Lesson: Heat

# Question: 1

A marble tile would feel cold as compared to a wooden tile on a winter morning, because the marble tile

(a)   is a better conductor of heat than the wooden tile

(b)   is polished while wooden tile is not polished

(c)   reflects more heat than wooden tile

(d)   is a poor conductor of heat than the wooden tile

## Solution:

(a) is a better conductor of heat than the wooden tile.

# Question: 2

A beggar wrapped himself with a few layers of newspaper on a cold winter night. This helped him to keep himself warm because

(a)   Friction between the layers of newspaper produces heat.

(b)   Air trapped between the layers of newspaper is a bad conductor of heat.

(c)   Newspaper is a conductor of heat.

(d)   Newspaper is at a higher temperature than the temperature of the surrounding.

## Solution:

(b) Air trapped between the layers of newspaper is a bad conductor of heat.

# Question: 3

Paheli and Boojho measured their body temperature. Paheli found her’s to be $98.6°\text{F}$ and Boojho recorded $37°\text{C}$. Which of the following statement is true?

(a)   Paheli has a higher body temperature than Boojho.

(b)   Paheli has a lower body temperature than Boojho.

(c)   Both have normal body temperature.

(d)   Both are suffering from fever.

## Solution:

(c) Both have normal body temperature.

# Question: 4

Boojho has three thermometers as shown in the figure given below. He wants to measure the temperature of his body and that of boiling water. Which thermometer (s) should he choose?

(a)   Thermometer (i) or (iii) for measuring body temperature and (ii) for measuring the temperature of boiling water.

(b)   Thermometer (i) for measuring temperature of both.

(c)   Thermometer (ii) for measuring temperature of both.

(d)   Thermometer (iii) for measuring temperature of both.

## Solution:

(a) Thermometer (i) or (iii) for measuring body temperature and (ii) for measuring the temperature of boiling water.

# Question: 5

Four arrangements to measure temperature of ice in beaker with laboratory thermometer are shown in the figure given below (a, b, c and d). Which one of them shows the correct arrangement for accurate measurement of temperature?

(a)

# Question: 6

Fig below shows a student reading a doctor’s thermometer.

Which of the figure indicates the correct method of reading temperature?

(a)

# Question: 7

Figure given below (a$–$d) shows the readings on four different thermometers. Indicate which of the reading shows the normal human body temperature?

(c)

# Question: 8

Shopkeepers selling ice blocks usually cover them with jute sacks. Explain why.

## Solution:

Ice blocks exposed to heat will melt. A jute sack is an insulating material. Covering ice blocks with jute sacks will prevent the ice blocks from melting.

# Question: 9

A laboratory thermometer A is kept 7 cm away on the side of the flame while a similar thermometer B is kept 7 cm above the flame of a candle as shown in figure below.

Which of the thermometers, A or B, will show a greater rise in temperature? Give reason for your answer.

## Solution:

Air on top of the lit candle will become hotter due to convection and hot air rises up. Therefore, thermometer B will show greater rise in temperature

# Question: 10

To keep her soup warm, Paheli wrapped the container in which it was kept with a woollen cloth. Can she apply the same method to keep a glass of cold drink cool? Give reason for your answer.

## Solution:

Yes. Woolen cloth is poor conductor of heat. A glass of cold drink will remain cool if wrapped with woolen cloth.

# Question: 11

In a mercury thermometer, the level of mercury rises when its bulb comes in contact with a hot object. What is the reason for this rise in the level of mercury?

## Solution:

On heating, mercury expands uniformly. Therefore, in a mercury thermometer, when the bulb comes in contact with a hot object, the level of mercury rises in the capillary tube.

# Question: 12

A circular metal loop is heated at point O as shown in the given figure.

(i)                 In which direction would heat flow in the loop?

(ii)              In which order the pins at points P, Q and R fixed with the help of wax fall if points O, P, Q and R are equidistant from each other?

## Solution:

(i)                 Heat will flow in both the directions i.e., from O to P and O to R.

(ii)              At first R and P will fall simultaneously, followed by Q.

# Question: 13

In the arrangements A and B shown in the figure, pins P and Q are fixed to a metal loop and an iron rod with the help of wax. In which case are both the pins likely to fall at different times? Explain.

## Solution:

In case B, pin P will fall before pin Q because, heat will first reach pin P. In case A, heat will travel in both directions, therefore pins P and Q will fall simultaneously.

# Question: 14

For setting curd, a small amount of curd is added to warm milk. The microbes present in the curd help in setting if the temperature of the mixture remains approximately between $35°\text{C}$ to $40°\text{C}$. At places, where room temperature remains much below the range, setting of curd becomes difficult. Suggest away to set curd in such a situation.

## Solution:

In order to maintain the desired temperature of the warm milk and curd mixture, the container should be wrapped with a poor conductor of heat; or the mixture should be kept in a heat resistant container.

# Question: 15

You may have noticed that a few sharp jerks are given to clinical thermometer before using it. Why is it done so?

## Solution:

A clinical thermometer is sharply jerked a few times before using to allow the mercury in or above the kink to flow back into the blub. This ensures that the mercury level is below normal temperature.

# Question: 16

Why is it advised not to hold the thermometer by its bulb while reading it?

## Solution:

While reading the temperature of a thermometer, if we hold the thermometer by its bulb, the mercury in the bulb will expand or contract due to our body temperature. This will result in incorrect reading of temperature.

# Question: 17

At a camp site there are tents of two shades $–$ one made with black fabric and the other with white fabric. Which one will you prefer for resting on a hot summer afternoon? Give reason for your choice. Would you like to prefer the same tent during winter?

## Solution:

On a hot summer afternoon, the tent made up of white fabric is preferred because white colour is a good reflector of heat and will keep the tent cool from inside. However, during winter, tent made up of black fabric is preferred because, black is a good absorber of heat and will keep the tent warm from inside.

# Question: 18

While constructing a house in a coastal area, in which direction should the windows preferably face and why?

## Solution:

The windows of houses in coastal area should preferable face towards the sea, because sea breeze will keep the houses cool during day time.

# Question: 19

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.

## Solution:

(i)            Position P will feel warmer

(ii)        Convection

$\text{T}\to$ Radiation

# Question: 20

Look at the figure given below.

The length of wire PQ in case of A is equal to the diameter of the semicircle formed by the wire CDE, in case B. One pin is attached to each wire with the help of wax as shown in figure. Which pin will fall first? Explain.

## Solution:

The pin P on the wire will fall first in case A because heat will reach it before it reaches the pin C in case B.