Q&A - PSLE Science

[Verysuperkiasu]

peichun prelim 2011 section b q39b


Mr Leong wanted to use a metal barbecue grill P or Q, that would result in the food being less burnt.

Picture of P shows some thinner metal strips with bigger gaps between the strips.
Picture of Q shows some thicker metal strips with narrower gaps between them.

Based on the diagram, which grill should he use. Give a reason.

Answer key is Q. thicker strips = large surface area in contact = conduct heat away faster to the air so food won't get burnt. My question ; but won't the bigger surface area of the strips also conduct heat faster to the food as well which will burn the food?

If choose P, also not right coz the bigger gaps means fire in direct contact with the food which will get burnt.

Pls enlighten me....

[atutor2001]

Tang:

atutor2001:

[quote=\"P5G\"]Please help.


http://i56.tinypic.com/16jj4mg.gif\">


TIA.

I will pick (2) because its twisted look appears to be more suitable for clinging to tree trunks.

Hi,

I have done an online search for pictures of woodpecker, the photophoto shows that the correct answer should be (4).[/quote]Hi Tang
The key word in the question is \"opposable claws\" which I take to mean that one claw can be moved to touch one or more of the remaining claws. Therefore, (2) appears to match the description.

[atutor2001]

Verysuperkiasu:

peichun prelim 2011 section b q39b


Mr Leong wanted to use a metal barbecue grill P or Q, that would result in the food being less burnt.

Picture of P shows some thinner metal strips with bigger gaps between the strips.
Picture of Q shows some thicker metal strips with narrower gaps between them.

Based on the diagram, which grill should he use. Give a reason.

Answer key is Q. thicker strips = large surface area in contact = conduct heat away faster to the air so food won't get burnt. My question ; but won't the bigger surface area of the strips also conduct heat faster to the food as well which will burn the food?

If choose P, also not right coz the bigger gaps means fire in direct contact with the food which will get burnt.

Pls enlighten me....

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.

[chrisu]

Well I'm totally clueless about such questions from Preparing for PSLE Science by EPH. Anyone can explain why the answer?


Q6 : Book answer is (3)
http://img.photobucket.com/albums/v160/guppiesonli/Misc/Q6.jpg\">

7 : Book answer is (4)
http://img.photobucket.com/albums/v160/guppiesonli/Misc/Q7.jpg\">

Thank you.

[atutor2001]

I think the first thing to do is to find the relationship between A&B; A&vegetation and B&Vegetation.


From graph of A & B, it is quite ok to see that A eats B because when A goes up, B comes down and vice versa.

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. This trend changes when A increase rapidly which causes B to decrease which then leads to the vegetation increasing as there are now less B eating them.

The graph of A and vegetation does not show any relationship.

With that we can confirm the food chain to be
\"Vegetation -> B -> A\"

The graph shows a sudden increase in A due to unknown reason (maybe migration from other places). Therefore, to prevent extinction of B (i.e. need to increase B), technically, we can either reduce A or increase vegetation. However, the graph also shows that vegetation is already increasing which means the death rate of B (killed by A) is way much faster than birth rate and the option of increase vegetation is not feasible.

For Q7, in the wild life reserve, the food chain is usually
plant -> herbivore (e.g. deer) -> carnivore (lion)

Hope my method is ok

[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.

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.

[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.

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.

[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.