Physics 2008 JAMB Past Questions - Question 25: In a good thermos Flask, the main cause of heat loss is

Correct Answer

C

Explanation

Since the cork is in contact with the walls of the flask, little conduction takes place.In a well-designed thermos flask, the main cause of heat loss is typically conduction. A thermos flask is specifically designed to minimize all forms of heat transfer, but conduction can still occur through the materials used in its construction. Here's how it works:

1. Conduction: Heat is transferred from the inside of the flask to the outside through the materials of the flask itself. While the materials used in a thermos are typically good insulators, like glass or stainless steel, they are not perfect insulators. Heat can slowly move from the inner layer to the outer layer through the material. To minimize conduction, the thermos flask is designed with double-walled or vacuum-insulated construction. The space between the walls is either a vacuum or contains a low-conductivity gas, reducing heat transfer through the material.

Other forms of heat transfer, such as convection and radiation, are minimized in a thermos flask:

2. Convection: Convection is the transfer of heat through the movement of fluids (liquids or gases). In a thermos flask, the air is removed from the space between the walls, creating a vacuum or low-pressure environment. Without air, convection currents are significantly reduced, minimizing heat loss through this mechanism.

3. Radiation: Radiation is the transfer of heat in the form of electromagnetic waves (infrared radiation). Thermos flasks often have reflective surfaces on the inner walls to reduce heat loss through radiation. These reflective surfaces help to keep the heat inside the flask by reflecting the thermal radiation back into the flask.

In summary, while all three modes of heat transfer (conduction, convection, and radiation) can play a role in heat loss in a thermos flask, a well-designed thermos flask is primarily focused on minimizing conduction. The vacuum or low-conductivity gas between the double walls of the flask is the key element that reduces heat transfer through the material and helps to keep the contents hot or cold for an extended period of time.