✔ 最佳答案
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.
