F.3 phy. about heat,,

2008-06-07 3:03 am

An immersion heater rated at 200W is immersed ina cup of water at 0度 . After the heater is switched on for 200s , the water is heated to 25度. Given that the room temperature is 25度.

Which of the following statements is/are correct?

1) If the immersion heater uesd is rated at 100W , the time needed to heat the water from 0度 to 25度 must be longer tyhan 400s.

2) If the polystyrene cup os replaced by a copper container , the time needed to heat the water from 0度 to 25度 must be longer.

3) The time needed to heat the water from 25度 to 50度 is longer than 200s.

and ONLY no.3 is CORRECT.

but why?

回答 (1)

天同

2008-06-07 4:31 am

✔ 最佳答案

Statement (1) is wrong
It is because there is heat inflow from the surroundings (which is at 25 deg.C) into the cup of water (which is below 25 deg.C).
Although the heat supplied by the heater is halved, the time needed is shorter than 400s due to the heat gained from the surroundings.
[Note: even if the heater is turned off, the water will eventually reach room temperature after some time because of heat flow from the surroundings].

Statement (2) is wrong
A copper container increases the rate of heat gain from the surroundings. The time will be shorter.

Statement (3) is right.
When water is heated above the room temperature of 25 deg.C, there is heat flowing out from the water to the surroundings. More heat from the heater is thus required to compensate the heat lost to the environment, thus needing a longer time to raise the temperature by 25 deg.C above the room temperature.

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