A 300 g block of an unknown substance is initially at –20 °C
PHYSICS · Subjective · A Level · Physics · Further mechanics and thermal physics · Further mechanics and thermal physics
Problem
A 300 g block of an unknown substance is initially at –20 °C. A constant-power electric heater is switched on, and the following observations are recorded:
• The temperature rises uniformly from –20 °C to 0 °C in 2 min.
• The temperature remains steady at 0 °C for the next 5 min.
• Thereafter the temperature rises uniformly from 0 °C to 80 °C in 8 min.
(i) Draw a neat, qualitative temperature–time graph for the entire heating process and clearly mark the three distinct regions.
(ii) For each region identified in part (i), state the physical state of the substance and the thermodynamic process taking place.
(iii) Explain why the temperature stays constant in the second region even though the heater continues to supply energy at the same rate.