Physics問題-heat!

You did not give a description on how your experiment is performed, but I think you measured the latent heat by noting the difference between the mass of water before and after boiling, provided that the heat supplied was known or estimated.

The simple equation used in evaluating the latent heat is,
mL = Q
where m is the decrases of mass of water in the container on heating,
L is the latent heat of vapourization of water
Q is the heat supplied to heat the water during boiling.

Thus, L = Q/m

(I) Replacing the beaker with a polystyrene cup would reduce the heat lost to the surroundings. Provided that the amount of heat supplied by the heater is the same as before (i.e. Q remains constant), then more water will boil away than if there is heat lost to the surroundings. As a result, the mass decrease due to boiling is more (i.e. value of m increases), this leads to a reduction on the value of L.

(II) When the heater is completely immersed, more heat can be transferred from the heater to boil the water in a unit time. As a result, more water will be evapourated in a unit time than before. Thus the value of m increases, which leads to a lower value of L.

Q: ......, 咁唔係要用更多energy去change state咩??
The concept is completely wrong. The energy needed to change the state of unit mass of a given substance is about constant. The putting in of more energy means more (mass of) substance will have its state changed.

Q: 同埋physics成日都話heat 咩loss to surrounding, 其實點解??
Simply speaking, this can be interpreted as the heat energy absorbed by the surrounding air, which subsequently carries the heat away.

答案好明顯係A.

首先你要明白Heat loss 係咩先~ heat loss 應該可以理解為一d energy 到唔到佢ge目的地啦. for example 去左beaker呀, enviorment呀等等. 咁去左另一d 地方而唔係杯水.or 泳(fushion果陣)---> 咁咪啫係有一部份heat 用多左?!


你話:個beaker入面存既熱就愈黎愈多....尼句好似有D問題，理論上你heat佢緊係會愈來愈多energy嫁啦...無論有冇poly cup 都係。


你應該理解為: 因為减少左外界因素，變相多d 快左 energy transfer<-- 想像一下本來去左出邊ge heat都俾你用番。


你話: "咁beaker入面存既熱就愈黎愈多, 愈熱就愈易evaporate, 所以latent heat of vaporization 就decreses" 好似怪怪地...唔係愈易..俾人感覺到個value of vapourization 少左...可以試下話快左..



如事者咪value decrease, 答案係A。


PS 計latent heat of vapourization 一定唔可以加蓋, 會大左value，因為d水condense番尼個mass大番.

希望幫到你啦.