Gravitational force is required for:

Options

A. Stirring of liquid
B. Convection
C. Conduction
D. Radiation

Answer

To solve the question "Gravitational force is required for:", we need to analyze the different modes of heat transfer and determine which one relies on gravitational force.

### Step 1: Understand the modes of heat transfer
There are three primary modes of heat transfer:
1. **Conduction**: Heat transfer through direct contact.
2. **Convection**: Heat transfer through the movement of fluids (liquids or gases).
3. **Radiation**: Heat transfer through electromagnetic waves, does not require a medium.

### Step 2: Analyze conduction
In conduction, heat is transferred through direct contact between materials. The molecules in the hotter object vibrate and pass their energy to the adjacent cooler molecules. This process does not require gravitational force because it occurs at the molecular level without the need for movement of the material as a whole.

**Hint**: Think about how heat travels through solids without any movement of the solid itself.

### Step 3: Analyze convection
In convection, heat transfer occurs through the movement of fluids. When a fluid is heated, it becomes less dense and rises, while cooler, denser fluid sinks. This movement is influenced by gravity, as it causes the denser fluid to fall and the less dense fluid to rise. Therefore, gravitational force is essential for convection to occur.

**Hint**: Consider how hot air rises and cool air sinks in a room; this is driven by gravity.

### Step 4: Analyze radiation
Radiation is the transfer of heat through electromagnetic waves. It does not require a medium and can occur in a vacuum. Since radiation does not involve the movement of matter and does not rely on gravity, it does not require gravitational force.

**Hint**: Remember that sunlight can travel through the vacuum of space without needing gravity.

### Conclusion
Based on the analysis, gravitational force is required for **convection**. Therefore, the correct answer to the question is convection.

**Final Answer**: Gravitational force is required for convection.