When does convection occur?

Convection is a process that occurs due to the movement of fluids (liquids or gases) caused by a temperature difference. When one part of a fluid is heated, it becomes less dense and rises, while the cooler, denser fluid sinks. This movement creates a circulation pattern that transfers heat throughout the fluid. Therefore, a temperature difference is essential for convection to take place, as it drives the movement of fluid necessary for the transfer of thermal energy.

The other options do not directly define the condition for convection. For instance, while heat pads can facilitate heat transfer, they do not specifically describe the natural process of convection itself, which is fundamentally reliant on temperature gradients. Solidification refers to the process by which a material transitions from a liquid to a solid and is not directly linked to convection, although convection can occur in the fluid phase prior to solidification if temperature differences exist. Vacuum conditions lack a fluid medium, making convection impossible as there are no particles to move and circulate heat.