Heat Transfer in Nature

 

A  Fill in the blanks.

1.    Land heats up and cools down                        than water.

2.    Heat  cannot  travel  by  convection  through

                       .

3.    All metals are good                       and plastics are

                       of heat.

4.                           currents are produced when liquids and gases are heated.

5.    We receive heat from the Sun by mode of heat transfer.

Ans.

1.    Land heats up and cools down faster than water.

2.    Heat cannot travel by convection through solids.

3.    All metals are good conductors and plastics are insulators of heat.

4.    Convection currents are produced when liquids and gases are heated.

5.    We receive heat from the Sun by radiation mode of heat transfer.

 

B  State True or False. Also correct the false statements.

6.    Planting more trees will reduce the groundwater.

Ans. False

7.    Solids are heated mainly by conduction.

Ans. True

8.    Gunny bags are used to prevent melting of ice because a lot of air is trapped in them.

Ans. True

9.    Two thin blankets are usually warmer than one thick one.

Ans. True

10.   In radiation mode of heat transfer, the particles of the medium move from one place to another, carrying heat with them.

Ans. False

 

Tick (ü) the odd-one out giving reason.

      1.    Rain, Snow, Cloud, Hail

Ans. Odd one: Cloud

Reason: Cloud is not a form of precipitation.

2.    Rainwater harvesting, Deforestation, Recharge pits, Bawris

Ans. Odd one: Deforestation

Reason: It causes groundwater depletion, others help conserve water.

3.    Metals, Plastics, Water, Air, Wood

Ans. Odd one: Metals

Reason: Metals are good conductors; others are poor conductors.

4.    Sea breeze, Ventilation, Thermal radiations, Ocean currents

Ans. Odd one: Thermal radiations

Reason: It is not caused by convection currents.

   II  LOGIC-BASED QUESTIONS

         Short and Long Answer

1. You are enjoying hot kesar milk on a cold January evening in your favourite silver glass. But you are unable to hold the glass as it is very hot. Which process of heat transfer has taken place here and why ?

You decide to put your glass in another silver glass to avoid heat on your palms. Is it a good idea ? Suggest another solution.

    Ans. Heat transfer occurs by conduction because silver is a good conductor. Using another silver glass is not effective.

Better solution: Use a wooden holder or cloth.

 

2. Why should one wear light coloured clothes in summer and dark clothes in winter ?

Ans. Light-coloured clothes reflect heat in summer; dark clothes absorb heat in winter.

 

3. Why is heat transfer by convection possible only in liquids and gases ?

Ans. Convection is possible only in liquids and gases because their particles can move freely.

 

4. You wish to choose a car that will get low fuel consumption on air conditioning on a bright sunny day. Which coloured car will you prefer out of the two - black or white ? Why ?

Ans. White-coloured car, because it reflects heat and reduces heating.

 

5. Why are the ventilators provided near the ceiling of rooms ?

Ans. Ventilators are near the ceiling because hot air rises and escapes easily.

 

6. Sarthak went swimming around 4:00 p.m. in May. He noticed the pool water was dirty and quickly got out. He observed that while the Sun had heated the top surface, sunlight couldn’t reach the bottom due to the dirt, yet the bottom was slightly warm. Which mode of heat transfer caused the warmth at the bottom ?

Ans. Heat reached the bottom by conduction through water.

 

7. What is the direction of flow of heat in the following cases ? Explain the difference in the two processes in both the cases.

Case I : Keep your palm on a wooden box kept inside a room and observe the feeling on your hand.

Case II : Open your mouth wide. Bring your palm close to your mouth and blow hot air over it.

Ans. Case I : No heat flow as both are at room temperature.

      Case II : Heat flows from hot air to palm by convection.

 

8.    Observe the picture given in the figure. Water is being boiled in a pan of wide base.

                 (i)   Which position P or T will feel warmer?            

                (ii)   Fill up the boxes P and T to indicate the mode of flow of heat to the hand.

            Ans.   (i) Position P feels warmer

(ii) P – Radiation, T – Convection

III HIGHER ORDER THINKING SKILLS

      1. Why are the pipes of solar heater and the containers of solar cooker painted black ?

Ans. Black surfaces absorb more heat, increasing efficiency.

 

2. Two objects at the same temperature, one smaller than the other, are placed together. In which direction will the heat be transferred ? Give reason.

Ans. No heat transfer occurs as both are at the same temperature.

 

3. Why is a room heater placed near the floor and an air-conditioner near the ceiling ?

      Ans. Heater near floor warms air upward; AC near ceiling cools air downward.