Q&A - PSLE Science

[atutor2001]

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 vlim

Congratulation, your ds has very good application skill for physics. His answer is correct. Let me try to explain using math :

Temperature in the box depends on the \"Net heat gain/loss in the box\"

Net heat gain/loss in the box = Heat/light produce by bulb - Heat loss through box.

Heat Loss through box depends on the \"Difference in temperature between the box and the external temperature\".

If the external temperature is very low, heat loss is will be very high.

In the external temperature is equal to temperature in the box, there will be no heat flow.

If external temperature is higher than temperature in the box, then the box will gain heat instead.

By having a black surface, the box will absorb heat faster, causing the temperature to increase higher than a white surface. With higher temperature, the difference in temperature with the external will be bigger and heat loss will be greater. Therefore, the temperature in the black box will be lower.

[chrisu]

Oh no, my daughter came back from her Science Prelim just now asking me;

"What is the structural difference between a female bird and a female human reproductive organ?"

She answered - the female bird has 1 ovary while female human has 2. Not so sure as femal bird has 2 just that only 1 is functional.

Wonder why such a question? Any answers?

[atutor2001]

chrisu:

Oh no, my daughter came back from her Science Prelim just now asking me;

\"What is the structural difference between a female bird and a female human reproductive organ?\"

She answered - the female bird has 1 ovary while female human has 2. Not so sure as femal bird has 2 just that only 1 is functional.

Wonder why such a question? Any answers?

Wow! This is extreme. Last time, while coaching my kids, I always advise them to \"think simple\" and answer within the confine of her knowledge for Pr Sch. Many PSLE questions are like that.

As for this question, my guesses would be :

1. Human normally produces only 1 egg each time while birds can produce more than 1 egg at a time.
2. Human has a womb for the fertilised egg to develop into a baby while bird does not.

Honestly, I don't know the answer but would use (2). But I think your girl has made a very good guess.

Cheers

PS
Just googled,

\"Female birds in most families have only one functional ovary (the left one), connected to an oviduct - although two ovaries are present in the embryonic stage of each female bird. Some species of birds have two functional ovaries, and the order Apterygiformes always retain both ovaries.\"

[smartmummy]

tianzhu:

smartmummy:

I am so sorry to disturb u all.The original qn I posted here.
I confused,for the sponge they said the air escaped when we squeezed,but here they said air compressed when we squeezed.

Hi

Thank you for posting the question.

On the highlighted portion,are you referring to answers given on 2 different questions?

Best wishes

Yup. the next qn I will post later.thanks for ur time.

[Nebbermind]

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:

try to relate this to a vacuum flask which is usually lined with some silvery material for the inner container. This helps to keep the heat in.

[atutor2001]

Nebbermind:

try to relate this to a vacuum flask which is usually lined with some silvery material for the inner container. This helps to keep the heat in.

Hey Nebbermind

This is a very good example. No need all my grandmother story. Thank you

[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