Q&A - PSLE Science

[chrisu]

Thanks for your explanation.

atutor2001:

Sorry forgot to explain this part.
Under normal situation, the increase in population size of an organism is usually triggered by increase in food supply and vice versa. Therefore, the graph of the food source should always change direction BEFORE the graph of the consumer. i.e. there must be more food first which will lead to more B being produced as it takes time to reproduce and vice versa. Increase in vegetation can be due weather.

If both the vegetation and B are increasing at the same rate or vegetation is increasing at a faster rate than B, then both organisms will keep increasing.

However, if the increase in B is faster than increase in vegetation, it will reach a point where vegetation will start to decrease (graph of food source will change direction first) and then B will also start to decrease (graph of consumer will change direction later)

chrisu:

Thank you atutor2001 for the time and effort, really appreciate it.

Just a little more explanation on this;
Regarding the graph of B and vegetation. If vegetation increases, B aslo increases (and vice versa) - why B eats vegetation then? If B eats vegetation, then shoudn't be B increases, vegetatin decreses and vice versa? Thanks.

[quote=\"atutor2001\"]From the graph of B and vegetation, the relationship was quite ok at first, showing that when vegetation increases, B also increases (and vice versa). Therefore, B eats the vegetation.

[/quote]

[Verysuperkiasu]

atutor2001:

I will pick the thicker strips because large surface area in contact = faster cooking AND also more even distribution of heat = prevent localised burning of food.

I never thought of more even distribution of heat which does make sense..

Still on the same topic on 'heat' and its conductivity,

1) Tom has a metal sieve with holes. He put some scraps of paper on it and place a flame below the sieve. Then he drill the holes bigger on the same sieve and repeat the experiment. Which set of scraps of paper will be more brown?

2) Mr Leong uses two metal sieves L and M (same no. of holes). He wants to find out which of the two sieves is a better conductor of heat. He place same scraps of paper on top of the 2 sieves. He observes that the paper inside sieve M looked more burnt than those inside L.

(i) Which sieve is made of a material that is a better conductor of heat.

(ii) Explain your answer in (i).

It appears that M is a better conductor of heat because paper inside it turns browner means more heat get conducted through M. but answer is L. Why? same surface area in contact because same no of holes.

[atutor2001]

Verysuperkiasu:

1) Tom has a metal sieve with holes. He put some scraps of paper on it and place a flame below the sieve. Then he drill the holes bigger on the same sieve and repeat the experiment. Which set of scraps of paper will be more brown?

2) Mr Leong uses two metal sieves L and M (same no. of holes). He wants to find out which of the two sieves is a better conductor of heat. He place same scraps of paper on top of the 2 sieves. He observes that the paper inside sieve M looked more burnt than those inside L.

(i) Which sieve is made of a material that is a better conductor of heat.

(ii) Explain your answer in (i).

It appears that M is a better conductor of heat because paper inside it turns browner means more heat get conducted through M. but answer is L. Why? same surface area in contact because same no of holes.

These 2 questions are very similar question to the burning of paper cup question. Understanding heat transfer alone is not enough. We also need to know :
1. Burning and flash point.
2. Temperature distribution.

My reasoning is as follows (sorry very lengthy) :

1. A material will burn only when its TEMPERATURE reaches a level we call \"flash point\". As long as the temperature on the sieve is less than the flash point of the paper, it will not turn brown. Flash point of paper is 232 degree C.

2. The temperature distribution of a metal sieve should roughly be highest at the centre where the flame is directly below and drops radially outwards. The temperature of a candle flame can be as high as 1400 degree C. However, the part of the metal sieve placed directly above the flame will be much lower because being a good conductor of heat, the metal sieve can transfer the heat away quickly from the source.

3. Therefore, a good conductor will have a lower temperature at the point where the flame source is. With lower temperature, the paper above it will not get burnt so easily.

4. A mesh with smaller holes will also have a lower temperature at the point where the flame source is because it has more metal surface and will be able to transfer heat away more quickly than one with big holes.

5. Therefore, if the same flame is used, the peak temperature of a good conductor will be lower compared to one that is a lousier conductor of heat. The temperature of a metal sieve with more metal surface will also have a lower temperature. They will cause less of the paper to get burnt.

Actually this is out of the syllabus but what to do.

[Verysuperkiasu]

atutor2001:

2. The temperature distribution of a metal sieve should roughly be highest at the centre where the flame is directly below and drops radially outwards. The temperature of a candle flame can be as high as 1400 degree C. However, the part of the metal sieve placed directly above the flame will be much lower because being a good conductor of heat, the metal sieve can transfer the heat away quickly from the source.

3. Therefore, a good conductor will have a lower temperature at the point where the flame source is. With lower temperature, the paper above it will not get burnt so easily.

