Questions Answers

Evaluation

I. Choose the appropriate answer

1. When an object is heated, the molecules that make up the object

a. begin to move faster

b. lose energy

c. become heavier

d. become lighter

Answer: a. begin to move faster

2. The unit of heat is

a. newton

b. joule

c. volt

d. Celsius

Answer:  b. joule

3. One litre of water at 30ºC is mixed with one litre of water at 50ºC. The temperature of the mixture will be

a. 80ºC

b. More than 50°C but less than 80ºC

c. 20ºC

d. around 40ºC

Answer:  d. around 40°C

4. An iron ball at 50°C is dropped in a mug containing water at 50ºC. The heat will

a. flow from iron ball to water.

b. not flow from iron ball to water or from water to iron ball.

c. flow from water to iron ball.

d. increase the temperature of both.

Answer:  b. not flow from iron ball to water or from water to iron ball.

II. Fill in the blanks

1. Heat flows from a hot body to a cold body.

2. The hotness of the object is determined by its total kinetic energy

3. The SI unit of temperature is kelvin

4. Solids expand on heating and contract on cooling.

5. Two bodies are said to be in the state of thermal Equilibrium if there is no transfer of heat taking place.

III. True or False. If False, give the correct statement

1. Heat is a kind of energy that flows from a hot body to a cold body. [True]

2. Steam is formed when heat is released from water. [False]

Ice is formed when heat is released from water 

3. Thermal expansion is always a nuisance. [True]

4. Borosilicate glass do not expand much on being heated. [True]

5. The unit of heat and temperature are the same. [False]

The SI unit of heat is joule. The SI unit of temperature is kelvin

IV. Give reasons for the following

1. An ordinary glass bottle cracks when boiling water is poured into it, but a borosilicate glass bottle does not.

Glass is a poor conductor of heat. When hot liquid is poured into the tumbler, the inner surface of the tumbler becomes hot and expands while the outer surface remains at the room temperature and does not expand. Due to this unequal expansion, the tumbler cracks.

Glassware used in kitchen and laboratory are generally made up of Borosilicate glass (pyrex glass). The reason is that the Borosilicate glass does not expand much on being heated and therefore they do not crack.

2. The electric wire which sag in summer become straight in winter.

3. Rivet is heated before fixing in hole to join two metal plates.

Rivets are used to join two steel plates together. Hot rivet is driven through the hole in the plates. One end of the rivet is hammered to form a new rivet head. When cooled, the rivet will contract and hold the two plates tightly together. 

V Match the following

1. Heat - 0ºC

2. Temperature - 100ºC

3. Thermal Equilibrium - kelvin

4. Ice cube - No heat flow

5. Boiling water – joule

Answer: 1. joule 2. kelvin 3. No heat flow 4.  0°C 5. 100°C

1. Heat - joule

2. Temperature - kelvin

3. Thermal Equilibrium - No heat flow

4. Ice Cube - 0°C

5. Boiling Water - 100°C

VI. Analogy

1. Heat : Joule :: Temperature : _____________

Heat: Joule:: Temperature: kelvin

2. ice cube : 0^ºC :: Boiling water:_____________

Ice cube: 0°C:: Boiling water: 100°C

3. Total Kinetic Energy of molecules: Heat :: Average Kinetic Energy : __________

Total Kinetic Energy of molecules: Heat::Average Kinetic Energy : Average heat

VII. Give very short answer

1. Make a list of electrical equipments at home which we get heat from.

At home we get heat from water heater, electric iron, electric kettle and room heater.

2. What is temperature?

The measurement of warmness or coldness of a substance is known as its temperature.

3. What is thermal expansion?

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

4. What do you understand by thermal equilibrium?

Thermal equilibrium exists when two objects in thermal contact no longer affect each other's temperature.

VIII. Give short answer

1. What difference do you think heating the solid will make in their molecules ?

Molecules in objects are constantly vibrating or moving inside objects. We cannot see that movement with our naked eye. When we heat the object this vibration and movement of molecules increases and temperature of the object also increases.

2. Distinguish between heat and temperature.

Heat and temperature are not the same thing, they in fact mean two different things;

• Temperature is related to how fast the atoms or molecules move or vibrate within the substance.

• Heat not only depends on the temperature of the substance but also depends on how many molecules are there in the object.

• Temperature measures the average kinetic energy of molecules. Heat measures the total kinetic energy of the molecules in the substance.

IX. Answer in detail

1. Explain thermal expansion with suitable examples.

1. Gaps are left in between rails while laying a railway track.

During summer season the rails made of iron expand due to hot sunshine. The ends of the rails come closer and they may hit each other. To .avoid this, expansion gaps are left between the ends of rails. 

2. Gaps are left in between two joints of a concrete bridge.

The concrete slabs expand during hot summer. Their edges may hit each other and cause dislocation. To prevent this gaps are left in between two joints of a concrete bridge

X. Questions based on Higher Order Thinking Skills

1. When a window is accidentally left open on a winter night, will you feel uncomfortable because the cold is getting in, or because the heat is escaping from the room?

When the window is accidentally left open the heat inside the room escapes from it through the open window. So the room becomes cold and we feel uncomfortable.

2. Suppose your normal body temperature were lower than what it is. How would the sensation of hot and cold change?

As the high temperature from outside affects our body we feel that it is hot.

3. If you heat a circular disk with a hole, what change do you expect in the diameter of the hole? Remember that the effect of heating increases the separation between any pair of particles.

The molecules of the solid matter around the hole expand on heating. As a result the circumference of the hole decreases. Naturally, the diameter of the hole also gets reduced.