Give reason for the following:
a. Iron rims are heated red-hot before selecting them on cartwheels
b. A clinical thermometer has a sight bend or kink in its capillary tube.
c. A new quilt is warmer than an old one
d. A brass tumbler Feels much cooler than a wooden tray on a Chilly day
e. The bottoms of cooking utensils are often kept black.

Academic Physics NCERT Class 7

a) Metals expand on heating and contracts on cooling. When the Iron rim is heated it expands and fits easily on the cartwheels. When they cool afterward they contract and fit tightly to the wheel.

b) Since the clinical thermometer has a kink the temperature does not fall on its own after removing from the person's mouth. This helps in taking reading after removing from the person. To lower the mercury level again, a small jerk is needed.

c) A new quilt is warmer because the air which is trapped inside the cotton or woolen clothing acts as an insulator and does not allow the transfer of heat. However, when the quilt gets older, the air spacing of the cotton or wool gets compressed.

d) Brass is a good conductor of heat. When one touches a brass tumbler, heat is conducted from the body to the brass tumbler easily. So one feels cooler. Wood is a bad conductor of heat, so when touched it doesn't absorb much heat from us. Thus, a brass tumbler feels colder than a wooden tray on a chilly day.

e) The bottoms of the cooking utensils are kept black because black colors absorb heat faster than any other color. For cooking, we need the supply of heat to be fast so that the food cooks easily.

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Updated on: 10-Oct-2022

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