Problem 978 - JEE Physics

Question

When there is no external force, the shape of liquid drop is determined by (A) Surface tension of liquid (B) Density of Liquid (C) Viscosity of liquid (D) Temperature of air only

Step-by-Step Solution

Answer

The shape of a liquid drop when there is no external force is determined by the surface tension of the liquid. Therefore, the correct answer is (A) Surface tension of liquid.

Step 1: Understanding Surface Tension

Surface tension is a property of liquids that allows them to resist any external force. It's the force acting across the surface of a liquid that causes it to contract and minimize its surface area. Surface tension is what causes liquid drop to form a spherical shape in the absence of external forces.

Step 2: Understanding Density

Density is the mass per unit volume of a substance. It determines how heavy or light a substance is for a given volume. The density of a liquid does not have a direct impact on the shape of a liquid drop when there is no external force acting on it.

Step 3: Understanding Viscosity

Viscosity is a measure of a fluid's resistance to flow. It indicates how "thick" or "sticky" a liquid is. While viscosity might affect the speed at which a liquid drop forms, it does not determine the shape of the liquid drop in the absence of external forces.

Step 4: Understanding Temperature of Air

The temperature of the air surrounding a liquid drop can influence the rate of evaporation and condensation and can cause the liquid to form drops more quickly or slowly. However, it does not directly determine the shape of the liquid drop when no external forces are acting on it. Based on the analysis of these factors:

Step 5: Conclusion

The shape of a liquid drop when there is no external force is determined by the surface tension of the liquid. Therefore, the correct answer is (A) Surface tension of liquid.

Key Concepts

Liquid Density

Liquid density is a fundamental physical property that represents how much mass is contained in a given volume of a liquid. It is commonly expressed in units of kilograms per cubic meter (kg/m³).
The higher the density, the more "tightly packed" the molecules are in the liquid.

To understand density, consider a heavy liquid like mercury compared to a less dense liquid like water:

• High density: Mercury has a density of about 13,534 kg/m³, meaning it is much heavier than water for the same volume.
• Low density: Water's density is approximately 1,000 kg/m³, making it lighter than many other liquids.

The density of a liquid can influence its behavior, but it does not affect the shape of a droplet that forms in the absence of external forces. Instead, density plays a significant role in concepts like buoyancy and separation of liquids in mixtures.

Viscosity

Viscosity is a measure of a liquid's resistance to flow, often referred to as the "thickness" or "stickiness" of a liquid. Thick liquids like honey or syrup have high viscosity, while thin liquids like water or alcohol have low viscosity.

Viscosity can impact how fast a droplet forms and its flow characteristics. Imagine pouring honey and water:

• High viscosity: Honey flows slowly and takes longer to form droplets because of its high resistance to flow.
• Low viscosity: Water flows quickly and forms droplets rapidly due to its low resistance to flow.

However, when no external forces are present, viscosity will not change the droplet's shape. It is more related to the liquid's flow behavior and interactions with surfaces rather than droplet formation.

Temperature Effects on Liquids

Temperature can have several effects on liquids, influencing properties like viscosity and density. When the temperature of a liquid changes, it can lead to expansion, contraction, and changes in flow and form characteristics.

Consider these effects:

• Viscosity reduction: Increasing temperature typically lowers the viscosity, making the liquid flow more easily. This is why heating honey makes it less sticky.
• Density variation: Temperature can cause liquids to expand and thus decrease density as the molecules move apart.

While temperature influences the above properties, it does not fundamentally change the shape of a droplet formed due to surface tension. Instead, temperature affects the conditions under which droplets might form, such as through evaporation rates or condensation.