Q&A - PSLE Science

[Cockroach]

Verysuperkiasu:

Which one of the following is not a source of light?


1. stars
2. firefly
3. burning torch
4. fluorescent light bulb

ans key is 4. My question : I thought all the 4 choices above are sources of light - can emit light. So why is fluorescent light bulb not a source of light?

I think that it is because the light bulb needs to paired with electricity to emit light, hence, i think that the answer is correct? Ha ha no confidence. The burning torch emit light as fire is present for combustion to take place.

[Cockroach]

Can someone explain to me why dry ice will be seen with white smoke when it is in contact with water?

[Verysuperkiasu]

Cockroach:

Verysuperkiasu:

Which one of the following is not a source of light?


1. stars
2. firefly
3. burning torch
4. fluorescent light bulb

ans key is 4. My question : I thought all the 4 choices above are sources of light - can emit light. So why is fluorescent light bulb not a source of light?

I think that it is because the light bulb needs to paired with electricity to emit light, hence, i think that the answer is correct? Ha ha no confidence. The burning torch emit light as fire is present for combustion to take place.

hmm....guess it makes sense. fluorescent bulb - different from the other 3 choices in that they are already lighted up. thanks!

[Cockroach]

Can someone explain to me why dry ice will be seen with white smoke when it is in contact with water?

[atutor2001]

Verysuperkiasu:

Which one of the following is not a source of light?


1. stars
2. firefly
3. burning torch
4. fluorescent light bulb

ans key is 4. My question : I thought all the 4 choices above are sources of light - can emit light. So why is fluorescent light bulb not a source of light?

In science, the definition of light source is an object that can produce light on its own. Unfortunately, the meaning of \"on its own\" is very vague.

A clearer way to identify a light source is to first find out, \"where did the energy that is used to produce the light comes from\". If potential energy from an object is used to produce (convert to) the light then that object is a light source.

For example, a paper with a silver shining surface is not a light source when it is reflecting light from a torch because no energy from the paper is being used to produce the light.

However, if we burn this same paper, it becomes a light source because the chemical potential energy of the paper is now being converted to light.

With the same reasoning, the lighted bulb in the question is not a light source because no energy from the bulb (itself) is being used to produce the light. The light comes from electrical energy which is provided by either batteries or power plant.

[atutor2001]

Cockroach:

Can someone explain to me why dry ice will be seen with white smoke when it is in contact with water?

Carbon dioxide, water vapour (i.e. air), are colourless and cannot be seen.

In Pr science, students are thought \"smoke\" is usually either water droplets or dust particles. Therefore, my deduction is that the white smoke from dry ice must be water droplets.

When the carbon dioxide sublimates, the gaseous molecules are very cold and water vapour will condense on them into tiny water droplets. These water droplets will appear as white smoke.

[Verysuperkiasu]

atutor2001:

In science, the definition of light source is an object that can produce light on its own. Unfortunately, the meaning of \"on its own\" is very vague.

A clearer way to identify a light source is to first find out, \"where did the energy that is used to produce the light comes from\". If potential energy from an object is used to produce (convert to) the light then that object is a light source.

For example, a paper with a silver shining surface is not a light source when it is reflecting light from a torch because no energy from the paper is being used to produce the light.

However, if we burn this same paper, it becomes a light source because the chemical potential energy of the paper is now being converted to light.

With the same reasoning, the lighted bulb in the question is not a light source because no energy from the bulb (itself) is being used to produce the light. The light comes from electrical energy which is provided by either batteries or power plant.

Thanks, atutor for your detailed explanation.

But I wonder

(1) have u come across such a question in section b? where students are asked to explain why they choose 'light bulb' as not a light source,

(2) If there is really this type of question, can they bring the conversion of chemical PE into light engergy as you've explained above? Or just answer simply 'it cannot emit light on its own but needs another source. What are the key words here?

Paiseh, I'm full of questions... :oops:

[atutor2001]

Verysuperkiasu:

But I wonder

(1) have u come across such a question in section b? where students are asked to explain why they choose 'light bulb' as not a light source,

(2) If there is really this type of question, can they bring the conversion of chemical PE into light engergy as you've explained above? Or just answer simply 'it cannot emit light on its own but needs another source. What are the key words here?

Paiseh, I'm full of questions... :oops:

I don't think I have come across such a question because source of light is usually tested in P4 (I think) where energy is not taught yet.

Anyway, my answer to such a question would be :

For P4 :
It is not a light source because it cannot produce (don't use emit) on its own as the light emitted by the bulb is produced by the electric current flowing through the bulb.

For P6 :
It is not a light source because it cannot produce (don't use emit) on its own as the light emitted by the bulb is converted from electrical energy that is supplied to the bulb by batteries or other power sources.

[Alchemist]

Atutor and others - sincere thanks upfront for answering these queries. I have 3 more queries.


1) ACS prelim 2011. The following circuit is in parallel arrangement - answer is No 1. But why? http://i54.tinypic.com/28khahx.jpg\">


2) This is From Chongfu Prelim 2011 - In this question my son initially answered 2 - because air in flask X will expand - and hence the ink will move right. HOwever after reading the given answer - we were wondering if it is due to the word immediately - he then reasoned that for the first instant the flask will expand - that would provide more space for the air in the flask, which would lead the ink to move leftwards. Firstly is the answer (2) - and if it is - is his reasoning correct? http://i56.tinypic.com/330clra.jpg\">


3) This is from Chongfu as well. My son answered (1). Answer key is (2). But he argues that we cannot assume that layer of oil and cover will prevent evaporation at the same rate - there may be differences there. So he will not choose Set-up D as a fair test. http://i52.tinypic.com/foocib.jpg\">

[chrisu]

Yes answer is No.1 Bulb X will not light up.

We assume both batteries are identical thus the charges cancel one another. But if there are 3 identical batteries (arrange like +-+--+ or -+-++-), then the bulb will be lighted up dimly.

In the diagram, the batteries are arranged in -++-, so it cancel each other and bulb will not light up.

Hope it's clear.

vidhyalog:

Atutor and others - sincere thanks upfront for answering these queries. I have 3 more queries.

1) ACS prelim 2011. The following circuit is in parallel arrangement - answer is No 1. But why? http://i54.tinypic.com/28khahx.jpg\">