8th Std Science Solution in English | Lesson.1 Heat

Lesson 4. Heat

Lesson 4. Heat

I. Choose the correct answers:

1. Heat is a form of _______

1. electrical energy
2. gravitational energy
3. thermal energy
4. None of these

Ans : thermal energy

2. If you apply some heat energy to a substance, which of the following can take place in it?

1. Expansion
2. Increase in temperature
3. Change of state
4. All the above.

Ans : All the above.

3. Which of the following substances will absorb more heat energy?

1. Solid
2. Liquid
3. Gas
4. All the above

Ans : Solid

4. If you apply equal amount of heat to a solid, liquid and gas individually, which of the following will have more expansion?

1. Solid
2. Liquid
3. Gas
4. All of them

Ans : Gas

5. The process of converting a liquid into a solid is called _______

1. sublimation
2. condensation
3. freezing
4. deposition

Ans : freezing

6. Conduction is the heat transfer which takes place in a _______

1. solid
2. liquid
3. gas
4. All of them

Ans : solid

II. Fill in the blanks.

1. A calorimeter is a device used to measure the ________.

Ans : heat capacity of water

2. _______ is defined as the amount of heat required to raise the temperature of 1kg of a substance by 1°C.

Ans : specific heat capacity

3. A thermostat is a device which maintains _________.

Ans : sensing

4. The process of converting a substance from gas to solid is called _________.

Ans : deposition

5. If you apply heat energy, the temperature of a system will ________.

Ans : increase

6. If the temperature of a liquid in a container is decreased, then the interatomic distance will ___________.

Ans : decrease

III. State True or False. If false, correct the statement.

1. The applied heat energy can be realized as an increase in the average kinetic energy of the molecules.

Ans : True

2. The dimensions of a substance are increased if the temperature of the substance is decreased.

Ans : False

Cu. St. : The dimensions of a substance are increased if the temperature of the substance is increased.

3. The process of converting a substance from solid to gas is called condensation.

Ans : False

Cu. St. : The process of converting a substance from solid to gas is called sublimation.

4. Convection is the process by which the thermal energy flows in solids.

Ans : False

Cu. St. : Convection is the process by which the thermal energy flows in liquids and gases.

5. The amount of heat gained by a substance is equal to the product of its mass and latent heat.

Ans : True

6. In a thermos flask, the silvered walls reflect and radiate the heat to the outside.

Ans : False

Cu. St. : In a thermos flask, the silvered walls reflect and radiate the heat back to the liquid in the bottle.

IV. Match the following

V. Read the directions given below and answer the questions.

1. If both assertion and reason are true and the reason is the correct explanation of the assertion.
2. If both assertion and reason are true, but reason is not the correct explanation of the assertion.
3. If the assertion is true, but the reason is false.
4. If the assertion is false, but the reason is true.

1. Assertion : Radiation is a form of heat transfer which takes place only in vacuum.

Reason : The thermal energy is transferred from one part of a substance to another part without the actual movement of the atoms or molecules.

Ans : Both assertion and reason are true, but reason is not the correct explanation of the assertion.

2. Assertion : A system can be converted from one state to another state.

Reason : It takes place when the temperature of the system is constant.

Ans : Both assertion and reason are true and the reason is the correct explanation of the assertion.

VI. Answer briefly.

1. What are the applications of conduction in our daily life?

Conduction in daily life :

• 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.

2. What are the effects of heat?

• Expansion.
• Increase in temperature.
• Change in state.

3. Name three types of heat transfer.

• Conduction
• Convection
• Radiation

4. What is conduction?

Conduction is the process of heat transfer in solids from the region of higher temperature to the region of lower temperature without the actual movement of molecules.

5. Write a note on convection.

The form of heat transfer from places of high temperature to places of low temperature by the actual movement of liquid or gas molecules is called convection.

6. Define specific heat capacity.

• The amount of heat energy required to raise the temperature of 1 kg of a substance by 1°C or 1K.
• It is denoted by the symbol C.

7. Define one calorie.

• One calorie is the amount of heat energy required to raise the temperature of 1 gm of water through 1°C.
• 1 Calorie = 4.186 J

VII. Answer in detail.

1. With the help of a neat diagram explain the working of a calorimeter.

• A calorimeter is a device used to measure the amount of heat gained or lost by a substance.
• It consists of a vessel made up of metals like copper or aluminium which are good conductors of heat and electricity.
• The metallic vessel is kept in an insulating jacket to prevent heat loss to the environment.
• There are two holes in it. Through one hole a thermometer is inserted to measure the temperature of the contents.
• A stirrer is inserted through another hole for stirring the content in the vessel.
• The vessel is filled with liquid which is heated by passing current through the heating element.
• Using this device we can measure the heat capacity of water.

2. Write a note on thermostat.

• A thermostat is a device which maintains the temperature of a place or an object constant.
• Thermostats 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.
• Sometimes, a thermostat functions both as the sensor and the controller of a thermal system.
• Examples for thermostat include, building heater, central heater in a room, air conditioner, water heater, oven and refrigerators.

3. Explain the working of thermos flask.

• 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

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