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Difficulty: EasyHeat Transfer: Convection

In a typical domestic water boiler, the heating element is installed near the bottom of the tank rather than the top. Which of the following best explains the reason for this placement?

  1. A
    To transfer heat upward through the fluid primarily by direct physical conduction from molecule to molecule.
  2. To allow the heated water to expand, become less dense, and rise, creating a natural convection current.Answer
  3. C
    To cause the heated water to contract and become denser, allowing it to efficiently collect heat at the bottom.
  4. D
    To compress the liquid water at the bottom of the tank so it can absorb thermal energy more rapidly.

Answer

To allow the heated water to expand, become less dense, and rise, creating a natural convection current.
When water at the bottom of a tank is heated, it undergoes thermal expansion and its density decreases. The warmer, less dense water naturally rises due to buoyancy, while the cooler, denser water from the top sinks to take its place near the heating element. This creates a continuous, self-sustaining circulation called a natural convection current, which efficiently heats the entire volume of fluid.

Step-by-Step Solution

1
Identify what happens to the fluid immediately surrounding a heat source.
The water at the bottom of the tank absorbs heat from the heating element.
Heat transfer begins with the interaction between the heat source and the adjacent fluid.
2
Apply the principle of thermal expansion.
As the water warms, it expands, which causes its density to decrease relative to the surrounding cooler water.
Temperature and fluid density are inversely related due to thermal expansion.
3
Determine the resulting fluid movement caused by buoyancy.
The warmer, less dense water rises to the top of the tank, displacing cooler, denser water that sinks to the bottom to be heated.
This continuous circulation pattern, driven by density differences, is known as a natural convection current.

Key Concept

Natural Convection
Estimated Time:45s
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Topics

Heat Transfer: Convection

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