# Heat Transfer Class-8th Goyal Brothers ICSE Physics Solution Chapter-6 Unit-1

Heat Transfer Class-8th Goyal Brothers Physics Solutions Chapter-6 (Heat Transfer), unit-1 (Heat Transfer ). We Provide Step by Step Answers of Objective, True False, Incorrect and Correct, Fill in the blanks, Match the following , Short/Long Answer Type of Chapter-6 (Heat Transfer ), unit-1 (Heat Transfer ) Visit official Website  CISCE  for detail information about ICSE Board Class-8.

## Heat Transfer Class-8th Goyal Brothers ICSE Physics Solution Chapter-6 Unit-1

 Board ICSE Class 8th Subject Physics Book Name Goyal Brothers Chapter-6 Heat Transfer Unit-1 Heat Transfer Topic Solution of exercise questions Session 2023-24

### OBJECTIVE QUESTION

Heat Transfer Class-8th Goyal Brothers ICSE Physics Solution Chapter-6 Unit-1

#### Que: A. Fill in the blank spaces by choosing the correct words from the list given below :

List : contracts, heating, expand, linear, inflated

1. The increase in length of an iron rod on heating is called linear expansion.

2. When ice is placed in a thick glass tumbler it cracks, because the inner surface of glass contracts more than the outer surface.

3. The electric transmission wires expand and sag during summer.

4. A partially inflated balloon is placed near burning wood. After some time the balloon gets inflated due to the expansion of air in it.

5. A straight bimetallic strip on heating bands.

#### Que: B. Statements given below are incorrect. Write the correct statements :

##### Question: 1. When a flat sheet of copper is heated the increase in its area is called cubical expansion.

Answer: When a flat sheet of copper is heated the increase in its area is called superficial expansion.

##### Question: 2. A bimetallic strip expands equally on heating and does not bend.

Answer: A bimetallic strip expands unequally on heating and bend.

##### Question: 3. An iron rim of a slightly smaller diameter than a wooden wheel is cooled so that it easily slips over wooden wheel.

Answer: An iron rim of a slightly smaller diameter than a wooden wheel is cooled so that it easily fit over wooden wheel.

##### Question: 4. Small gaps are provided between the railway lines to provide for easy transportation and laying of railway lines.

Answer: Small gaps are provided between the railway lines to fill gap in summer laying of railway lines.

##### Question: 5. New electric wires for power transmission are fixed tightly, if laid during summer. This prevents wires from snapping in winter.

Answer: New electric wires for power transmission are fixed slightly, if laid during summer. This prevents wires from snapping in winter.

#### Que: C. Write ‘True’ or ‘False’ for the following statements :

 Statements True/False 1. A bimetallic strip is based on the principle that different metals have different thermal expansions for the same rise in temperature. T 2. While laying concrete slabs on the floor tiny gaps are left so as to provide for the cubical expansion. F 3. A thick glass tumbler does not crack, when boiling hot tea is poured in it. F 4. Different liquids have same value of cubical expansion. F 5. All gases on heating have same value of cubical expansion. F

#### Que: D. Tick (√) the most appropriate answer:

##### 1. A thick glass tumble cracks when boiling hot water is poured into it because :

(a) glass is a bad conductor of heat

(b) inner surface of glass expands more than outer surface

(c) outer surface of glass expands more than inner surface

(d) both (a) and (b)

Answer: option (b) inner surface of glass expands more than outer surface is correct.

##### 2. A bimetallic strip of brass and iron is straight at room temperature. When this bimetallic strip is heated it bands, such that:

(a) iron takes the outer surface of bend

(b) brass takes the outer surface of bend

(c) the bimetallic strip does not bend

(d) none of these

Answer: option (b) brass takes the outer surface of bend is correct.

##### 3. Small gaps are left between railway lines because :

(a) we cannot have rails of very long length

(b) it allows for expansion of rails during summer

(c) it allows for contraction of rails during summer

(d) both (b) and (c)

Answer: option (b) it allows for expansion of rails during summer is correct.

##### 4. Iron tyres are mounted on the wooden wheels of a tonga. Initially the diameter of the iron tyre is:

(a) slightly more than the wooden wheels

(b) slightly less than the wooden wheels

(c) of same diameter as wooden wheel

(d) none of these

Answer: option (b) slightly less than the wooden wheels is correct.

##### 5. A bimetallic strip is used in :

(a) pressure cooker

(b) automatic fire alarm

(c) ceiling fan

(d) flash light

Answer: option (b) automatic fire alarm is correct.

#### Que: E. Match the statements in Column A, with those in Column B :

 Column A Column B 1. Expansion produced in matter due to the absorption of heat energy. (a) Cubical expansion 2. Increase in the length of a solid only on the absorption of heat energy. (b) Bimetallic strip 3. A flat and straight strip of metal made by riveting two flat and straight strips of brass and iron respectively. (c) Linear expansion 4. Increase in the area of a solid only on the absorption of heat energy. (d) Thermal expansion 5. Increase in the volume of a substance on the absorption of heat energy. (e) Superficial expansion

 Column A Column B 1. Expansion produced in matter due to the absorption of heat energy. (a) Thermal expansion 2. Increase in the length of a solid only on the absorption of heat energy. (b) Linear expansion 3. A flat and straight strip of metal made by riveting two flat and straight strips of brass and iron respectively. (c) Bimetallic strip 4. Increase in the area of a solid only on the absorption of heat energy. (d) Superficial expansion 5. Increase in the volume of a substance on the absorption of heat energy. (e) Cubical expansion

### STUDY QUESTION

Heat Transfer Class-8th Goyal Brothers ICSE Physics Solution Chapter-6 Unit-1

#### Question: 1. Define boiling and evaporation. State difference between boiling and evaporation.

Answer: Boiling Point is the temperature at which liquid starts to boil. It is the temperature at which the vapor pressure of liquids becomes equal to its atmospheric pressure.

