SCIENCE MADE EASY FOR STUDENTS

HEAT

Heat- is a form of energy that gives us the sensation of hot or cold.

Temperature- The degree of hotness or coldness of a body.

1. With the help of a diagram, explain the structure of a clinical thermometer.

Ans A clinical thermometer is used for measuring human body temperature. It consists of a long narrow uniform glass tube. It has a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining thread of mercury can be seen. A kink prevents mercury level from falling on its own. the thermometer scale is marked from 98°F to 108°F or 35°C to 42°C on the glass.

2. State the precautions to be taken while using a clinical thermometer.

Ans

Thermometer should be washed before and after use, preferably with an antiseptic solution.
Ensure that before use the mercury is below 35°C or 104°F.
Read the thermometer keeping the level of mercury along the line of sight.
Handle the thermometer with care. If it hits against some hard object, it can break.
Don't hold the thermometer by the bulb while reading it.

3. What is the normal temperature of human body?

Ans The normal human body temperature is 37°C. The normal human body temperature is the average body temperature of large number of healthy people.

4. Why is the range of a clinical thermometer is from 35°C to 42°C?

Ans The clinical thermometer is designed to measure the temperature of human body only. the temperature of human body normally doesn't go below 35°C or above 42°C.

5. Explain the structure of a laboratory thermometer with the help of a diagram.

Ans Laboratory thermometer is used to measure temperature of different substances in the laboratory. the scale in this thermometer ranges from 0° to 100°C.

The lower fixed point is the temperature at which pure ice melts (melting point of ice/ 0°C). And the upper fixed point is the temperature at which pure water boils. (boiling point of water/ 100°C)

Mercury/ thermometric liquid

Mercury is taken in a thermometer because:-

It doesn't stick to the glass.
It is a good conductor of heat.
Its silvery shine helps us to see the temperature accurately.
Its expansion is uniform.
It doesn't vaporise easily.

Transfer of Heat

Heat transfer from a hot body to a cold body. It can take place in three different ways-

Conduction
Convection
Radiation

Conduction

is the process of transfer of heat from one particle to the next particle in a solid without the movement of particles from their position.

Conductors and Insulators

Materials which allow heat to pass through them easily are called Conductors. Eg Iron, Aluminium, Mercury (liquid metal)
Materials which don't allow heat to pass through them easily are called insulators or poor conductors. eg plastic, cotton, wool, etc.

Applications for Conductors and Insulators

Cooking utensils are made of different metals which are good conductors of heat. This helps in saving fuel.

The handles of cooking utensils are made of wood or plastic as both are insulators of heat.
Two thin blankets are warmer than one thick blanket because a layer of air is trapped in between the thin blankets and doesn't allow body heat to escape.
We wear woolen cloths in winter because wool is a poor conductor of heat. The wool fibres trap air inbetween them which prevents lose of body heat.
Insulators like bricks and mud are used in the construction of our houses.

Convection

The process of transfer of heat through a fluid (in liquids and gases) due to the actual movement of particles of the medium is called convection.

2. With the help of a diagram explain the formation of sea breeze and land breeze.

Ans

During the day the land gets heated faster than the water. The air over the land becomes hotter and rises up. The cooler air from the sea rushes in towards the land to take its place. The warm air from the land moves towards the sea to complete the cycle. This cycle is called Sea breeze.
At night the water cools down more slowly than the land. So, the cool air from the land moves towards the sea. This is called Land breeze.

Applications of Convection

Ventilation are provided in the room near the ceiling so that hot air escapes and cold air from outside enters the room through windows and cold air from outside enters the room through windows and doors.
Room heaters are installed at the ground level. The warm air rises up, cold air comes down making the romm warm.
Chimneys are provided for hot air to escape out.

Radiation

The process of transfer of heat in which a medium in not necessary and heat is directly transferred from a hot body to the cold body.

Application of Radiation

Light coloured clothes are preferred in summer because they absorb less heat energy than darker clothes or reflect most of the heat that falls on them.
Solar cookers and heaters are painted black to absorb maximum heat.
During summer buildings are painted in light colours because they reflect maximum heat that falls on them.
Room heaters have shiny reflectors to reflect heat into the room.
The radiators of auto mobile engines are painted black because black surfaces are good radiators of heat.