Tutor Niedino: Pri Science Questions and Concepts

[atutor2001]

rkumar:

Thanks Dharma and Haley,

My nephew did his P5 CA2 recently. Some question relating to thermos flask and why they can keep heat longer. He answered it correctly quoting poor conductor of heat. Got one 1 mark out of 2. When my sister asked the teacher. the Teacher told her (very curtly) that the appropriate answer would be to talk about the insulating properties of the flask. My sister has a bone to pick with her since.

being the ignoramus as i was, told her to leave it, as i thought the teacher was right.

Now that i know that its officially discouraged in other schools...... :x. Lets just say the teacher is gonna need all the \"insulation\" she can get her hands on.

Appreciate your timely comments. You people rock!

Hi rkumar

Hate to add to the confusion but the teacher's answer is correct in this case.

The question is not testing on \"heat conductivity of a material\". The topic tested is \"insulating property of vacuum (no material in between)\" i.e. heat cannot be conducted if there is no material (air) in between. Heat cannot pass through a thermo flask because the inner layer and outer layer are separated by a vacuum. Therefore, vacuum can insulate heat. So we need to apply different concept for this question.

Regards

[Dharma]

Slides 15 to 19 may interest some of you.


http://www.yewteepri.moe.edu.sg/uploads/P6%20Science.pdf

[rkumar]

hi atutor,


Thanks for your input.
My mistake for adding thermos to flask. It was not a vacuum(thermos) flask. it was a flask, diagram (cross section). The walls were hollow and filled with air.
The question was why it could retain heat better than the other container (flask) same material no air in between.

In any case whats with the 1 mark. you either give the 2 marks or nothing at all. Another thing that i have to clarify with her.

cheers

[snowman.022851697]

For 2-mark explanation questions, first you explain your answer, then explain why the other options/possibilities are not the answers.

[Tang]

rkumar:

Dharma:

[quote=\"rkumar\"]



Thanks Dharma and Haley,

My nephew did his P5 CA2 recently. Some question relating to thermos flask and why they can keep heat longer. He answered it correctly quoting poor conductor of heat. Got one 1 mark out of 2. When my sister asked the teacher. the Teacher told her (very curtly) that the appropriate answer would be to talk about the insulating properties of the flask. My sister has a bone to pick with her since.

being the ignoramus as i was, told her to leave it, as i thought the teacher was right.

Now that i know that its officially discouraged in other schools...... :x. Lets just say the teacher is gonna need all the \"insulation\" she can get her hands on.

Appreciate your timely comments. You people rock!

[/quote]Hi,

For a 2-mark question, one can't expect to get full mark by just stating that it is a poor conductor of heat.

You need to elaborate further....

In fact 'a poor conductor of heat' is not a complete answer, comparison is required since question stated 'keep heat longer'.

It slows down the heat loss better from the hot water to the surroundings.

Hope it is useful and timely for the PSLE 2010.

[yongjianrong]

Just found some confusing concepts.


1. When something is at a greater height, it has more gravitational potential energy but the gravity pull is weaker, why is that so?

2. Do we consider 'air resistance' as 'friction'?

3. a) When duckweeds cover the surface of pond completely, do they literally mean that there is no gap between each duckweed?

b)If so, will fishes suffocate? I have one revision paper which says the fish will suffocate. However, when duckweeds photosynthesise, they will produce oxygen and the oxygen will also exit through the stomata on the underside of the leaves followed by entering the pond. So how can the fishes suffocate?

[Tang]

yongjianrong:

Just found some confusing concepts.


1. When something is at a greater height, it has more gravitational potential energy but the gravity pull is weaker, why is that so?
No, gravity pull is constant wherever you are as it depends on the mass of Earth which is a constant.

2. Do we consider 'air resistance' as 'friction'?
Yes. The object is in contact with air which slows it down.

3. a) When duckweeds cover the surface of pond completely, do they literally mean that there is no gap between each duckweed?
Don't quite understand

b)If so, will fishes suffocate? I have one revision paper which says the fish will suffocate. However, when duckweeds photosynthesise, they will produce oxygen and the oxygen will also exit through the stomata on the underside of the leaves followed by entering the pond. So how can the fishes suffocate?
Duckweeds are floating plants, so oxygen released during photosynthesis does not necessarily be dissolved into the water.

Hi,

See my comments in bold.

[yongjianrong]

Tang:

yongjianrong:

Just found some confusing concepts.


3. a) When duckweeds cover the surface of pond completely, do they literally mean that there is no gap between each duckweed?
Don't quite understand

Hi,
See my comments in bold.

What I meant was:

When a question says 'Duckweeds cover the surface of the pond completely', does it want pupils to think that the pond is covered completely without the SLIGHTEST gap, or are there gaps for oxygen (from above water) and Sunlight (from Sun) to enter the pond?

[atutor2001]

1. When something is at a greater height, it has more gravitational potential energy but the gravity pull is weaker, why is that so?


Energy is NOT force.
Energy is the \"ability to do work\"
Force is \"something\" that can change the state of motion, shape, position of an object.

Energy = Force x Distance travelled by the object (lets not talk about heat energy)

The gravitational potential energy of an object is the amount of energy \"stored in the object due to the existence of gravitational force\". The amount of such energy that an object has depends only on its distance from the centre of the Earth (and its mass).

The gravitation force acting on an object becomes smaller when it is further away from Earth. This means that the Kinetic Energy that an object gained (which is converted from PE) when it falls through a distance of 1km at a height of 1000 km from the earth will be less than the kinetic energy gained by an object that falls through also a distance of 1m, but at a height of only 100km from the Earth. However, the total PE of the object at 1000km is definitely more than when it is at 100km above the Earth as the distance travelled by the object in the 1st case is 1000km while in the 2nd case it is only 100km.


3. a) When duckweeds cover the surface of pond completely, do they literally mean that there is no gap between each duckweed?

Realistically should not be, there will sure to be gaps.

b)If so, will fishes suffocate? I have one revision paper which says the fish will suffocate. However, when duckweeds photosynthesise, they will produce oxygen and the oxygen will also exit through the stomata on the underside of the leaves followed by entering the pond. So how can the fishes suffocate?

For floating plants like duckweeds, the stomata are found on the upperside of the leaf, not the underside as in normal leafs. Therefore, the oxygen released will not enter the water.

The fish suffocate because the RATE of introduction of dissolved oxygen into the water becomes much slower than the RATE of consumption of dissolved oxygen (which actually did not change).

Dissolved oxygen comes from oxygen entering through the water surface - which is greatly reduced because the surface is covered by duckweed

Dissolved oxygen also comes from oxygen released by submerged plants - which is also greatly reduced because most of the sunlight is blocked (i.e. reduced photosysnthesis)

However, the amount of oxygen consumed by the living organism in the pond remains the same and soon all the dissolved oxygen in the water will be depleted because OUT is faster than IN.

[atutor2001]

deleted sorry double posting