Heat

heat flows from water to ice

Reason — Heat flows from a hot body to cold body when they are kept in contact.

0 °C

Reason — The temperature of pure melting ice is 0 °C.

mercury

Reason — Mercury is used in a thermometer.

A little space is left between two rails on a railway track

Reason — A little space is left between two rails on a railway track to allow expansion of rails during summer otherwise rails will bend sideways.

convection

Reason — Heat in a liquid is mainly transferred by convection.

upwards

Reason — In convection the transfer of heat is always vertically upwards.

conduction and convection

Reason — The vacuum kept in between the walls of a thermos flask reduces the heat transfer by conduction and convection.

Both A and R are true and R is not the correct explanation of A

Explanation

Assertion (A) is true because a temperature of 32 °F is equal to 0 °C, which is the freezing/melting point of water on both scales.

Reason (R) is true because the Celsius scale is widely used in most countries for measuring temperature.

Both Assertion and Reason are correct, but the Reason does not explain the Assertion, since it does not relate to the conversion between temperature scales.

Therefore, both A and R are true and R is not the correct explanation of A.

A is false but R is true

Explanation

Assertion (A) is false because the bottoms of cooking utensils should not be smooth and shiny; instead, they are usually made dull or blackened to absorb heat more effectively.

Reason (R) is true because smooth and shiny surfaces are good reflectors of heat and poor absorbers.

Therefore, A is false but R is true.

(a) Heat is a form of energy.

(b) Temperature determines the degree of hotness or coldness of a body.

(c) On heating a body, its temperature rises.

(d) We use a thermometer for measuring the temperature of a body.

(e) The S.I. unit of temperature is kelvin.

(f) In a thermometer, the commonly used liquid is mercury.

(g) The temperature of a normal human body is 37 °C.

(h) A person is said to have fever if his body temperature is more than 98.6 °F.

(i) A hot metallic piece is placed in tap water contained in a bucket. Heat will flow from metallic piece to water.

(j) The temperature of boiling water is 100 °C.

(k) Liquids expand more than solids.

(l) Gases expand more than liquids.

(m) Heat transfer in solids is by conduction.

(n) Heat transfer in liquids and gases is by convection.

(o) Metals are conductors of heat.

(p) Still air is an insulator of heat.

(q) Black and dull surfaces are good absorbers of heat.

(a) True

(b) True

(c) False
Correct Statement — All solids expand by the different amount when heated to the same rise in temperature.

(d) False
Correct Statement — Telephone wires are kept loose between two poles in summer.

(e) True

(f) True

(g) True

(h) False
Correct Statement — Water contracts on heating from 0 °C to 4 °C.

(i) True

(j) False
Correct Statement — Medium is not necessary for the transfer of heat by radiation.

(k) True

(l) False
Correct Statement — Liquids are heated by conduction and convection.

(m) False
Correct Statement — Black surfaces are good absorbers of heat.

Heat is a form of energy which flows from a hot body to a cold body when they are kept in contact. This process of heat transfer continues till the temperature of both the bodies becomes equal.

The S.I. unit of heat is joule(J).

Temperature is a quantity which tells us the degree of hotness or coldness of a body. When a body is heated its temperature rises and when it is cooled its temperature falls.

The three units of temperature are degree Celsius (°C), degree Fahrenheit (°F) and Kelvin (K).

Thermometer is used to measure the temperature of a body.

On the Celsius scale the ice point is 0 °C and the steam point is 100 °C. The interval between the ice point and steam point is divided into 100 equal parts.

On the Fahrenheit scale the ice point is 32 °F and the steam point is 212 °F. The interval between the ice point and steam point is divided into 180 equal parts.

On Kelvin scale ice point is marked as 273 K and steam point is marked as 373 K. The interval between the ice point and steam point is divided into 100 equal parts. 0 K is the lowest possible temperature on this scale when no molecular motion can occur.

°C = $\bold {\frac{5}{9}}$ (°F - 32)

The interval between the ice point and steam point is divided into 100 equal parts and each division is called a degree on a Celsius scale (1 °C).

The interval between the ice point and steam point is divided into 180 equal parts and each division is called a degree on a Fahrenheit scale (1 °F).

(i) On the Celsius scale the ice point is 0 °C.

(ii) On the Celsius scale the steam point is 100 °C.

(i) On the Fahrenheit scale, lower fixed point is 32 °F.

(ii) On the Fahrenheit scale, upper fixed point is 212 °F.

