NCERT Class VII Science Solutions: Chapter 4 – Heat Part 4

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Question 13:

In the arrangements A and B shown in 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.

As showing in images is a pins is heated

Pins is Heated

As showing in images is a pins is heated

Answer:

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.

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 . 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.

Answer:

  1. In order to maintain the desired temperature of the mixture, the container can be wrapped either by woolen material or any other poor conductor of heat. Alternately, the mixture can be kept in a heat resistant container.

  2. The container can be kept in the sun or near the gas stove while cooking.

Question 15:

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

Answer:

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.

Question 16:

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

Answer:

If we hold a thermometer by its bulb, the mercury in the bulb will expand due to our body temperature.

Question 17:

At a camp site there arc 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?

Answer:

  1. 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.

  2. No. the black fabric tent will be preferred during winter.

Question 18:

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

Answer:

The windows of houses in coastal areas should preferably face towards the sea as sea breeze will keep it cool during day time.