4. A mesh with smaller holes will also have a lower temperature at the point where the flame source is because it has more metal surface and will be able to transfer heat away more quickly than one with big holes.

5. Therefore, if the same flame is used, the peak temperature of a good conductor will be lower compared to one that is a lousier conductor of heat. The temperature of a metal sieve with more metal surface will also have a lower temperature. They will cause less of the paper to get burnt.

Actually this is out of the syllabus but what to do.

Thanks, atutor! So, the sieve with bigger holes, due to smaller surface area in contact with the heat source, paper turns more brown coz heat conducted away slower.

For comparing the 2 sieves L & M, the one with the browner paper is a poorer conductor coz heat conducted away slower.

And is this really out of the syllabus?

[Verysuperkiasu]

breanne:

On the topic of conductor of heat, I have a question on Pei Chun's question 39b. Sorry could not post an image. The question asked which of 2 grills should be used so that the food is less burnt. The answer given was the grill with the thicker metal strips. If more heat is conducted away from the strips wouldn't it mean that food gets mire heat so wouldn't the food be more burnt ?

Hi Breanne, this question has been raised on page 155/156 and kindly addressed by atutor.

Basically, thicker strips = bigger surface area= more even distribution of heat throughout the food. I also think that bigger strips also mean thinner gaps which reduces direct contact between food and flame hence reduces overburning or what atutor says 'localised burning'.

[atutor2001]

Verysuperkiasu:

Thanks, atutor! So, the sieve with bigger holes, due to smaller surface area in contact with the heat source, paper turns more brown coz heat conducted away slower.

For comparing the 2 sieves L & M, the one with the browner paper is a poorer conductor coz heat conducted away slower.

And is this really out of the syllabus?

I feel that it is out of syllabus because students are taught that good conductor will gain heat faster than poorer conductor. The syllabus did not cover heat (temperature) distribution (i.e. localised heating)

To be able to answer the question, a student needs to know that although the total heat in a good conductor is more than a poorer conductor, the poorer conductor has poorer distribution of heat which results in higher \"peak temperature\" than good conductor.

On top of that, a student must also know that a material will only starts to burn when the temperature reaches the \"flash point\". Therefore, burning is dependent on localised heat accumulation (resulting in temperature reaching the flash point) and not the overall total amount of heat in the metal sieve.

Just for the discussion - imo, the question on boiling water in paper cup is also not properly covered in the syllabus. Students are only taught that water is a poor conductor of heat. However, water can actually transfer heat away very quickly through another way called convection - which I think is not in the syllabus. Students were only taught that warm air/water rises but there is no emphasis on the effect of convection current in heat transfer. Therefore, it create a misconception that heat travels through water very slowly (true if we only consider heat conduction) but in reality, heat can travel through water very quickly through convection current.

Well there are many more such issues and I guess we just have to accept it as setters of exam questions do not like students to get 100 marks.

[Verysuperkiasu]

atutor2001:

Well there are many more such issues and I guess we just have to accept it as setters of exam questions do not like students to get 100 marks.

My sentiments exactly. Even with the other subjects like Math also, they keep coming up with 'new' types of questions in PSLE that the children have not been taught the method of solving or come across.

My son was told that he should do assessment or past year questions (not sure if he was just referring to science alone or other subjects) only for the sake of practising how to answer and not hoping the same question will appear because it will never happen.

[hildangirl]

I posted a new topic accidentally.


Does anyone have the answer key to Booklet A for Rosyth Prelims Science 2011? Would really appreciate if you can share. Thanks

[atutor2001]

Verysuperkiasu:

My sentiments exactly. Even with the other subjects like Math also, they keep coming up with 'new' types of questions in PSLE that the children have not been taught the method of solving or come across.

My son was told that he should do assessment or past year questions (not sure if he was just referring to science alone or other subjects) only for the sake of practising how to answer and not hoping the same question will appear because it will never happen.

I really feel bad for the kids. What used to be a 5-mark question for math in the 1990s (my kids' time) is now worth only 3 marks. I really can't see how kids can survive math with just model skill. Skill in simple algebraic manipulation and solving simultaneous equation are essential but most schools are not teaching.

Imo past year questions are better than assessment books. They are more up to date with the latest craze and also less errors.

All the best to your son.

Cheers

[Verysuperkiasu]

atutor2001:

I really feel bad for the kids. What used to be a 5-mark question for math in the 1990s (my kids' time) is now worth only 3 marks. I really can't see how kids can survive math with just model skill. Skill in simple algebraic manipulation and solving simultaneous equation are essential but most schools are not teaching.

Imo past year questions are better than assessment books. They are more up to date with the latest craze and also less errors.

All the best to your son.

Cheers

Thanks for the advice & well wishes, atutor!