The heat will flow from hot milk glass tumbler to tub of cold water.

The body temperature of a healthy person is 37 °C or 98.6 °F.

Iron and brass.

Telephone wires sag in summer because they expand due to rise in temperature during summer.

A little gap is left between successive rails on a railway track to allow expansion of rails during summer otherwise rails will bend sideways.

In summer when temperature rises cement expand. So a cement floor is laid in small pieces with gaps in between to allow expansion in summer and contraction in winter, otherwise the cement floor will crack.

Thermal expansion of liquids is used in the working of mercury thermometer.

Gas will expand more as compared to solid and liquid when heated to the same temperature.

The three modes of transfer of heat are:

1. Conduction
2. Convection
3. Radiation

(a) conduction

(b) conduction and convection

(c) conduction and convection

(d) radiation

Mercury

Copper

An oven is made of double walls with the space in between filled with cork because cork is insulator of heat and prevent the heat of the oven to escape.

We use cooking utensils made up of copper because it is a very good conductor of heat and gets heated up rapidly.

Ebonite is an insulator of heat. It does not pass heat from the tea kettle to our hand, so we can hold the tea kettle comfortably by the handle. Therefore, the tea kettle is provided with an ebonite handle.

The air filled in the fine pores of the gunny bag is insulator of heat. The air does not allow heat from outside to pass through it to the ice thus preventing the ice from melting rapidly. So in summer, ice is kept wrapped in a gunny bag.

The transfer of heat is always vertically upwards in the process of convection.

Ventilator is provided in a room for proper ventilation because when we breathe out in a room it becomes warm and air inside the room becomes impure. The warm air is less dense and rises up and moves out through the ventilator. So cold, fresh air comes inside the room through windows to take its place. Thus, the continuous circulation of fresh air keeps the air in the room fresh.

Chimneys are provided over furnaces in factories because the hot gases are less dense and rises up and comes out through the chimneys. Smoke, fumes etc. around the furnace rush in to take their place and they are sucked out. Thus chimneys remove undesired smoke, fumes from the premises.

We feel warm when we sit near fire. Heat from fire reaches us by radiation process.

We prefer to wear white or light coloured clothes in summer because white clothes reflect most of sun's heat and absorb very little of it, thus keep our body cool. But we wear black or dark coloured clothes in winter because they absorb most of the heat of sun and keep us warm.

Labelled diagram of a laboratory thermometer is shown below:

The expansion of a substance when heated is called thermal expansion.

The iron rims are made slightly smaller in diameter than the wheel. So they are heated because iron expands and is slipped over the wheel easily, so that on cooling it contracts and makes a tight fit on the wheel.

When hot water is poured on the neck of the bottle, the neck expands and the glass stopper stuck in it gets space to loosen and then can be easily removed.

The following experiment demonstrates that liquids expand on heating:

1. Take an empty bottle with a tight fitting cork with a hole drilled in its middle, a drinking straw, two bricks, a wire gauze and a burner.
2. Fill the bottle completely with water. Add few drops of ink to make it coloured.
3. Fix the cork in the mouth of the bottle. Pass the drinking straw through the cork. Put some molten wax around the hole to avoid leakage of water.
4. Pour some more water in the drinking straw so that water level in the straw can be seen. Mark the water level in the straw as shown in figure.
5. Place the bottle on the wire gauze kept over the two bricks. Heat the bottle using a burner as shown in figure.
6. Look at the level of water in the straw.

Conclusion — When water is heated more and more, the level of water in the drinking straw rises. This shows that water expands on heating.

The following experiment demonstrates that air expands on heating:

1. Take an empty bottle. The empty bottle contains air.
2. Attach a rubber balloon to its neck as shown in figure.
3. The balloon is deflated initially.
4. Place the bottle in a water bath containing boiling water. It heats the air present in bottle.
5. After sometime it is noticed that the balloon gets inflated as shown in figure.

Conclusion — This shows that on heating, air present inside bottle expands and fills the balloon. So the balloon gets inflated. This shows that air expands on heating.

The empty glass bottle is filled with air. When the bottle is heated, the air inside the bottle expands and it escapes out through the tube into water which is seen in the form of bubbles.

The substances which allow heat to pass through them easily are called good conductors of heat. Examples — silver, copper.

The substances which do not allow heat to pass through them easily are called bad conductors of heat or insulators. Examples — wood, plastic.

