equilibrium if the systems are not in thermal equilibrium at the start. That’s why ice is packed in insulating materials so that the insulation delays the process of achieving thermal equilibrium with the surrounding. Zeroth law of thermodynamics If system A is in thermal equilibrium with system C, and system B is in thermal equilibrium with System C, then System A and System B must be in thermal equilibrium. Important implication:
THERMAL ENERGY & HEAT Thermal energy is also frequently described as internal energy of a system. The internal energy of a system is the sum of kinetic energy and potential energy possessed by the molecules of the system. Heat is the amount of thermal energy transferred from a region of higher temperature to a region of lower temperature. Important note: Heat and work are energy in transit. A body does not contain work; a force is needed to transfer energy between two interacting mechanical system. Likewise, a body does not contain heat; heat is the transfer of energy between two systems due to a temperature gradient. Does your body contain heat? If not, what does it contain?
Hwa Chong Institution Sec 3 (SMTP) Example 1 The diagram shows two interacting bodies A and B. How would the heat flow; from A to B or B to A? A Internal energy = 100000 J Temperature = 40oC B Internal energy = 4000 J
Temperature = 100oC
Direct contact and thermal contact Two systems in direct contact mean that they are physically touching each other. Two systems in thermal contact may not always be in direct contact. Can you think of a situation where two objects are in thermal contact but not touching each other?
CONDUCTION Conduction is the process by which thermal energy is transferred in a medium from faster vibrating particles (solids) or faster moving particles (liquids/gases) to slower ones through collisions. When one end of the rod is heated, the local molecules gain energy and vibrate faster. These molecules collide with their neighbouring molecules and transfer some energy to them. This process carries on with the next layer of neighbouring molecules and the energy is passed from the region of high temperature to regions of lower temperature on the rod.
Why are metals good conductors of heat?
Hwa Chong Institution Sec 3 (SMTP) Example 2 The tiled floor feels colder than the carpeted floor even though both surfaces are at the same temperature. Why?
Example 3 Why will the snow melt faster on some parts of the roof than others?
Example 4 Sometimes during the winter, it is possible for the temperature to drop below freezing point during the night. In this case, farmers spray water on the fruits to prevent them from freezing. Explain how spraying water on the fruits may help.
Question: Using the logic above, can we boil water in a test tube which is placed inside a beaker of boiling water?
Hwa Chong Institution Sec 3 (SMTP) CONVECTION Convection is the transfer of heat energy from a hot region to a cold region by the transfer of...
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