Q&A - PSLE Science

[kelly1]

Question about Gravitational Potential Energy:

1) Object on ground, near a well
2) Object in the well.
Do they both have no GPE, or does the object on the ground still have some GPE (just because the object shown relative to it is actually below ground level, in the well)?

What about objects lying on the floor, but say in a high-rise apartment?
TIA

[atutor2001]

kelly1:

Question about Gravitational Potential Energy:

1) Object on ground, near a well
2) Object in the well.
Do they both have no GPE, or does the object on the ground still have some GPE (just because the object shown relative to it is actually below ground level, in the well)?

What about objects lying on the floor, but say in a high-rise apartment?
TIA

You have brought up a very good question which highlights the inadequacy of the way science is thought in primary school.

By definition, an object will possess GPE if it has the potential to do work (i.e. to move to the centre of the Earth) due to gravity.

Therefore, as long as an object is not at the centre of the Earth, it possesses GPE.

Unfortunately, primary students are told that an object at the bottom of a slope has zero GPE. The correct term should be \"it has minimum GPE\" as compared to positions above the slope.

In higher study, we usually need to set a \"reference level\" for purpose of calculation. This level is set as \"zero GPE\". However, I believe many students didn't realise that the absolute/actual GPE at the that level is not zero. What they are usually calculating is the \"change in GPE\" and not the actual GPE.

When I was faced with this problem with my kids, I tried my best to explain that for simplicity, we set a certain level for GPE to be zero. That level is up to us. However, the actual GPE there is not zero. I can only pray that they get to understand it later in life.

[emerald]

When a metal ring expands, it only expands outwards. Does it not expand inwards too? Why?


How much carbon dioxide is produced during combustion relative to the amount of oxygen used?

Please enlighten.

[atutor2001]

emerald:

When a metal ring expands, it only expands outwards. Does it not expand inwards too? Why?


How much carbon dioxide is produced during combustion relative to the amount of oxygen used?

Please enlighten.

Question 1 is tough :x let me try :

Imagine the ring is cut and straightened into a metal strip. The length is much longer then its thickness. When heated, the amount of expansion is the same in all directions for each \"unit length\"

The amount of expansion \"lengthwise\" is much more than the expansion in the direction of the \"width/breadth/thickness...\" because the length is so much longer (i.e. more unit lengths) than the width,breadth...

When this metal strip is rolled into a ring, the only way to allow it to expand \"lengthwise\" is to increase its circumference which means that the diameter must be increased. Therefore, the most obivous change is the increase in the diameter of the ring.

As to your question on whether the ring expand \"inwards\", I would like to re-phrase it as \"Does the thickness of the ring increase?\"

My answer is yes. If we measure the thickness of the ring before and after heating, the thickness should also increase.

Question 2 is \"Chemistry\"

1 molecule of oxygen (O2) + 1 carbon = 1 carbon dioxide (CO2)

So the ratio of oxygen used to carbon dioxide produced should be 1 : 1

[emerald]

Thanks to atutor2001 but 1 more question pertaining to ans 2. Imagine an inverted beaker in a basin of water. A candle is burning inside the beaker. After the candle had finished burning, some water from the basin entered the beaker.


My girl’s teacher said that water entered the beaker to take up the space of the oxygen that was used up by the burning candle. He also said that little carbon dioxide was produced when the candle was burning.

Could you please explain? Thanks.

[Full.010894Cream]

emerald:

Thanks to atutor2001 but 1 more question pertaining to ans 2. Imagine an inverted beaker in a basin of water. A candle is burning inside the beaker. After the candle had finished burning, some water from the basin entered the beaker.


My girl's teacher said that water entered the beaker to take up the space of the oxygen that was used up by the burning candle. He also said that little carbon dioxide was produced when the candle was burning.

Could you please explain? Thanks.

perhaps carbon dioxide is more soluble than oxygen in water

[atutor2001]

emerald:

My girl's teacher said that water entered the beaker to take up the space of the oxygen that was used up by the burning candle. He also said that little carbon dioxide was produced when the candle was burning.

This another fine example of the inadequacy in our teaching.

In Sec 3, students will be thought that 1 mole of O2 combines with 1 mole of C to give 1 mole of CO2.

Therefore, it is not true that the number of molecules of CO2 is less than the number of molecules of O2, causing a decrease in the volume that allows water to enter the beaker.

However, the size of 1 molecule of O2 is not the same as 1 molecule of CO2.

The size of one O2 molecule is slightly bigger than that of one molecule of CO2. (one O2 molecule is about 3.5 angstroms while one CO2 molecule is about 3.3 angstoms - 1 angstroms = 1/10,000,000,000 metre)

Therefore, when O2 is converted to CO2, the volume of the gas decreases allowing the water to enter the beaker.

I was equally troubled by the explanation given by my kids teacher. Anyway, I remembered my teacher telling me the same thing and it just goes on being passed down till present day. What to do? This is how science is thought here.

[atutor2001]

Full.Cream:

emerald:

Thanks to atutor2001 but 1 more question pertaining to ans 2. Imagine an inverted beaker in a basin of water. A candle is burning inside the beaker. After the candle had finished burning, some water from the basin entered the beaker.


My girl's teacher said that water entered the beaker to take up the space of the oxygen that was used up by the burning candle. He also said that little carbon dioxide was produced when the candle was burning.

Could you please explain? Thanks.

perhaps carbon dioxide is more soluble than oxygen in water

Great point, in fact this is the main contributing factor for the water to enter because the change in molecular size is negligible.

[Full.010894Cream]

atutor2001:

Great point, in fact this is the main contributing factor for the water to enter because the change in molecular size is negligible.

think both are probably outside PSLE scope.

I was just thinking that since the candle is really a hydrcarbon and not carbon, then both water and CO2 (and a whole lot of other oxides) will be formed. Therefore the CO2 formed is definitely lesser than the O2 used. But think this is also beyond PSLE.

[atutor2001]

Full.Cream:

atutor2001:

Great point, in fact this is the main contributing factor for the water to enter because the change in molecular size is negligible.

think both are probably outside PSLE scope.

I was just thinking that since the candle is really a hydrcarbon and not carbon, then both water and CO2 (and a whole lot of other oxides) will be formed. Therefore the CO2 formed is definitely lesser than the O2 used. But think this is also beyond PSLE.

Thanks, have not thought of that. Fully agree with you. Furthermore most of the gaseous oxides are more soluble. Wish you are from MOE