(a) Woollen clothes have fine pores which are filled with air and both wool and air are insulators of heat. They do not allow the heat to escape through them and keep us warm. So we wear woollen clothes in winter.

(b) Cotton has air trapped in its fine pores. Both cotton and air are insulators of heat which do not pass heat from the water inside the pipes to the outside atmosphere and prevents the water from freezing. So the water pipes are covered with cotton during very cold weather

During summer in places near the sea a breeze blows from land to sea during night which is called land breeze and during the day time a breeze blows from sea to land known as sea breeze. They are formed by the process of convection.

Sea breeze — During day time, land becomes more warmer than sea. So air above the land is warmer, less dense and rises up. Cold air from the sea blows towards the land to take its place. This is called sea breeze.

Land breeze — During night, land cools much faster than sea. So sea water is warmer than land and air above sea is also warmer. It rises up being less dense and cold wind from land blows towards sea to take its place. This is called land breeze.

Radiation is the process of heat transfer in which heat directly passes from one body at high temperature to another body at low temperature without affecting the medium. In vacuum heat transfer occurs by radiation process. We get heat from the sun by radiation.

Labelled diagram of a thermos flask is shown below:

The vacuum present between the walls of the bottle prevents transfer of heat by conduction and convection. The shining surface on the outer side of the inner wall and inner side of the outer wall reduces the transfer of heat by radiation. The cork stopper prevents the transfer of heat by convection. In this way the transfer of heat by conduction, convection and radiation is reduced to a minimum in it.

(a) 100 divisions on the Celsius scale = 180 divisions on Fahrenheit scale

⟹ 1 division on Celsius scale = $\frac{\text{180}}{\text{100}}$ = 1.8 divisions on Fahrenheit scale

So the corresponding rise on the Fahrenheit scale = 1.8 °F.

(b) 100 divisions on the Celsius scale = 100 divisions on Kelvin scale

⟹ 1 division on Celsius scale = $\frac{\text{100}}{\text{100}}$ = 1 division on Kelvin scale

So the corresponding rise on the Kelvin scale = 1 K.

180 divisions on the Fahrenheit scale = 100 divisions on Celsius scale

⟹ 1 division on Fahrenheit scale = $\frac{\text{100}}{\text{180}}$ divisions on Celsius scale

⟹ 18 divisions on Fahrenheit scale = $\frac{\text{100}}{\text{180}}$ x 18 =10 divisions on Celsius scale

So the corresponding rise on the Celsius scale = 10 °C.

t° C = $\frac{\text{5}}{\text{9}}$(F - 32)

= $\frac{\text{5}}{\text{9}}$(5 - 32)

= $\frac{\text{5}}{\text{9}}$ x (-27)

= -15 °C

So 5 °F = -15 °C

(a) t °F = $\frac{\text{9}}{\text{5}}$C + 32

= $\frac{\text{9}}{\text{5}}$ x 40 + 32

= 9 x 8 + 32

= 72 + 32

= 104 °F

So 40 °C = 104 °F

(b) t°C = (273 + t) K

= 273 + 40

= 313 K

So 40 °C = 313 K

t °C = $\frac{\text{5}}{\text{9}}$ (F - 32)

= $\frac{\text{5}}{\text{9}}$ (-40 - 32)

= $\frac{\text{5}}{\text{9}}$ x (-72)

= -40 °C

So -40 °F = -40 °C.

The solved crossword puzzle is given below:

One end of a steel girder in a bridge is kept on rollers instead of fixing it in a pillar because in summer or in winter when there is rise or fall in temperature, the girder may expand or contract freely without affecting the pillar or bridge.

Cotton has air trapped in its fine pores. Both cotton and air are insulators of heat which prevent heat from our body to escape and thus keep us warm. So quilts are filled with fluffy cotton.

The freezing chest in a refrigerator is fitted near its top because air in the top when contact with the freezing chest gets cooled and become denser and descends down. The hot air from lower part rises up thus convection currents are formed.The cold air coming down from the cooling chest cools the entire space along with the things present in the refrigerator.

Black surface absorbs more heat and the contents inside cooking utensil gets cooked rapidly, so the bottom of a cooking utensil is painted black.

(i) The black umbrella absorbs more heat because dark colours are good absorbers of heat radiation.

(ii) The shiny silver umbrella reflects more heat radiation because shiny surfaces are good reflectors and poor absorbers of heat.

(iii) Heat from the Sun reaches us by radiation, as it can travel through vacuum without any medium.