Convection in water driven by density differences from bottom heating

Steps:

• Heat water at the bottom to increase its temperature.
• Heated water expands, becoming less dense than surrounding cooler water.
• Less dense water rises, displacing cooler, denser water downward.
• This density-driven circulation establishes the convection current around the tube.

Why A is correct:

• Point A is at the bottom; heating there causes hot water to rise per the convection principle where buoyancy drives less dense fluid upward.

Why the others are wrong:

• B (side): Heating midway disrupts flow without initiating full density gradient for circulation.
• C (upper side): Localized heating here causes minimal vertical movement, insufficient for loop-wide convection.
• D (top): Heating at the top makes water sink if denser, but fails to create rising hot fluid needed for current.

Final answer: A