S4 Physic Energy

2011-10-08 10:56 pm

唔該我想問下
計energy系用呢d式架啦Q=mc△T
呢條系我書既EXAMPLE 我有D地方唔明
A 1200W electric kettle is used to heat 0.6kg of water.The water temperature changes fro 25℃ to 100℃ in 3 minutes. The mass and the specific heat capacity of the electric kettle are 0.5kg nad 500Jkg-1℃-1 respectively
甘計佢個energy就會系
Q=0.5x500x80+0.5x4200x80
我想問下點解佢呢一條個energy要計埋比kettle既energy既??
我見其他example 計比water 既energy 系唔需要計比埋個煲既energy架喎??

回答 (1)

天同

2011-10-09 1:51 am

✔ 最佳答案

Your equation: Q=0.5x500x80+0.5x4200x80 is NOT right.

The correct equation should be:
Q = (0.5 x 500 x 75 +0.6 x 4200 x 75 ) J

The kettle is also subject to a temperature rise, and hence it absorbs heat. It needs to be taken into account in the calculation. This just follws from the Law of Conservation of Energy.

Only in exceptional circumstance that the mass and/or specific heat capacity of the container is low compared to that of water, the heat absorbed by the container is much less than that absorbed by water that the former could be neglected.

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