Lesson.1 Heat
Lesson.1 Heat
I. Choose the correct answer:
1. When an object is heated, the molecules that make up the object
- begin to move faster
- lose energy
- become heavier
- become lighter
Ans : begin to move faster
2. The unit of heat is
- newton
- joule
- volt
- celsius
Ans : joule
3. One litre of water at 30oC is mixed with one litre of water at 50oC. The temperature of the mixture will be
- 80oC
- More than 50oC but less than 80oC
- 20oC
- around 40oC
Ans : around 40oC
4. An iron ball at 50°C is dropped in a mug containing water at 50°C. The heat will
- flow from iron ball to water.
- not flow from iron ball to water or from water to iron ball.
- flow from water to iron ball.
- increase the temperature of both.
Ans : not flow from iron ball to water or from water to iron ball.
II. Fill in the blanks
1. Heat flows from a _________ body to a _________ body.
Ans : higher,low
2. The hotness of the object is determined by its _________
Ans : Temperature
3. The SI unit of temperature is ________
Ans : kelvin
4. Solids _________ on heating and _________ on cooling.
Ans : expand,contract
5. Two bodies are said to be in the state of thermal _________ if there is no transfer of heat taking place.
Ans : equilibrium
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.
Ans : True
2. Steam is formed when heat is released from water.
Ans : True
3. Thermal expansion is always a nuisance.
Ans : True
4. Borosilicate glass do not expand much on being heated.
Ans : True
5. The unit of heat and temperature are the same
Ans : False
Cu. An : The unit of Heat is joule (J) 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.
Borosilicate glass (pyrex glass). The reason is that the Borosilicate glass do not expand much on being heated and therefore they do not crack.
2. The electric wire which sag in summer become straight in winter.
- During summer,the wires heat up and expand in length. Hence it sag down.
- During winter,the temperature drops and wires cool down. Due to decrese in temperature,it contracts in length and hence become straight.
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 fixed.
V. Match the Following
| 1. Heat | 0oC | |
| 2. Temperature | 100oC | |
| 3. Thermal Equilibrium | kelvin | |
| 4. Ice cube | No heat flow | |
| 5. Boiling water | joule | |
| Ans : 1 – E, 2 – C, 3 – D, 4 – A, 5 – B | ||
VI. Analogy
1. Heat : Joule :: Temperature : _________
Ans : kelvin
2. ice cube : 0oC :: Boiling water :_________
Ans : 100oC
3. Total Kinetic Energy of molecules: Heat :: Average Kinetic Energy : _________
Ans : Temperature
VII. Give very short answer
1. Make a list of electrical equipments at home which we get heat from.
Heater, iron box, electric kettle etc., work on this principle.
2. What is temperature?
- The measurement of warmness or coldness of a substance is known as its Temperature.
- SI unit of temperature is kelvin.
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?
Two objects are said to be in thermal contact if they can exchange heat energy. 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:
Heat is an energy that raises the temperature of a thing by causing the molecules in that thing to move faster.SI Unit of Heat is joule. The unit calorie is also used
Temperature:
The measurement of warmness or coldness of a substance is known as its Temperature. SI unit of temperature is kelvin.
IX. Answer in detail
1. Explain thermal expansion with suitable examples.
Fitting the iron rim on the wooden wheel
The diameter of the iron ring is slightly less than that of the wooden wheel. Therefore, it cannot be easily slipped on from the rim of wooden wheel.
The iron ring is, therefore, first heated to a higher temperature so that it expands in size and the hot ring is then easily slipped over to the rim of the wooden wheel. Cold water is now poured on the iron ring so that it contracts in size and holds the wooden wheel tightly.
Rivetting
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.
Cracking of a thick glass tumbler
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.
Electric wires
Electric wires between electric posts contract on cold days and sag in summers. To solve this prob lem, we leave wires slack so that they are free to change length.
