In an experiment to compare how rapidly different metals conduct thermal energy, thermocouples are placed at 5 cm intervals along bars of copper, iron and g...

(a) From Table 7.1, the temperature at thermocouple T3 is 45 °C. [1]

(b) Temperature difference between T1 and T4 on the copper bar:

\[ \Delta T = 78 - 31 = 47\,°\text{C} \quad [1] \]

(c) Graph of temperature against distance for the copper bar:

All four points correctly plotted [2]; smooth curve drawn through the points [1].

(d) As the distance from the hot end increases, the temperature decreases. [1]

The curve shows that the temperature drop is steepest near the hot end and becomes more gradual further away. Energy is lost to the surroundings along the length of the bar.

(e) For the glass bar after 3 minutes: [2]

• The temperatures would be much lower at all positions, and those furthest from the hot end would be close to room temperature. [1]
• The curve would drop more steeply near the hot end and flatten much sooner, because glass conducts thermal energy very poorly. Little energy reaches the far end. [1]

(f) Copper conducts thermal energy much faster than glass because: [2]

• Copper contains free (delocalised) electrons that move rapidly through the metal lattice and transfer kinetic energy quickly along the bar. [1]
• Glass has no free electrons and relies only on slow vibrations passed from particle to particle through its rigid atomic structure. [1]

(g) One advantage of thermocouples: they respond faster to temperature changes than liquid-in-glass thermometers, giving more accurate readings of rapidly changing temperatures. [1]

They can also be placed more precisely at exact positions along the bar, and provide a digital output for easy recording.