F.3 Physics Latent Heat

Wai Keung

2009-11-26 11:48 pm

A.
How much energy is needed to heat up and vaporize 0.5 kg of water at 25 degrees Celsius?

Energy needed
=0.5*4200*(100-25)+0.5*2.26*10^6
=1.2875MJ

B.
Cathy is steaming fish in a wok. Initially there is 0.5 kg of water at 25 degrees Celsius in the wok. The wok is covered but steam can still escape through a hole in the cover. Compare the energy required to vaporize all the water in the wok with the result in (A). Which one is larger? Explain your answer briefly. You can ignore the energy loss to the surroundings.

回答 (1)

天同

2009-11-27 5:16 am

✔ 最佳答案

The energy required in (B) is larger than 1.2875 MJ.
It is because the wok is covered, although some steam can escpae through the hole on the cover, most steam is trapped by the cover and which will condensed back to water. Hence, extra heat energy is needed to vapourize the condensed water into steam again.
In the actual fact, covering the wok during steam cooking is a way to lengthen the time of boiling all the water away.

👍 0 👎 0