Question

A good insulator of heat is
A. Copper
B. Mercury
C. Trapped air
D. Iron

Answer 1

Hint: Good insulators are high in resistivity and conductivity. The electrons are static in insulators and those electrons cannot roam freely. So, current cannot easily pass. All the metals are almost conductors. Vacuum is also a kind of insulator.

Complete answer:
The insulators are those substances, through which heat energy cannot flow. Insulators contain large and strong dielectric strength. Their electrons are tightly bonded together. But they do not contain any free electrons. Good insulators contain high air permeability. That is why air itself is an insulator. Any material that does not allow heat or any electricity to go through it is known as an insulator.
Here in the question copper and mercury both are good conductors. They allow heat and current to go through them and they are also metals. Iron is also a good conductor. It also helps heat and electricity to travel through it. So the only remaining option is c. which is trapped air. This is the right answer. Because only trapped air or any kind of air cannot allow heat or electricity to pass through it. Because it has high permeability.

Hence, option C is the correct answer.

Note: We can get confused by mercury and trapped air. But only trapped air would be the right answer. Because it has high permeability and it does not have electrons for free roaming. Only trapped air has strong dielectric strength. So this option c holds all the properties of a good insulator. That is why option c would be the right answer.