Fig. 1.1 shows two identical aluminium cans, each containing 200 cm³ of hot water. Can A has a matte black painted surface. Can B has a polished silver surf...

(a) Reading the thermometer in Fig. 1.2: the liquid column sits at the second minor division above the 80 °C mark. Each major division spans 20 °C with one minor division in between (every 10 °C), so the reading is:

\[ \text{Initial temperature} = 82\,°\text{C} \quad [1] \]

(b) Temperature drop over 10 minutes:

\[ \text{Can A: } 82 - 52 = 30\,°\text{C} \quad [1] \]

\[ \text{Can B: } 82 - 61 = 21\,°\text{C} \quad [1] \]

(c) Plotting both cooling curves on the grid:

Can A points plotted correctly [1]; Can B points plotted correctly [1]; two smooth curves drawn and labelled A and B [1]. Both curves start at 82 °C and decrease, with A falling more steeply.

(d) Can A (matte black surface) cools at a faster rate. [1]

Its temperature drops by 30 °C in 10 minutes, compared to only 21 °C for Can B.

(e) Average rate of cooling for Can A:

\[ \text{rate} = \frac{\text{temperature drop}}{\text{time}} = \frac{30\,°\text{C}}{10\,\text{min}} \quad [1] \]

\[ \text{rate} = 3.0\,°\text{C per minute} \quad [1] \]

(f) Dark, matte surfaces are better emitters of infrared radiation than shiny, polished surfaces. [1]

Can A (matte black) radiates thermal energy to its surroundings at a higher rate. Can B (polished silver) reflects radiation internally and emits poorly, retaining more of its thermal energy. This is why Can A loses heat faster. [1]

The same surface properties that make a good absorber also make a good emitter. This is a general principle of thermal radiation.