Q&A - PSLE Science (Page 2)

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PostPosted: Mon Jan 05, 2009 9:16 am 
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tianzhu wrote:
Electricity

Q1) Joe decided to make a bulb light up brighter. What should he do?

Q2) Using the same number of batteries, does arranging two bulbs in parallel cause the bulbs to be brighter as compared to arranging the two bulbs in series?

Q3) For bulbs arranged in parallel, does it require more batteries for them to shine as brightly as compared to bulbs arranged in series?

Q4) Do batteries arranged in parallel have a longer life span as compared to those arranged in series?


1. He can either use more batteries arranged in series, or use a higher voltage battery, however, he has to be careful not to exceed the maximum voltage, else the bulb will fuse.

2. Yes. Bulbs arrange in parallel will shine brighter than bulbs arranged in series, with the same no. and battery arrangement. The current flowing through bulbs in series are lower due to higher resistance than the current flowing through bulbs in parallel.

3. No. same explanation as 2.


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PostPosted: Mon Jan 05, 2009 12:04 pm 
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P6 Science

In the experiment set up, there are two air tight containers. A contains a piece of bread mixed with a few drops of water. B has a piece of bread. After three days, it was given that the bread at the container A was mouldy and the bread at container B was less mouldy.

Q1) The containers are airtight. How does this make the experiment a fair test?

Q2) A third piece of bread was heated in an oven. What do you expect to see when this bread was put in another airtight container after three days?


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PostPosted: Mon Jan 05, 2009 1:42 pm 
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tianzhu wrote:
P6 Science

In the experiment set up, there are two air tight containers. A contains a piece of bread mixed with a few drops of water. B has a piece of bread. After three days, it was given that the bread at the container A was mouldy and the bread at container B was less mouldy.

Q1) The containers are airtight. How does this make the experiment a fair test?

Q2) A third piece of bread was heated in an oven. What do you expect to see when this bread was put in another airtight container after three days?


1. When the containers are airtight, no water vapour will be able to enter or leave the container. So the result of the experiment is only due to the test variable, ie. the few drops of water in A. The other factors / variables remain same or constant. This makes the experiment a fair test.

2. Moisture / water is removed when the bread is heated. Since bread turns mouldy due to the presence of water, the 3rd piece of bread will not turn mouldy after 3 days in an airtight container.


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PostPosted: Mon Jan 05, 2009 6:06 pm 
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Hi lizawa

Thanks for sharing.

If the bread is dried, mould will not develop over time. However, the question did not state if the bread was hot or cold when placed in the container. Should bread be enclosed in a container straight from the oven, water droplets would appear, causing mould to develop?

The question then will be whether the condensed water droplets are sufficient to turn the bread mouldy or whether the high temperature of the oven will kill the fungi and therefore no bread mould will be formed even if there is some water present.

This is why cookies made by loving mothers/wives and not forgetting some fathers during Chinese New Year are left to cool before packaging. Interesting indeed.


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PostPosted: Mon Jan 05, 2009 6:08 pm 
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Rate of evaporation

One of the factors affecting the rate of evaporation is temperature.

When we are talking about temperature, does it refer to temperature of the source or temperature of the surroundings? For example a cup of hot water as compared to a cup of warm water in airconditioned/non airconditioned room, clothes left to dry under shade or out in the sun.
Please share your view.


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PostPosted: Mon Jan 05, 2009 6:36 pm 
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lizawa wrote:
...
2. Yes. Bulbs arrange in parallel will shine brighter than bulbs arranged in series, with the same no. and battery arrangement. The current flowing through bulbs in series are lower due to higher resistance than the current flowing through bulbs in parallel.

3. No. same explanation as 2.


Oops. I didn't read question 2 properly. Looks like my son's carelessness is hereditary :) . Thought it was batteries in parallel rather than bulbs. Your answer for 2 is absolutely right.

But I stand by my answer for Q3. Batteries in parallel will last longer, assuming that the batteries are all of the same capacity.


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PostPosted: Tue Jan 06, 2009 7:56 am 
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BrownBelt
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tianzhu wrote:
Hi lizawa

Thanks for sharing.

If the bread is dried, mould will not develop over time. However, the question did not state if the bread was hot or cold when placed in the container. Should bread be enclosed in a container straight from the oven, water droplets would appear, causing mould to develop?

The question then will be whether the condensed water droplets are sufficient to turn the bread mouldy or whether the high temperature of the oven will kill the fungi and therefore no bread mould will be formed even if there is some water present.

This is why cookies made by loving mothers/wives and not forgetting some fathers during Chinese New Year are left to cool before packaging. Interesting indeed.


you are right. if it is cooled down then left in air tight container, mould will not develop. but if it is put in when hot, then, maybe mould will be formed. so how to answer this question ? maybe the child needs to put in the qualifier first. I did notice that sometimes even for a 2 mark question, the expectation is quite a lot to get the full 2 marks.
Y


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PostPosted: Tue Jan 06, 2009 9:23 am 
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lizawa wrote:
I did notice that sometimes even for a 2 mark question, the expectation is quite a lot to get the full 2 marks.


It’s difficult to know what went through the mind of the question setter. There are certainly some alternative answers if you look at this question through different angles.

Perhaps, we should state some assumptions before going on to answer the question. Yes, it’s tough getting those marks. Putting in the right details is indeed challenging.


Last edited by tianzhu on Tue Jan 06, 2009 9:35 am, edited 1 time in total.

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PostPosted: Tue Jan 06, 2009 9:25 am 
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Rate of Evaporation

Q1) Why do you feel cool when you stand next to a fan?

Q2) Many people are familiar with the daily routine of washing, drying, and styling their hair. Have you ever wondered what are the factors affecting the speed of drying up your hair?


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PostPosted: Tue Jan 06, 2009 6:57 pm 
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tianzhu wrote:
Rate of Evaporation

Q1) Why do you feel cool when you stand next to a fan?

Q2) Many people are familiar with the daily routine of washing, drying, and styling their hair. Have you ever wondered what are the factors affecting the speed of drying up your hair?


A1) Fan produces air movement which accelerates process of moisture evaporating from human skin, which in turn lowers the temperature as heat is absorbed to transform water to vapor.

A2) Factors
- Wind
- Heat
- Surface area


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