In this experiment, you will investigate which component of a vacuum flask is most important in reducing heat loss. You sets up five identical containers of...

(a) The thermometer reads 84 °C for container E. The completed table:

[1]

(b) Temperature drop for container A:

\[ \Delta T = 90 - 52 = 38\;^\circ\text{C} \] [1]

With no insulation features, container A loses heat rapidly through conduction, convection, and radiation simultaneously.

(c) The partial vacuum (container D) is the most effective single feature at reducing heat loss. [1] It retains 71 °C, the highest temperature of any single-feature container. Its temperature drop is only \( 90 - 71 = 19\;^\circ\text{C} \), compared to 30 °C for the stopper and 26 °C for the silvered wall. [1]

This result makes physical sense: conduction and convection through air are the dominant heat-loss mechanisms for a container of hot water, and removing most of the air tackles both at once.

(d) The stopper reduces heat loss by preventing hot water vapour and steam from escaping from the top and by stopping convection currents from carrying hot air away from the surface. [1]

(e) The silvered wall is a poor emitter and poor absorber of infrared radiation, so it reduces heat loss by radiation. [1] Shiny metallic surfaces have low emissivity and reflect most infrared radiation back towards the hot water instead of letting it radiate outward.

(f) The partial vacuum contains very few air molecules, so there is very little heat transfer by conduction or convection across the gap between the walls. [1] Both conduction and convection require particles to transfer kinetic energy; with most of the air removed, these transfer routes are greatly reduced.

(g) Container E retains the most heat (84 °C, a drop of only 6 °C) because it combines all three methods of reducing heat loss simultaneously. [1] The stopper reduces evaporation and convection from the top, the silvered wall reduces radiation, and the partial vacuum reduces conduction and convection through the walls. Each feature targets a different heat-transfer pathway.

(h) From Fig. 29.1, the gap between the outer wall (at x = 80) and the inner wall (at x = 95) is approximately 15 mm at the diagram scale. [1]

(i) A true vacuum contains no molecules at all, so there is zero conduction and zero convection across the gap. [1] A partial vacuum still has some residual air molecules that can transfer heat by colliding with each other and the walls, making the insulation less effective.