A vacuum flask has a double-walled vessel with a vacuum in the space between the walls. How does this vacuum help to keep the contents of the flask hot?
It prevents loss of thermal energy by radiation.
It prevents loss of thermal energy by conduction and convection.
It increases the rate of conduction through the walls.
It acts as a good thermal conductor to keep the outer surface warm.
301 exam-style questions on CIE IGCSE Physics 2.3 Transfer of thermal energy, covering 2.3.1a Thermal conductors and thermal insulators, 2.3.1b Explaining thermal conduction, 2.3.2 Convection, 2.3.3a Thermal radiation, 2.3.3b Surface colour and texture, 2.3.3c Rate of emission and absorption of radiation, and 2.3.4 Consequences of thermal energy transfer. Each one has a worked solution and a mark scheme showing where the marks go.