Samacheer Class 8 Science - Heat

We cook food in vessels made up of metals. When the vessel is heated, heat is transferred from the metal to the food.
When we iron dresses heat is transferred from the iron to the cloth.
Handles of cooking utensils are made up of plastic or wood because they are poor conductors of heat.
The temperature inside igloo (snow house) is warm because snow is a poor conductor of heat.

The form of heat transfer from places of high temperature to places of low temperature by the actual movement of molecules is called convection. Convection takes place in liquids and gases.

Specific heat capacity of a substance is defined as the amount of heat energy required to raise the temperature of 1 kilogram of a substance by 1°C or 1 K. It is denoted by the symbol C.

1. A calorimeter is a device used to measure the amount of heat gained or lost by a substance.
2. It consists of a vessel made up of metals like copper or aluminium which are good conductors of heat and electricity.
3. The metallic vessel is kept in an insulating jacket to prevent heat loss to the environment.

4. There are two holes in it. Through one hole a thermometer is inserted to measure the temperature Of the Contents.
5. A stirrer is inserted through another hole for stirring the content in the vessel.
6. The vessel is filled with liquid which is heated by passing current through the hating element.
7. Using this device we can measure the heat capacity of the liquid in the container.

1. A thermostat is a device which maintains the temperature of a place or an object constant.
2. The word thermostat is derived from two Greek words, ‘thermo’ meaning heat and ‘static’ meaning staying the same.
3. Thermostats are used in any device or system that gets heated or cools down – to a pre – set temperature. It turns an appliance or a circuit on or off when a particular temperature is reached.
4. Devices which use thermostat include building heater, central heater in a room, air conditioner, water heater, as well as kitchen equipment including oven and refrigerators.
5. Sometimes, a thermostat functions both as the sensor and the controller of a thermal 6 system.

A thermos flask has double walls, which are evacuated.
It is silvered on the inside.
The vacuum between the two walls prevents heat being transferred from the inside to the outside by conduction and convection.
With very little air between the walls, there is almost no transfer of heat from the inner wall to the outer wall or vice versa.
Conduction can only occur at the points where the two walls meet, at the top of the bottle and through an insulated support at the bottom.
The silvered walls reflect radiated heat back to the liquid in the bottle.

Lakes don’t completely freeze because the ice (and eventually snow) on the surface acts to insulate die water below. To freeze water into ice, a large quantity of heat is to be withdrawn. This heat cannot be

1. A. Steel > Wood > Water
2. B. Steel > Water > Wood
3. C. Water > Steel > Wood
4. D. Water > Wood > Steel

(b) Steel > Water > Wood
Reason:
Thermal conductivity is defined as the heat flow per unit time.
Steel has a higher thermal conductivity than water and wood.
[Thermal conductivity of steel = 50.2 w/mk
Thermal conductivity of water = 0.6 w/mk Thermal conductivity of wood = 0.12 w/mk]

Heat capacity C’ = \(\frac{Q}{∆T}\)
Here, A = 1000 J
T = 20°C – 0°C = 20°C = 20 k
C = \(\frac{1000}{20}\) = 50 JK -1
The heat capacity of the ball = 50 JK -1

Specific heat capacity, C = \(\frac{Q}{mx∆T}\)
Here, m = 100 kg
Heat capacity = \(\frac{Q}{∆T}\) = 8000 J/°C = 8000 J/K
C = \(\frac{Q}{mx∆T}\) = 100 x 8000 J = 8,00,000 JKg -1 K -1

When the water is heated, water molecules receives heat energy, increases the kinetic energy of the molecules.
Question (ii)
What caused the temperature change?
When the molecules receive more energy, the temperature of the water increases.
Heat energy causes increase in temperature.

In ice cubes, the force of attraction between the water molecules is more. So they are close together.
When we heat them the force of attraction decreases and the ice cubes become water.
when we heat the water, the force of attraction between the molecules decreases further.
Hence they move away from one another and become vapor.
Since water vapor escape to the surrounding,water level decreases.
From this Activity we understand that heat energy causes change in the state of the substances.

Yes, we feel hot.
It is because heat in the hot water is transferred from one end to other end of the spoon.
In solid substances such as silver spoon, atoms are arranged very closely.
So heat transfer takes place from the higher temperature region to lower temperature region.
This is due to conduction.

When water in the vessel is heated, water molecules at the bottom receive heat energy and move upward.
Then the molecules at the top comes down and get heated.
This kind of heat transfer is known as convection.

Heat transfer depends on the nature of the substance.
Water has high specific heat capacity than that of cooking oil.
A substance with high specific heat capacity absorbs a large quantity of heat.
Thus, it takes long time to heat up.