Distinction Between Heat And Temperature

DISTINCTION BETWEEN HEAT AND TEMPERATURE

                    When we place a lighted burner under a beaker of water, an amount of heat is transferred to the water. As the internal energy of the water increases, we observe a difference in sensation when we touch it. Internal energy is the total kinetic and potential energies of all the molecules. The energy that is transferred by the source of heat (bun son flame) is hear energy and the sensation of hotness or coldness experienced by the fingers corresponds to high or low temperature. In other words temperature  is the degree of hotness of a body. The increase in the kinetic energy of the molecules of the body is proportional to increase in temperature.

               

                    Suppose we keep a hot body in contact with a cold body. Let us try to explain what happens in terms of temperature and heat. The temperature of the hot body is higher than that of the cold body. When they are kept in contact energy is transferred from the hot body to the cold body in the form of heat. 

THERMAL EQUILIBRIUM ZEROTH LAW OF THERMODYNAMICS

                      When there is no flow of heat between bodies kept in contact they are said to be in thermal equilibrium. Consider three bodies A, B, and C. suppose that the body C is in  thermal equilibrium with A and also B. Then it is found experimentally that the bodies A and B are also in thermal equilibrium. That fact is sometime knows as the Zero law of Thermodynamics. This law enables us to use a thermometer (T) to find out whether two bodies A and B are in thermal equilibrium. Bring T into thermal equilibrium with A and note the reading in T. Repeat it for the body B. If the thermometer reading are the same in both cases, we can deduce that A and B are in thermal equilibrium . Using that law, temperature can also be defined as that property of a body which decides whether it is in thermal equilibrium with another body or not.

THERMOMETERS

                    Differences of temperature may be estimated roughly by our sense of touch. These sensations are not reliable enough for scientific work. Therefore, We make use, of a device which indicates a change in the reading when it is subjected to a change in temperature. Such a device is know as a thermometer.

THERMOMETRIC PROPERTIES

                        In General, all thermometers make use a measurable property of a substance which in sensitive to changes in temperature. Such a property is know as a "thermometric property".

                        "Mercury-in-glass thermometer" is the most common of all, the thermometers. In this type of thermometer the apparent expansion of a volume of mercury is used as the thermometric property. Here a change in  temperature is measured by observing the change in length of mercury column of uniform area of cross-section. In some other type of thermometers, change of pressure with temperature of a gas at constant volume is used as the thermometric property. This is known as the constant volume gas thermometer. Similarly, a constant pressure gas thermometer makes use of the volume of a gas at constant pressure as this property.

TEMPERATURE SCALES

                         In order to assign a number to specify a particular temperature, a "temperature scale" has to be defined. To establish a temperature scale we need to know the followings.

1. Physical Property of a substance should change in such a manner as to increase continuously with increase in temperature and to be constant at constant temperature. This condition is fulfilled if we use a good thermometric property such as the volume of gas or the electrical resistance of pure platinum. 
2. It should be always possible to reproduce certain chosen temperature' accurately whenever we require. These are known as fixed points. 

FIXED POINTS 

Temperature Scale were defined by making use of two fixed points. They are as follows.

1. Ice point : The temperature of ice in equilibrium with air saturated with water at 1 atmospheric pressure.
   
   
2. Steam point : The equilibrium temperature of pure water and pure steam at 1 atmospheric pressure.

                 However, the use of two fixed points was found to be unsatisfactory, partly due to the difficulty of achieving the equilibrium between pure ice and saturated water. When ice melts, it surrounds itself with pure water and this prevents intimate contact of Ice with air saturated water. Also, the steam point is extremely sensitive to a change in pressure. Thus , the temperature scale now in use is based on one fixed point only.  This fixed point is called the "triple point of water."

TRIPLE POINT OF WATER

                     Water can exist as solid, liquid and gas at the same time in the same vessel at one temperature and one pressure only. This temperature at which saturated water vapor, pure water and melting ice are all in equilibrium is known as "triple point of water".

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