Q&A - PSLE Science

[shine]

Water Cycle


Hi, can someone please help to explain the following? It's from my child's P5 science CA paper.

We know that water evaporates and condenses at higher parts in the sky where temperature is much lower. When we put dishes of sea water and enclose them in plastic sheets (the plastic sheets are being propped up by sticks so it's like placing the dishes in a house of plastic), water droplets can been seen on the underside of the roof of the plastic sheets. The whole set-up is placed under the sun. I think water evaporates from the dishes and condenses on the plastic sheet, similar to our eco system. What I don't quite understand is that if the whole set-up is placed in the sun, why would the plastic sheet be of lower temperature for condensation to occur?

Thanks much in advance!

[atutor2001]

shine:

Water Cycle


What I don't quite understand is that if the whole set-up is placed in the sun, why would the plastic sheet be of lower temperature for condensation to occur?

When light passes through the plastic sheet it is absorbed by the ground/container below and is then converted to heat energy. The wave lengths of light and heat energy are different. Wave length of heat energy is longer than that of light energy and cannot pass through the plastic sheet. Therefore, heat is trapped below. This is the same principle used in \"greenhouse\". (same as when you park a car under the hot sun) Therefore, the temperature of air/water vapour below the plastic sheet is higher than that of the plastic sheet as the plastic sheet can lose heat to external air by conduction.

[tianzhu]

shine:

Water Cycle


Hi, can someone please help to explain the following? It's from my child's P5 science CA paper.

We know that water evaporates and condences at higher parts in the sky where temperature is much lower. When we put dishes of sea water and enclose them in plastic sheets (the plastic sheets are being propped up by sticks so it's like placing the dishes in a house of plastic), water droplets can been seen on the underside of the roof of the plastic sheets. The whole set-up is placed under the sun. I think water evaporates from the dishes and condenses on the plastic sheet, similar to our eco system. What I don't quite understand is that if the whole set-up is placed in the sun, why would the plastic sheet be of lower temperature for condensation to occur?

Thanks much in advance!

Hi

First, is this a complete question? It is helpful to have the complete question from the test paper, because vital information may be omitted during the summary.

Let us assume the setup is left for a day. During the day, water evaporates from the seawater to form water vapour. At night when the weather is cooler, the water vapour condenses on the plastic sheet above, forming water droplets.

[atutor2001]

[quote]Water Cycle


What I don't quite understand is that if the whole set-up is placed in the sun, why would the plastic sheet be of lower temperature for condensation to occur?


During the day, water evaporates from the seawater to form water vapour. At night when the weather is cooler, the water vapour condenses on the plastic sheet above, forming water droplets.[/quote]
The water vapour will also condense on the underside of the sheet in the daytime under the hot sun as the temperature of the sheet will be lower than the temperature of the water vapour below it. The temperature of the sheet will always be lower because it is in contact with external air and heat can be conducted away while the air/water vapour below the sheet is trapped together with the heat below the plastic sheet.

[shine]

Hi atutor2001 and tianzhu, thanks for the prompt response! Really appreciate it.

tianzhu:

First, is this a complete question? It is helpful to have the complete question from the test paper, because vital information may be omitted during the summary.

Don't have the exact question with me as my child came home after the Science CA and discussed the question with me. The question is not exactly what I've written but the \"essence\" is there. Will probably post the question up once they get back their CA paper.

tianzhu:

Let us assume the setup is left for a day. During the day, water evaporates from the seawater to form water vapour. At night when the weather is cooler, the water vapour condenses on the plastic sheet above, forming water droplets.

According to my child's description, the set-up has not been left for a day. It was like for a few hours.

[atutor2001]

[quote]
According to my child's description, the set-up has not been left for a day. It was like for a few hours.[/quote]We don't need to wait for the weather to cool down in order for the water to condense on the plastic sheet.

There is another factor that I did not mention - \"the water vapour below the plastic sheet is saturated\" \"Water vapour is saturated\" means that the amount of water vapour in the air below the sheet is at the maximum allowable amount (at that temperature). Any slight drop in temperature on any surface will result in condensation. (this is covered only in sec or JC) The temperature of the sheet is the only surface that can get cooled down slightly because it is in contact with the external air.

Sadly, many Pr school teachers copy questions 'blindly' without understanding the many other theories behind that are not covered in the syllabus.

[tianzhu]

http://www.wendykoh.com/08/primary6-nanyangsa1-science.pdf


MCQ Q2, from the options available, the answer is pretty obvious, that is option(2).But, I am wondering on how an iron converts electrical energy to any useful forms of kinetic energy.

[Dharma]

tianzhu:

http://www.wendykoh.com/08/primary6-nanyangsa1-science.pdf


MCQ Q2, from the options available, the answer is pretty obvious, that is option(2).But, I am wondering on how an iron converts electrical energy to any useful forms of kinetic energy.

Good Morning Tianzhu,

The useful forms of energy in the qn is referring to heat energy and not kinetic energy.

[tianzhu]

Good morning Dharma


Thank you for your quick reply. Sorry, typed in useful for kinetic energy erroneously.

Should be read as I am wondering how an iron converts electrical energy into kinetic energy.

[Dharma]

tianzhu:

Good morning Dharma


Thank you for your quick reply. Sorry, typed in useful for kinetic energy erroneously.

Should be read as I am wondering how an iron converts electrical energy into kinetic energy.

The kinetic energy refers to the movement of the iron when u iron clothes. W/o the KE, u will not get the useful form of heat energy. Clothes will be burnt.