Q&A - PSLE Science

[vlim]

tianzhu:

vlim:

Just now my ds was doing this qn... In an enclosed aluminum box with a light bulb on. They want to test with what inner wall Colour, the temperature will be the highest with the bulb on for 15 Min. The answer is black follow by grey then white... But my ds answer to it is exactly opp as he said tt black absorb heat and conducted away fast thru the aln box while white reflect light within box so make the temp high ... So I was confuse ... :?

So can someone pls enlighten me :please: :please:

Hi

This question is pretty similar to one from PSLE 2009, remember the one with a toy, white and black coloured vanes and Ahmad shining a torch.

More light energy from the light bulb is absorbed by the black inner wall than the light coloured ones. The absorbed light energy is then transformed to heat energy. As the black inner wall absorbs the most light energy,there is more light energy being changed into heat energy,it becomes hotter than the rest.

Why do mothers tell their kids to wear light coloured clothes when they are out in the sun?

This link helps.
http://www.sciencekids.co.nz/experiments/lightcolorheat.html

Best wishes

PS --- Students should note that conduction of heat takes place when two objects are touching one another. (PSLE Science Guide, MC)

thanks tianzhu for your explaination..
But his reasoning is..the heat energy being absorbed by the black inner wall is transmitted away(sorry i don't how to use the exact word) through the aln box to the air outside...whereas the box with white interior, reflected the light and hence the light energy-> heat energy remain in the box...so the temperature in the box with white col is higher then tt of black col inner wall... :?

[atutor2001]

vlim:

....
But his reasoning is..the heat energy being absorbed by the black inner wall is transmitted away(sorry i don't how to use the exact word) through the aln box to the air outside...whereas the box with white interior, reflected the light and hence the light energy-> heat energy remain in the box...so the temperature in the box with white col is higher then tt of black col inner wall... :?

Hi vlim

Your son's explanation is correct.

Part of the light absorbed by the box which is changed to heat will be lost due to conduction. The more it absorbs, the more heat it will lose. Therefore, a black surface which absorbs more light will lose more heat than a white surface.

The white surface absorb much less light. The light is reflected to and fro. In the process, this reflected light will be absorbed by the dust particles and air molecules in the box and is similarly converted to heat. But it is the dust particles and air that gained the heat energy (not the box) and needs a longer time to conduct through the material of the box. It will stay (longer) in the box as compared to a black surface box. Therefore, the AIR in the white surface box will have higher temperature than the black surface box.

[vlim]

atutor2001:

vlim:

....
But his reasoning is..the heat energy being absorbed by the black inner wall is transmitted away(sorry i don't how to use the exact word) through the aln box to the air outside...whereas the box with white interior, reflected the light and hence the light energy-> heat energy remain in the box...so the temperature in the box with white col is higher then tt of black col inner wall... :?

Hi vlim

Your son's explanation is correct.

Part of the light absorbed by the box which is changed to heat will be lost due to conduction. The more it absorbs, the more heat it will lose. Therefore, a black surface which absorbs more light will lose more heat than a white surface.

The white surface absorb much less light. The light is reflected to and fro. In the process, this reflected light will be absorbed by the dust particles and air molecules in the box and is similarly converted to heat. But it is the dust particles and air that gained the heat energy (not the box) and needs a longer time to conduct through the material of the box. It will stay (longer) in the box as compared to a black surface box. Therefore, the AIR in the white surface box will have higher temperature than the black surface box.

Thanks atutor2001 for your explanation..so the model answer of that past yr exam paper is wrong..

[tianzhu]

vlim:

thanks tianzhu for your explaination..
But his reasoning is..the heat energy being absorbed by the black inner wall is transmitted away(sorry i don't how to use the exact word) through the aln box to the air outside...whereas the box with white interior, reflected the light and hence the light energy-> heat energy remain in the box...so the temperature in the box with white col is higher then tt of black col inner wall... :?

Hi vlim

I am sorry, I made a mistake.

My answer refers to heat absorbed by the black surface, I read the question again, and it’s about the temperature of the air inside the box, not the surface.

Best wishes

[vlim]

tianzhu:

vlim:

thanks tianzhu for your explaination..
But his reasoning is..the heat energy being absorbed by the black inner wall is transmitted away(sorry i don't how to use the exact word) through the aln box to the air outside...whereas the box with white interior, reflected the light and hence the light energy-> heat energy remain in the box...so the temperature in the box with white col is higher then tt of black col inner wall... :?

Hi vlim

I am sorry, I made a mistake.

My answer refers to heat absorbed by the black surface, I read the question again, and it’s about the temperature of the air inside the box, not the surface.

Best wishes

Thanks tianzhu!!
I always have a hard time when going through given model ans for past yr science exam papers with my son...He often disagree with the answer and argue with me abt the answer given...So nowadays,..i give up liao..if he think he is right then ok lo...but if his argument is wrong then jialet liao, he will give tt wrong answer again if tt qn appear in the exam
:thankyou:

[atutor2001]

vlim:

....so the model answer of that past yr exam paper is wrong..

If there is only 1 correct answer and your son's answer is correct, then by logical deduction, I think the model answer should be incorrect.

[tianzhu]

smartmummy:

I confused,for the sponge they said the air escaped when we squeezed,but here they said air compressed when we squeezed.

Hi

I’ve come across another question, it’s about a sponge being squeezed to take up a smaller space in a box.
If I remember correctly, the answer is “The sponge has a lot of air spaces in it and air can be compressed\"

I agree with you that air is being forced out rather than compressed.

Maybe, this is acceptable in P4 science for such question.

Best wishes

[smartmummy]

http://i55.tinypic.com/653k39.jpg\">

Hi Tianzu!
here is the sponge qn with answer.

[smartmummy]

http://i55.tinypic.com/20hrsao.jpg\">

Answer gave like as follows in the Assessment book Science Process Skills P3/4,

Yes,the stress ball is in the solid state.There are tiny air spaces inside the foam rubber which can be compressed when the ball is squeezed.As soon as the hand squeezing it is removed,the stress ball is able to return to its original shape and size.

[smartmummy]

Hi Tianzu!

I have posted two qns with answers provided in the respective books.
thank you.