B. Fill in the blanks.
1. Temperature of boiling water cannot be measured with a ________ thermometer.
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. Clinical 2. Solids 3. Conductors, nonconductors 4. Convection 5. Radiation
C. State True or False. Also correct the false statements.
1. A laboratory thermometer can be used to measure the body temperature of humans.
2. Solids are heated mainly by conduction.
3. Gunny bags are used to prevent melting of ice because a lot of air is trapped in them.
4. Two thin blankets are usually warmer than one thick one.
5. In radiation mode of heat transfer, the particles of the medium move from one place to another, carrying heat with them.
Ans. 1. False 2. True 3. True 4. True 5. False
II. SHORT ANSWER TYPE QUESTIONS
1. Write one similarity and one difference between a laboratory thermometer and a clinical thermometer.
Ans. One similarity between a laboratory thermometer and a clinical thermometer is that both contain mercury as thermometric liquid. One difference between a laboratory thermometer and a clinical thermometer is that temperature range of laboratory thermometer is 0°C – 100°C While clinical thermometer is 35°C – 42°C.
2. Why should one wear white clothes in summer and dark clothes in winter ?
Ans. We wear white or light-coloured clothes during summer but dark-coloured clothes during winter. This is because white or light-coloured clothes absorb lesser heat and keep us cool during summer. On the other hand, black or dark-coloured clothes absorb more heat and keep us warm during winter.
3. Why is heat transfer by convection possible only in liquids and gases?
Ans. In liquids and gases, the molecules are not firmly bound together and can move freely. Hence heat transfer by convection is possible in liquids and gases. In a solid, the molecules are firmly bound together and cannot move freely. Hence convection is not possible in solids.
4. Differentiate between a conductor and a nonconductor of heat.
Ans. The materials which allow heat to flow through them are called good conductors of the heat. All the metals such as aluminium, copper etc. are good conductors of heat. The materials which do not allow heat to flow through them are called nonconductors of heat. Plastics, wood etc. are nonconductors of heat.
5. Why are the ventilators provided near the ceiling of rooms ?
Ans. Ventilators are provided near the ceiling of rooms so that warm air being lighter can rise up and leave the room through these ventilators and fresh air can enter through doors and windows of the room.
III. LONG ANSWER TYPE QUESTIONS
1. What is meant by a temperature scale ? Describe celsius scale.
Ans. The scale on which temperature is measured is called temperature scale. There are three temperature scales.
These are :
(i) Celsius scale
(ii) Fahrenheit scale and
(iii) Kelvin scale.
Celsius Scale : On this scale, temperature is described in degree Celsius (°C). The lower fixed point (ice point) of the Celsius scale is taken as 0°C and the upper fixed point (steam point) of this scale is taken 100°C. The interval between these two points is divided into 100 (=100 – 0) equal divisions. Thus, the unit division on this scale corresponds to a temperature equal to 1°C.
2. Describe a clinical thermometer. Why doesn’t a clinical thermometer has markings above 42°C?
Ans. A clinical thermometer is a device used to measure human body temperature. It typically ranges from 35°C to 42°C and has a constriction to hold the mercury in place, allowing accurate readings even after removal from the body.
The temperature of human body does not rise above 42°C, that is why the clinical thermometer does not contain markings above 42°C.
3. What are the good conductors and nonconductors of heat ? Give two examples of each.
Ans. The materials which allow heat to flow through them easily are called good conductors of heat. Examples : Iron, brass. The materials which do not allow heat to flow through them easily are called poor conductors of heat. Examples : glass, paper.
4. Why do woolen clothes keep us warm when it is cold outside ?
Ans. Wool fibre is porous. It has air trapped inside the pores. Air is a nonconductor of heat. It does not allow the body heat to escape (go out) to the surroundings. As a result, woolens keep us warm during winter.
5. Compare conduction, convection and radiation.
Ans.
Comparison of conduction, convection, and radiation based on key features:
| Feature | Conduction | Convection | Radiation |
|---|---|---|---|
| Definition | Transfer of heat through direct contact | Transfer of heat by movement of fluids | Transfer of heat through electromagnetic waves |
| Medium required | Yes, needs solids (mostly) | Yes, occurs in liquids and gases | No medium needed (can occur in vacuum) |
Mode of transfer | Molecule to molecule in solids | Movement of fluid particles | Infrared radiation |
Speed | Slow | Moderate | Fastest |
| Example | Heating a metal rod | Boiling water (hot water rises, cold sinks) | Sun heating the Earth |
IV. HIGHER ORDER THINKING SKILLS
1. Why are the pipes of solar heater and the containers of solar cooker painted black ?
Ans. The pipes of solar heaters and the containers of solar cooker are painted black because black colour absorbs heat more than any other colour and this will help to keep the pipes and containers warmer.
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. The direction of heat transfer would be from larger to smaller object because larger object has more surface area due to which it loses its heat much faster.
3. Why is a room heater placed near the floor and an air-conditioner near the ceiling ?
Ans. A room heater is placed near the floor because hot air is light. It will move up from the floor to the roof and this will make the room heated properly. An air conditioner is placed near the ceiling because cool air being heavy will move down and make the room cooler.
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