Class 7 Science

Heat

Heat is the transfer of energy from a hot body. The sense of touch can be used to understand the degree of hotness or coldness of something. But the sense of touch is not reliable and cannot be always used to say how much hot anything is. Moreover, using the sense of touch can be risky in case of something being very hot. Thus, hotness of anything is measured in terms of TEMPERATURE in reliable way. To measure temperature a device called THERMOMETER is used.

Unit of heat:

There are three units which are used to measure the temperature: Degree Celsius, Fahrenheit and Kelvin.

Degree Celsius: Degree Celsius is written as °C and read as degree Celsius or simply Celsius. For example 20°C; it is read as twenty degree Celsius.

Fahrenheit: Fahrenheit is written as °F and read as degree Fahrenheit. For example 25°F; it is read as twenty five degree Fahrenheit.

Kelvin: Kelvin is written as K. For example 100K; it is read as hundred Kelvin.


Thermometer:

Thermometer is a device which is used to measure temperature. Thermometer is made of a long narrow glass tube; with a bulb at one end. The narrow tube appears as a continuous silver line; because it is filled with mercury. Mercury is a metal which is in liquid state at room temperature and it readily expands or contracts at the slightest change in temperature. Hence, mercury is used in thermometer.

Types of thermometer:

Laboratory Thermometer: Laboratory thermometer is used to measure the temperature. The scale of temperature is graduated generally from –10°C to 110°C over the glass tube. Each division of temperature scale is further divided into 10 parts to read fraction of temperature.

structure of thermometer



Clinical Thermometer: Clinical thermometer is used to measure the body temperature. The scale of temperature is graduated from 35°C to 42°C and or from 94°F to 108°F. The temperature of human body always remains within this range and this is the range on the clinical thermometer. There is a kink near the bulb of clinical thermometer which prevents the automatic fall of mercury level.

Digital Thermometer: In digital thermometer, reading of temperature is displayed digitally as in digital watches. This is safer because no mercury is used in this. It is important to note that mercury is a highly toxic substance.

Maximum-Minimum Thermometer: Maximum – minimum thermometer is used to measure the daily temperature to prepare weather reports.

Reading of thermometer and measuring of temperature:
  • Take a clinical thermometer and hold it horizontally with reading scale towards your eye.
  • Do not hold the thermometer from the bulb.
  • Rotate the thermometer slightly clockwise and anticlockwise. By doing this you will see a shiny thin silvery thread.
  • The end of the silvery thread shows the reading of temperature. If mercury lining ends at 37, the reading is 37°C.
  • Wash the bulb end of thermometer with an antiseptic solution.
  • Give two or three jerks slightly. By doing this the mercury level would fall. When it falls to 35°C or below, put it below the tongue and wait for one minute.
  • Take out the thermometer and read the temperature. Temperature would be near 37°C.
  • The normal body temperature is 37°C. This can differ from person to person.




Use of Laboratory Thermometer:

  • Take some water in a beaker.
  • Take a laboratory thermometer and immerse its bulb end in water; holding it vertically. Ensure to dip whole portion of bulb end. The bulb end should not touch the bottom or side of the beaker.
  • Observe the movement of rise of mercury. When it becomes stable, take the reading of the thermometer.
  • Repeat this with hot water and take the reading.
Difference between clinical and laboratory thermometer:
Clinical ThermometerLaboratory Thermometer
Clinical thermometer is scaled from 35°C to 42°C or from 94°F to 108°F.Laboratory thermometer is generally scaled from -10°C to 110°C.
Mercury level does not fall on its own, as there is a kink near the bulb to prevent the fall of mercury level.Mercury level falls on its own as no kink is present.
Temperature can be read after removing the thermometer from armpit or mouth.Temperature is read while keeping the thermometer in the source of temperature, e.g. a liquid or any other thing.
To lower the mercury level jerks are given.No need to give jerk to lower the mercury level.
It is used for taking the body temperature.It is used to take temperature in laboratory.