Chapter 4: Heat
Multiple Choice Questions
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.
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.
Paheli and Boojho measured their body temperature. Paheli found her’s to be 98.6 °F and Boojho recorded 37°C. Which of the followng 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.
Boojho has three thermometers as shown in Figure 4.1. 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.
Four arrangements to measure temperature of ice in beaker with laboratory thermometer are shown in Figure 4.2 (a, b, c and d). Which one of them shows the correct arrangement for accurate measurement of temperature?
Fig 4.3 (a–d) shows a student reading a doctor’s thermometer. Which of the figure indicates the correct method of reading temperature?
Figure 4.4. (a–d) shows the readings on four different thermometers. Indicate which of the reading shows the normal human body temperature?
Very Short Answer Questions
Shopkeepers selling ice blocks usually cover them with jute sacks. Explain why.
They must use some insulating material like, sack, saw dust, newspaper, etc. to cover the ice.
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 4.5.
Which of the thermometers, A or B, will show a greater rise in temperature? Give reason for your answer.
Thermometer B will show a greater rise in temperature because hot air rises up or air on the top of the candle flame is getting heated by convection.
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.
Yes. Wool is poor conductor of heat.
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?
Mercury expands when heated. Hence, it rises in the capillary tube.
Short Answer Questions
A circular metal loop is heated at point O as shown in Figure 4.6.
(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?
(i) The heat will flow in both the directions i.e. from O to P and O to R.
(ii) At first the pins at R and P will fall simultaneously followed by the pin at Q.
In the arrangements A and B shown in Figure 4.7, 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.
In case ‘B’ the pin P will fall before the pin Q because the heat will reach pin P first. In case ‘A’, the heat travels in both the directions and pins P and Q will fall simultaneously.
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°C to 40°C. At places, where room temperature remains much below the range, setting of curd becomes difficult. Suggest a way to set curd in such a situation.
(i) In order to maintain the desired temperature of the mixture, the container can be wrapped either by woollen material or any other poor conductor of heat. Alternately, the mixture can be kept in a heat resistant container.
(ii) The container can be kept in the sun or near the gas stove while cooking.
You may have noticed that a few sharp jerks are given to clinical thermometer before using it. Why is it done so?
The jerk to the thermometer will allow the mercury in or above the kink to flow into the bulb so that the mercury level is below normal temperature.
Why is it advised not to hold the thermometer by its bulb while reading it?
If we hold a thermometer by its bulb, the mercury in the bulb will expand due to our body temperature.
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?
(i) On a hot summer afternoon the tent made up of white fabric will be preferred as white colour is a bad absorber and good reflector of heat.
(ii) No, the black fabric tent will be preferred during winter.
While constructing a house in a coastal area, in which direction should the windows preferably face and why?
The windows of houses in coastal areas should preferably face towards the sea as sea breeze will keep it cool during day time.
Observe the picture given as Figure 4.8. 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.
Position ‘P’ will feel warmer due to the hot air rising up
P ----->- Convection
T ----->- Radiation
Look at Figure 4.9.
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.
The pin on the wire in case A will fall first as heat will reach to it before it reaches the pin in case B.