Give reasons for the following:
(a) Iron rims are heated red hot before 'fixing them on cart wheels.
(b) A clinical thermometer has a slight 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.
Answers
Give reasons for the following:
(a) Iron rims are heated red hot before 'fixing them on cart wheels.
→ Iron rims are made slightly smaller than the wheels. They are heated red hot before fixing them on the cart wheels to expand them so that they can be easily fixed on the wheels and water is poured on them to cool them. As the rims cool they contract and take the shape of the wheel and get firmly fixed on the wheels.
(b) A clinical thermometer has a slight bend, or kink, in its capillary tube.
→ A clinical thermometer has a narrow kink in the capillary tube because when Mercury expands, it pushes through the kink, but when it's taken out of patient mouth, Mercury does not go back to the bulb beacuse if it. Hence, doctor can read the patients temperature easily .
(c) A new quilt is warmer than an old one.
→ A new quilt is warmer because the air which is trapped inside the cotton or woollen 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 it no longer remains a good insulator.
(d) A brass tumbler feels much cooler than a wooden tray on a chilly day.
→ The thermal conductivity of brass is high i.e., brass is a good conductor of heat. So, when a brass tumbler is touched, heat quickly flows from human body to tumbler. Consequently, the tumbler appears colder, on the other hand wood is a bad conductor.
(e) The bottoms of cooking utensils are often kept black.
→ The bottoms of the cooking utensils are kept black because black colors absorb heat faster than any other color. For cooking, we need that the supply of heat is fast so that the food cooks quickly. This is why the bottoms of the cooking utensils are kept black in color.