F4 Physics

1.)Wet clothes dry faster on a sunny day.
陽光加速水分蒸發的速率

2.)Wet clothes dry faster on a windy day.
風帶走水分 加快了水分流失的速率 即蒸發速率

3.)0.5kg of ice at 0'C is mixed with 1kg of water at 20'C.Assume that there is no heat transfer between the mixture and the surroundings.Find the final temp. of the mixture.
0.5x4200x(T-0)=1x4200x(20-T)
21000T=84000-4200T
25200T=84000
T=4度

2007-01-07 00:04:40 補充：
老眼昏花....冰的能量 - E10.5*4200 3.34*10^5=336100水的能量 - E20.5*4200*20=42000E1 E2so, the final temp of the mixture is 0'C

2007-01-07 00:05:09 補充：
E1＞E2

2007-01-07 00:10:52 補充：
對呢個情況係水比冰既能量吾夠所以冰未溶哂既時候水已經冇哂能量再比冰溶即係呢杯係一杯冰水冰水再要溶既話就要起空氣中取能量了

2007-01-07 00:13:50 補充：
所以呢題係一題幾陰人既題目即係可以話要水既能量大過冰先有溫度升由於混合左冇可能會有負數 所以設Unknow T係計吾到既呢題只要知道冰既能量大過水就知答案因為水最低既溫度就係0"C

2007-01-07 00:17:06 補充：
由於specific latent heat of fusion of ice is 3.34*10^5Jkg-1=334000所以水既能量一定要大過334000咁先有足夠既能量比冰去熔

2007-01-07 00:25:13 補充：
不如我咁講由於之前條數冰既能量大過水既能量 條式係( 0.5*4200*0 334000 ) = 334000水既能量係 ( 0.5*4200*20=42000 )所以冰既能量大過水既能量 混合後=0"C代數入公式係冇意思既 因為計出黎一定係負數

2007-01-07 00:35:51 補充：
要呀 因為我又打漏左XDD即係167000167000＞42000 都係好多XD

首先, rate of evaporation is affected by the following reasons:
1) Temperature of the environment
因為 higher temperature makes water molecules possess higher kinetic energy, then the change of the water molecules in escaping from the vicinity of water is higher.
(Example: A cup of water put in furnace evaporates much faster than a cup of water put in refrigerator.)

2) Surface area of liquid exposed to air
Larger area, more chance for water molecules to escape. (Since evaporation happens only at the liquid surface exposed to the atmosphere, unlike vaporizatino which can happen in the whole bulk of the liquid.)
(Example: A bowl of water evaporates faster than a test-tube of water.)

3) Air current
Faster air current, air can bring away those water molecules evaporated more quickly and hence raise the rate of evaporation.
(Example: When one sweats and a breezing wind blows on him/her, he/she feels cooler since higher evaporation rate brings heat energy away from him/her at a faster rate.)

4) Air humidity
Higher humidity means air is more saturated with water vapour, then water molecules are harder to escape into the air and hence the rate of evaporation is lower.
(Example: On a humid day, one will feel 唔爽 and 焗 since his/her sweat cannot evaporate and heat energy cannot be brought away.)

Paragraphs above can explain for your questions 1 and 2.

Question 3)
Specific latent heat of fusion of ice = 3.34 x 10^5 J/kg
Specific heat capacity of water = 4200 J K^-1 kg^-1

So when 0.5 kg of ice at 0C melts to water at 0C:
Energy absorbed = 0.5 x 3.34 x 10^5 = 167000 J

When 1 kg of water at 20C cools down to water at 0C:
Energy absorbed = 1 x 4200 x (20 - 0) = 84000 J

In word descriptions, NOT all ice are melted even when all water has been cooled down to 0C, i.e. the resulting mixture will be water + ice at 0C.

Moreover, mass of ice melted = 84000 / 334000 = 0.251 kg

Therefore, resulting mixture will consist of 0.5 - 0.251 = 0.249 kg of ice and 1 + 0.251 = 1.251 kg of water at 0C.

(Since the result is mixture of ice + water, it is for sure that the temperature should be 0C without other possibility.)

如果條問題一開始就set左塊冰係-20度o既話,咁仲計唔計到?)

Pls. find my explanation below:

Specific heat capacity of ice = 2100 J K^-1 kg^-1

when 0.5 kg of ice at -20C rises to water at 0C:
Energy absorbed = 0.5 x 20 x 2100 = 21000 J

when 0.5 kg of ice at 0C melts to water at 0C:
Energy absorbed = 0.5 x 3.34 x 10^5 = 167000 J

So total energy needed to melt all ice at -20C = 21000 + 167000 = 188000 J

咁就更加溶唔晒o的冰. 因為 20C 的水都係放到 84000 J 咁多 heat energy 出黎.

1) This is because the rate of evaporation is high when the temperature or when the humidity is low.

2) Wind can carry away the evaporation vapour and therefore increase the rate of evaporation.

3) Let T be the final temperature of the mixture
Applying E = mcT,we have
Energy lost by water = Energy gain by ice
1*4200*(20 -T) = 0.5*4200*(T - 0)
84000 - 4200T = 2100T
84000 = 6300T
T = 13.3'C

1. because the sun light gives energy to the water molecules in the clothes. Thus, it makes water evpourate faster.
2. because the wind gives energy to the water molecules in the clothes. Thus, it makes water evpourate faster.
3. energy absored by the ice = energy lost by the water at 20 oC
334000 * 0.5 + 4200 * 0.5 * T = 4200 * 1 * ( 20 - T )

understand?