ICSE Class 7 Answered

• Features of a thermos flask:

1. Double-walled glass vessel and vacuum prevents the loss of heat by conduction.
2. The heat loss by convection is minimized by the insulating stopper.
3. The heat loss by radiation is prevented by the silvered surface of the walls and reflects back any radiant energy that tends to escape into the bottle.

The silvering of the outer surface of the inner wall makes it a poor radiator of heat while the silvering of the inner surface of the outer wall makes it a good reflector of heat.

1. The thin glass walls at the neck transfers some heat by conduction, but is negligible. 

2) Water contracts when heated from 0°C to 4°C and above 4°C, it expands like any other liquids. This unusual behavior of water is known as Anomalous Expansion of Water.

3) The expansion of a substance on heating is called the thermal expansion of that substance.

4) Thermal expansion of solids

• Solids expand in all directions on heating.
• The solids have definite shape. Thus, its length, area and volume, all increase on heating.
• However, the increase is more along the largest dimension.

 Thermal expansion of liquids

Liquids expand on heating. The water contracts when heated from 0°C to 4°C and above 4°C, it expands like any other liquids. This unusual behavior of water is known as Anomalous Expansion of Water.

 Thermal expansion of gases:

When the temperature of a gas increases on heating, its volume increases and it expands.

Example: The tyres filled tightly with air, burst in hot summers. It is because the air inside it expands due to heat.

5) Conduction is the process by which heat is transferred from the hotter end of the object to the colder end of the object. It takes place mainly in solid objects, and occurs only when two materials are in contact.

Example: A metal spoon kept in hot water absorbs heat from the water and becomes hot.

Convection

• Convection is the transfer of heat by the movement of particles of a medium from one place to another.
• It occurs only in liquids and gases.

Example: Wind currents, the lower floor of a building is cooler than the upper floor and the atmosphere at the sea shore is always pleasant because of convection.

 Radiation

• Radiation is the transfer of heat that does not require a medium.
• All hot bodies emit heat by the process of radiation.
• We get sunlight because of the radiation as there is no medium present between the atmosphere of the Earth and the Sun.

Example:

1. When we sit before a room heater, we get warmth due to radiation.
2. A hot utensil kept away from the flame cools down as it transfers heat to the surroundings by radiation.
3. Our body produces heat and receives heat from the surroundings by radiation.