Physics - Heat & Thermo Question with Solution | TestHub
By varying the voltage applied to the kettle, you can change its power consumption P. Depending on P, the kettle with water can be heated to different maximum temperatures. This dependence is shown in Table 1.
Table 1
Power P (W) | 0 | 100 | 200 | 300 |
|---|---|---|---|---|
Temperature T (°C) | 20 | 40 | 60 | 80 |
Table 2 shows the variation of temperature with time when the kettle with a 300 W heater is switched off. The heat capacity of the empty kettle C₀ = 100 J/K; specific heat of water = 4200 J/(kg K); density of water = 1000 kg/m³. Assume Newton’s law of cooling.
Table 2
Time t (s) | 0 | 60 | 300 | 600 | 1200 | 2400 |
|---|---|---|---|---|---|---|
Temperature T (°C) | 80 | 75 | 60 | 45 | 30 | 20 |
If the power consumption is 400 W
Options:
Answer:
Solution:
At a steady maximum temperature, heater power equals heat loss to the room.
At 100°C all 400 W offsets loss; there is no net power to vaporize water.
It reaches boiling point but does not boil. Answer B.
