Tutor Niedino: Pri Science Questions and Concepts

[Niedino]

kancheongmum:

Hi Niedino


Need confirmation on the following

A metal cup and a plastic cup both contain hot water of the same temperature. After 15 mins the water in which cup will have a higher temperature. Answer: water in the plastic cup Reason: metal is a better conductor of heat than plastic, so heat is conducted away faster.

What if the water is changed into cold water?
After 15 mins the water in which cup will have a lower temperature.
Answer: water in the plastic cup. Reason: metal is a better conductor of heat so it gain heat faster from the surrounding.

Thank you

Hi KCmum,

There's been quite a few replies to it. The answer you provided already makes sense, and the phrasing is quite good except for one problem, the reason is not in line with the answer. Another problem I foresee is that your child does not understand the true meaning of conductivity.

Perhaps you can run the experiment with him at home with a simple thermometer?

My answer to the question would be

a) the metal cup, as metal is a better conductor of heat than plastic, so the heat loss from the water is faster than the the plastic cup.

b) the plastic cup, plastic is a poorer conductor of heat, so less heat is lost to the surroundings as compared to the metal cup.

Always answer to the question and in this case, both questions deal with heat loss / gain. I would always recommend that the reason be given in line with the earlier answer (plastic cup --> plastic cup because) and (plastic cup --> metal is a better) so that your child will not be confused.

[rkumar]

HaleyCopterz:

rkumar:

[quote=\"kancheongmum\"]Hi Niedino


Hi Niedino

Correct me if i am wrong but i think the reason for Q1 \"metal is a better conductor of heat than plastic, so heat is conducted away faster.\"
should be Metal is a poor insulator of heat so heat is lost to the surrounding faster

Hi,

We do not say that a material is an 'insulator of heat' that would be marked wrong. Nothing is an insulator of heat as everything can conduct heat to a certain degree. So we should say that metal is a better conductor of heat than plastic. If a material is a better conductor of heat, that means heat travels through that material very quickly.

[/quote]Hi Haleycopter and Bigdad

insulator of heat is indeed a valid term. Poor conductors of heat are good insulators of heat and vice versa. i believe it is still covered in the \"o\" level syllabus. \"Plastic\" heat properties question came out 2004 'O' level physics exam. Good Insulator/poor conductor both accepted.:D

I think you may have mistaken insulation to mean stop heat altogether.

definition i found on a website

Substances that do not conduct heat well are called insulators. The word insulator comes from the Latin word insula, which means island. Insulators have a low coefficient of conductivity; they do not conduct heat well. Nonmetals, such as wood, textiles, and plastic, are usually poor conductors. Gases are also poor conductors.

[Dharma]

rkumar:

HaleyCopterz:

[quote=\"rkumar\"]
Hi,

We do not say that a material is an 'insulator of heat' that would be marked wrong. Nothing is an insulator of heat as everything can conduct heat to a certain degree. So we should say that metal is a better conductor of heat than plastic. If a material is a better conductor of heat, that means heat travels through that material very quickly.

Hi Haleycopter and Bigdad

insulator of heat is indeed a valid term. Poor conductors of heat are good insulators of heat and vice versa. i believe it is still covered in the \"o\" level syllabus. \"Plastic\" heat properties question came out 2004 'O' level physics exam. Good Insulator/poor conductor both accepted.:D

I think you may have mistaken insulation to mean stop heat altogether.

definition i found on a website

Substances that do not conduct heat well are called insulators. The word insulator comes from the Latin word insula, which means island. Insulators have a low coefficient of conductivity; they do not conduct heat well. Nonmetals, such as wood, textiles, and plastic, are usually poor conductors. Gases are also poor conductors.

[/quote]For PSLE Science, I remember my dds Science teachers’ telling them not to use “insulator”……

Always use good conductor of heat or poor conductor of heat

or

Object A is a better conductor of heat than object B

Or

Object B is poorer conductor of heat than object A

[atutor2001]

There are other terms that must never be used. Another one is never use \"coldness\" - use loses heat. Oh no, my mind is blank. Will post more when I remembered. :oops:


Another one : Never use \"grow\" for non-living things such as metal when heated or shadow - use \"expands\" for metal and \"become\" for shadow.

Don't use \"soft\" if you need the material to bend - use \"flexible\"
Don't use \"flexible\" if you need the material to stretch - use \"elastic\"

[rkumar]

Dharma:

rkumar:

[quote=\"HaleyCopterz\"]

For PSLE Science, I remember my dds Science teachers’ telling them not to use “insulator”……

Always use good conductor of heat or poor conductor of heat

or

Object A is a better conductor of heat than object B

Or

Object B is poorer conductor of heat than object A

[/quote]


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!

[rkumar]

atutor2001:

There are other terms that must never be used. Another one is never use \"coldness\" - use loses heat. Oh no, my mind is blank. Will post more when I remembered. :oops:


Another one : Never use \"grow\" for non-living things such as metal when heated or shadow - use \"expands\" for metal and \"become\" for shadow.

Don't use \"soft\" if you need the material to bend - use \"flexible\"
Don't use \"flexible\" if you need the material to stretch - use \"elastic\"

Good list of words. I can see how the students might make the mistakes. Will be very useful for my son doing P3 now. Thanks

[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.