Aluminium is sometimes used as the leaf of an electroscope because it

Analysis

The question asks for the specific physical property of aluminium that makes it suitable for use as the leaf of a gold-leaf electroscope.

Concept Explanation

An electroscope is a device used to detect the presence of electric charge. It consists of a metal rod with a thin, flexible metal leaf (traditionally gold, but often aluminium in modern school laboratories) attached to the bottom.

For an electroscope to function effectively:

1. Charge Distribution: The material must be a conductor so that the electric charge can flow from the cap down to the leaf.
2. Sensitivity: The leaf must be extremely light so that even a very small amount of charge (which creates a small electrostatic force) can cause the leaf to deflect or move away from the metal rod.

Step-by-Step Solution

1. Evaluate Option C: Aluminium is a metal, not an insulator. An insulator would not allow charge to flow to the leaf, so the electroscope would not work. This is incorrect.
2. Evaluate Option D: While it is true that aluminium is malleable (can be made into thin sheets), this is a property of many metals. It explains how we make the leaf, but not why it is the best choice for the function of the device.
3. Evaluate Option B: Aluminium is a good conductor, which is necessary. However, if it were heavy, it would require a large force to move, making the electroscope insensitive to small charges.
4. Evaluate Option A: The primary reason for choosing aluminium (or gold) is its low density and ability to be beaten into an extremely thin, light foil. Because the leaf is so light, the electrostatic repulsion between the rod and the leaf is sufficient to cause a visible deflection even with a tiny charge. In the context of physics examinations, "lightness" is the critical factor that ensures high sensitivity.

Conclusion

While aluminium is a conductor, its primary advantage as an electroscope leaf—compared to other conductors—is its extremely low mass (lightness), which allows it to respond to the smallest electrostatic forces.

Exam Tip

When you see questions about electroscope leaves, remember the "Sensitivity Rule": The leaf must be light to detect small charges and a conductor to allow charge flow. If both "light" and "conductor" are options, "light" is usually the intended answer because it explains the sensitivity of the instrument.