Heat and Temperature

Heat and Temperature:

Heat is the agent which produces the sensation of hotness. It is measured in Joules. Temperature is a measure of the degree of hotness or coldness of a body. It is measured in degree Celsius.

Difference between Heat and Temperature:

Heat is a form of energy.Temperature is the degree of hotness or coldness of a body.
Heat is measured in Joules.Temperature is measured in degree Celsius.
Heat cannot be measured directly. It is measured indirectly by its effect on matter.Temperature can be read directly on a thermometer.

Measurement of Temperature:

We cannot rely upon our senses to give us the exact knowledge of temperature. We can compare the temperatures of two objects and decide which is higher by using our sense of touch. But even touching the objects will not provide a reliable estimate of their temperature. Substances expand on heating. This property may be used to measure the temperature of a body. Greater the expansion, greater will be the temperature of the body.

Definition of a Thermometer:

An instrument for measuring the temperature of a body is called thermometer. Liquids and gases are generally used in thermometers. Solids, in general, are not used as they expand very little on heating. The most commonly used thermometer is mercury thermometer. Such a thermometer is based on the change in length of mercury column. This change is due to thermal expansion of mercury as the temperature is raised. The clinical thermometer used to measure the temperature of the human body is an example of mercury thermometer.

A mercury thermometer consists of a glass tube having a fine bore. There is a glass bulb at one end of the tube and the other end is sealed.

Mercury is preferred, although its expansion is very little as compared to water and alcohol, because

  • Mercury expands uniformly.
  • Mercury remains a liquid at temperatures at which water and alcohol change into a vapour.
  • Mercury can be obtained pure.
  • Mercury takes the temperature of a body quicker than the other liquids i.e. it absorbs heat quicker than the other liquids.
  • Mercury does not wet the glass. (It does not stick to the walls of the tube).
  • Mercury is a shiny liquid.

Measurement of Heat:

We shall find the relation between the amount of heat and the temperatures it can produce in substances. For this purpose, we shall perform the following experiment.

Experiment: To find the change in temperature of water taken in two beakers.

Materials Required: One beaker of 200 ml, another beaker of 400 ml, Bunsen burner (or spirit lamp), tripod, stopwatch, thermometer (°C).

Method: Take 200 ml beaker full of water. This beaker contains 200 gram of water. Place it on a tripod. Find the temperature of water with the help of a thermometer. Heat it with a Bunsen burner for five minutes. Record the temperature of water. Find the rise in temperature.

Take the second beaker which has a capacity of 400 ml. The beaker can hold 400 grams of water. Fill it with water. Find its initial temperature. This temperature should be the same as that of the first beaker. Heat it for five minutes (same time) under the same burner so that heat supplied may be same in both cases. Record the rise in temperature of the water. Compare the changes in the temperature of the water in the two beakers. It will be seen that the rise in temperature of 200 grams of water is twice the rise in temperature of 400 grams of water, provided the same amount of heat is supplied for the same duration.

Units for measuring the amount of Heat:

The units for measuring the amount of heat is called calorie.

Definition of 1 calorie: 1 calorie is the amount of heat supplied to 1 gram of water to raise its temperature by 1°C.

Bigger unit is kilocalorie.

1 kilocalorie = 1000 calories

Definition of 1 kilocalorie: 1 kilocalorie is the amount of heat supplied to 1 kg of water to raise its temperature by 1°C.

Also, 1 calorie = 4.186 Joule

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