Four hot metallic articles which are coloured blue, red, black and white, and have the same temperature, are allowed to cool. Which will have the fastest cooling rate ?

Options

A. Black
B. Blue
C. Red
D. White

Answer

To determine which of the four colored metallic articles (blue, red, black, and white) will have the fastest cooling rate, we can analyze the properties of color and emissivity.

### Step-by-Step Solution:

1. **Understanding Emissivity**: Emissivity is a measure of an object's ability to emit energy as thermal radiation. It ranges from 0 to 1, where 1 is a perfect black body that emits the maximum amount of thermal radiation.

2. **Color and Emissivity**: Different colors have different emissive properties. Generally, darker colors have higher emissivity compared to lighter colors. Specifically, black surfaces are known to be the best emitters of thermal radiation.

3. **Comparing the Colors**:
- **Black**: Highest emissivity, best at emitting heat.
- **Blue**: Moderate emissivity.
- **Red**: Moderate to low emissivity.
- **White**: Lowest emissivity, poor at emitting heat.

4. **Cooling Rate**: According to the Stefan-Boltzmann law, the rate of heat loss of an object is proportional to its emissivity. Therefore, the object with the highest emissivity will cool down faster.

5. **Conclusion**: Since black has the highest emissivity among the colors listed, it will emit heat more effectively than the others. Thus, the black metallic article will cool the fastest.

### Final Answer:
The black metallic article will have the fastest cooling rate.

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