Evaporation is the process in which water evaporates from the liquid surface and changes into a gaseous state by the action of heat. It plays a very important role water cycle.

State difference between boiling and evaporation:

 Boiling Evaporation 1. During boiling, a liquid change into a gaseous state at a constant temperature, called its boiling point. During evaporation, a liquid changes into a gaseous state at any temperature below its boiling point. 2. This process needs an external source of heat energy. This phenomenon does not need any external source of heat energy. 3. It is a bulk process in which the gaseous state is formed from all parts of the liquid followed by the agitation of the liquid. It is not a bulk process in which the gaseous state is formed from only the exposed surface of the liquid. 4. The boiling does not cause cooling. The evaporation causes cooling.

#### Question: Define the following terms:

##### (a) Thermal expansion

Answer: Thermal expansion is the tendency of matter to change in volume in response to temperature alterations. When a substance is heated, its particles move more and thus maintain a greater average separation. Since thermosets are used in solid form they undergo linear thermal expansion.

##### (b) Linear expansion

Answer: Expansion means to change or increase in length. If the change in length is along one dimension (length) over the volume, it is called linear expansion.

##### (c) Superficial expansion

Answer: Superficial expansion is a type of thermal expansion in which whenever a solid is heated, its surface area increases. This type of expansion is known as superficial expansion or area expansion.

##### (b) Cubical expansion

Answer: Cubical Expansion: The increase in the volume of a substance (solid, liquid, or gas) when heated, is called cubical expansion.

#### Question: 3. Why do the liquids and gases have no linear or superficial expansion?

Answer: Liquids and gases occupy the same shape of vessel. They do not have individual length and area. Hence, liquids and gases have no linear and areal expansions.

#### Question: 4. Describe an experiment to prove that solids expands on heating and contract on cooling.

Answer: Ball and Ring Experiment: This well known experiment shows that the solids expand on heating and contract on cooling. The apparatus consists of a metallic ball and ring such that at room temperature the ball just passes through the ring.

#### Question: 5. What is a bimetallic strip? Why does a bimetallic strip of brass and iron bend on heating?

Answer: A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases steel and brass.

A bimetallic strip consists of a strip of two different metals welded together. When we heat it, different metals expand by different amounts. It bends due to thermal expansion. Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature.

#### Question: 6. How is bimetallic strip used in fire alarm?

Answer: An automatic fire alarm in a building’s security system is a heat detector that responds to the heat from a fire by setting off an alarm. Some heat detecting fire alarms rely on a bimetallic strip as the temperature sensor. This strip responds to heat by closing a normally open electrical circuit to activate the alarm.

#### Question: 7. Explain the following:

##### (a) Why are gaps left in between rails, while laying a railway line?

Answer: These gaps which are of the order of a few millimeters, are provided to allow room for the rails to expand the rise in temperature due to the atmospheric temperature as well as the friction caused by running of train. All materials expand when heated and contract when cooled.

##### (b) Why are electric transmission lines kept slightly loose, when they are laid in summer?

Answer: Telephone wires are kept slightly loose when they are laid because heat. Therefore, without a slack they could become too tight and break in winters or pull loose from their anchors in summers . This is the reason why wires are kept slightly loose when they are laid.

##### (c) Why is the concrete floor made in small slabs, rather than a single block?

Answer: A cement floor is not laid out in a single piece. The floor is put in small sections with gaps between them. It is done to avoid cracking of the floor due to expansion in summer and contraction in winter.

##### (d) Why are iron bridges mounted on steel rollers?

Answer: Bridges are mounted on rollers to prevent their collapse.

##### (e) Why is a small gap left behind the walls while mounting a girder for roofing?

Answer: So to allow the expansion and contraction, a small gap is left behind the walls while mounting a girder for roofing.

##### (f) Why does a thick glass tumbler crack when boiling hot water is poured into it?

Answer: When hot water is poured into thick glass tumbler, the inner surface of the tumbler becomes hot and expands. The outer surface of the tumbler does not expand quickly. Due to this uneven expansion, the glass tumbler cracks.

##### (g) Why does a pyrex glass not crack when boiling hot tea is poured into it?

Answer: Pyrex glass tumbler does not crack on adding hot water because Pyrex glass has a low coefficient of expansion. It does not expand less when hot water is added to the tumbler.

##### (h) Why is the diameter of an iron tyre kept slightly smaller than the wooden wheel on which this tyre is to be mounted?

Answer: It is kept a bit smaller so that while mounting it can be heated and mounted easily and on contracting it will just fix tightly to the wooden wheels but if kept the same diameter as the wheel , it will wull further expand on summers and will come off the wheels.

#### Question: 8. What is a rivet? How are two metal plates joined by a rivet?

Answer: Riveting is a forging process that may be used to join parts together by way of a metal part called a rivet. The rivet acts to join the parts through adjacent surfaces. A straight metal piece is connected through the parts. Then both ends are formed over the connection, joining the parts securely.

— : end of Heat Transfer Class-8th Goyal Brothers ICSE Physics Solution Chapter-6 Unit-1:–-