Thermal Equilibrium Essay Example for Free

Thermal Equilibrium Essay 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? 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? 2 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? 3 CONVECTION Convection is the transfer of heat energy from a hot region to a cold region by the transfer of matter. The flow of convection is called the convection currents. Natural convection is due to a temperature difference that causes the density at one place of the fluid to be different from the density at another. Hot fluid expands and becomes less dense and consequently will rise. Whether the convection currents form convection loops (or a convection cell) depend on the space available for the convection to take place. Forced convection uses an external device such as a fan or a pump to mix warmer and cooler portions of the fluid. Question: You can hold your finger beside a candle flame without getting hurt but not above the flame. Why? Question: Some ice is trapped at the bottom end of a test tube by steel wool. A flame is applied near the mouth of the test tube until the water at the mouth of the test tube boils. Yet, the ice cubes are still present for quite a while. Explain why a fluid like water, which can transport heat by conduction and convection, does not transfer energy quickly from the flame to melt the ice. 4 Hwa Chong Institution Sec 3 (SMTP) Example 5 A half-boiled egg is prepared by placing a fresh egg inside hot water (slightly less than 100oC). You have placed three eggs at the bottom of the container with hot water. After a minute, you decided there is enough space for more eggs so you put one more egg on top of the bottom three. 15 minutes later, which of the eggs will be the most cooked? RADIATION Radiation is the process by which energy is transferred by electromagnetic waves. Radiation can travel through vacuum. This form of energy is sometimes called radiant energy. A good radiator is also a good absorber but a poor reflector. Some factors that can affect the rate of absorption / emission of radiation include 1 2 3 Colour of surface The classic experiment that shows that dark colour surface is a better absorber of radiant energy than a shiny surface. Clearly, a shiny surface tends to reflect radiation that falls on it. Hot water Surface temperature The hotter the surface, the more radiation it emits. A surface can emit all types of radiation, but at a specific temperature, it has a dominant emission wavelength (a particular type of radiation). For example, at human body temperature, the dominant type is infrared (that’s why we have infrared thermometers). In general, the hotter the surface, the type of radiation emitted will tend to shift more to the higher energy type like ultraviolet, x-ray or gamma rays. Surface area The larger the surface area, the more radiation it can emit. There are two important physics law describing radiant energy: Stefan-boltzmann law and the Wien’s displacement law. However, these laws are not within our syllabus. 5 APPLICATIONS OF HEAT TRANSFER A thermos bottle, sometimes referred to as a Dewar flask, reduces the rate at which hot liquids cool down or cold liquids warm up. A thermos flask usually consists of a doubledwalled glass vessel with silvered inner walls. The space between the walls is evacuated to reduce energy losses due to conduction and convection. The silvered surfaces reflect most of the radiant energy that would either enter or leave the liquid in the flask. Finally, little heat is lost through the glass or rubberlike gasket and stopper because these materials have low thermal conductivities. Covering a space satellite with highly reflective material can prevent excessive temperature fluctuations that can damage the highly sensitive electronic circuitry. By reflecting much of the sunlight, the foil minimizes temperature rises. Being a poor absorber means it’s a poor emitter too. When the satellite is not shone by the sun (e.g. in Earth’s shadow), it will not emit a lot of radiation and causes the temperature to plummet. Halogen cooktops use radiant energy to heat pots and pans. A halogen cooktop uses several quartz-iodine lamps, like the ones in ultra-bright automobile headlights. These lamps are electrically powered and mounted below a ceramic top. The electromagnetic energy they radiate passes through the ceramic top and is absorbed directly by the bottom of the pot. Consequently, the pot heats up very quickly, rivaling the time of a pot on an open gas stove.