Suppose a test tube of water is heated from the top. Explain how the heat is transferred through the water, and why the person can continue to hold the test tube.

Answer 1

When a test tube of water is heated from the top, the process of heat transfer primarily occurs through conduction and convection.

1. Conduction: Initially, when the heat source warms the top of the water, the molecules at the surface gain energy and, as a result, begin to vibrate more vigorously. This increased kinetic energy is passed on to neighboring molecules through collisions, transferring heat downward through conduction. However, conduction is not the most efficient means of heat transfer in fluids like water, as it is generally slower compared to convection.

2. Convection: As the water molecules at the surface heat up and gain energy, they become less dense and start to rise. Conversely, the cooler, denser water molecules below will sink to take their place. This creates a convection current, where warmer water rises and cooler water descends, effectively distributing heat throughout the liquid. This circulation helps to transfer heat more efficiently throughout the body of water.

Regarding why a person can continue to hold the test tube, it is important to note a few things:

• Location of the Heat Transfer: Because the test tube is heated from the top, the bottom of the test tube—where the person is holding it—is not being directly heated. The heat from the water at the top does not immediately transfer to the glass or the area being held.

• Low thermal conductivity of glass: The glass of the test tube has relatively low thermal conductivity, meaning it does not transfer heat from the hot water to the hand rapidly. This provides insulation and allows the person to hold the test tube comfortably.

• Heat capacity of water: Water has a high specific heat capacity, which means it can absorb a lot of heat without a large increase in temperature. Therefore, even if the top layer of water becomes hot, the bulk of the water below may not reach high temperatures quickly.

Overall, the combination of conduction and convection facilitates heat distribution in the water, while the insulation properties of the glass and the positioning of the heat source explain why the person can comfortably and safely hold the test tube.