Biology; Chemistry; Physics: Post your doubts here!

[AbbbbY]

In the coming oct/november 2013 session... btw are u like gud at other subjects as well like eco?

Fire away for Econ! Just, in the Econ thread.

 

[Neil Shah]

Aly Emran usama321 Hey!!! Could u guys temme how to answer question 11 in this paper? http://papers.xtremepapers.com/CIE/Cambridge IGCSE/Physics (0625)/0625_s12_qp_33.pdf
I really need ur help with my doubts coz i have physics on 26th! (this friday)

 

[Nabeel.Ahmad]

Does anybody know the special criteria they have to give outstanding achievers award in biology?
For instance specific ways to answer the questions?

We have to use key words mentioned in the marking scheme and write accurate to-the-point answers.

 

[Yazka Shathir]

We have to use key words mentioned in the marking scheme and write accurate to-the-point answers.

how about the graphs. does it need to be curves? or ruled lines?

 

[Nabeel.Ahmad]

how about the graphs. does it need to be curves? or ruled lines?

I don't know about that.

 

[Saad Mughal]

how about the graphs. does it need to be curves? or ruled lines?

Best fit curves unless otherwise stated. This needs a LOT of practice.

 

[Saad Mughal]

today my teacher told us that we dont need to draw best fit curves in biology , that lines would do , but i really am not sure.

I didn't take Biology so don't know about that, but, in Physics and Chemistry, we ALWAYS needed to draw best fit curves unless otherwise stated.

Check out the mark schemes, it's stated in the mark schemes that whether the graph should be best fit or not.

Go through it and you'll find out how to gain full marks in graph questions (e.g. equally distributed points, even scaling, 1/2 of graph paper used, etc.)

 

[Neil Shah]

Although this is IGCSE and i did GCE but still i'll try to help
Q11.a) what this question gives idea to me that there is background radiation present which does not decreases the value below a certain limit! the background radiation might be caused by an other radioactive source present or the GM tube might be contaminated which causes the increase in readings! The sample might have been almost decayed NOT FULLY decayed!!
b) 52/2 = 26: 1st half life
15-13=2min,
15/26= 0.6,
=0.6+1
half life=1.6 (idk if this is correct ask someone)!!
c) although alpha would stop in air due to less penetrating power but gamma has very high pentrating power and it can reach the person and penetrate his body !

well the marking scheme does say that ur ans.s r correct... but could u EXPLAIN me how u got the and to b as i'm still a little confused

 

[Neil Shah]

for b) think like 1st half life =52/2=26, 2nd half life=26/2=13, now the bottom line is on 15 so the product must have decayed before the second half life!! so the product decayed 2mins before so 15/26=0.6 half life now add both of the half lives!!
a) so see the graph, its going down and then it becomes constant. now you have studied that the graph keeps on going down when the product is decayed and the constant graph show there is another radioactive source present near somewhere which makes the reading constant!!
b) definately u have to learn that alpha has less pentrating power while gamma have high penetrating power. alpha can be stopped by thin paper and few centimetres in air which gamma can reach the person and move deep into the body of the person. this would kill the person's body cells and he might have diseases like skin cancer, sores etc Hope u understand it cox i made it extemely easy

ohhhhh and what abt question 6 in this paper http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s07_qp_2.pdf (sorry got physics tmrw :'( and i REALLY need ur help!,
P.S--> sorry if I'm troubling u ...

 

[usama321]

ohhhhh and what abt question 6 in this paper http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s07_qp_2.pdf (sorry got physics tmrw :'( and i REALLY need ur help!,
P.S--> sorry if I'm troubling u ...

this is included in the syllabus, i suggest you read your syllabus, so at least you won't be surprised by such questions in the paper

i) http://upload.wikimedia.org/wikiped...p_rule.svg/220px-Right-hand_grip_rule.svg.png

use right hand rule here. Actually the picture in the question is a bit unclear. There is a coil wound around the two lines shown in the pic, so that the current goes into the paper from above as the coil winds around the line, and then as it comes around the bottom, it once again tells us the direction of the current in the wound coil. using the right hand rule, we can see that the north is to the right of the coil. The rest is pretty self explanatory

part b i is quite easy

b ii) We know when direct current is passed through a coil, there is magnetic field produced, with north and south poles formed accordingly to the direction of the current. The same principle applies here, except the current is alternating, meaning its direction constantly alternates, therefore causing the magnetic poles to alternate at high speeds at the ends of the coils. Due to the alternating current, the magnetic poles produced at the ends of the coil constantly change.

This results in a constant attraction and repulsion from the permanent magnet that is present nearby. As a result, vibrations are produced, that is sound waves are produced

 

[Neil Shah]

this is included in the syllabus, i suggest you read your syllabus, so at least you won't be surprised by such questions in the paper

i) http://upload.wikimedia.org/wikiped...p_rule.svg/220px-Right-hand_grip_rule.svg.png

use right hand rule here. Actually the picture in the question is a bit unclear. There is a coil wound around the two lines shown in the pic, so that the current goes into the paper from above as the coil winds around the line, and then as it comes around the bottom, it once again tells us the direction of the current in the wound coil. using the right hand rule, we can see that the north is to the right of the coil. The rest is pretty self explanatory

part b i is quite easy

b ii) We know when direct current is passed through a coil, there is magnetic field produced, with north and south poles formed accordingly to the direction of the current. The same principle applies here, except the current is alternating, meaning its direction constantly alternates, therefore causing the magnetic poles to alternate at high speeds at the ends of the coils. Due to the alternating current, the magnetic poles produced at the ends of the coil constantly change.

This results in a constant attraction and repulsion from the permanent magnet that is present nearby. As a result, vibrations are produced, that is sound waves are produced

usama321 thanks! And this is my FINAL doubt from an igcse paper...http://papers.xtremepapers.com/CIE/Cambridge IGCSE/Physics (0625)/0625_w10_qp_33.pdf Q-2

 

[Yazka Shathir]

Hey.
http://olevel.sourceforge.net/papers/5090/5090_s13_qp_22.pdf
I need help with Section B 6 (a)
And Section A Question 3 part b. which process do we have to refer, asexual reproduction (cutting grafting ) genetic engineering, artificial selection? And also, part c? Please..

 

[jhaime]

Hi,
Please enlighten me on the question no. 1. (d) and 4. (b) ii , (b) iii. of this paper http://olevel.sourceforge.net/papers/5054/5054_s13_qp_22.pdf
Thank you.

 

[Aly Emran]

Hi,
Please enlighten me on the question no. 1. (d) and 4. (b) ii , (b) iii. of this paper http://olevel.sourceforge.net/papers/5054/5054_s13_qp_22.pdf
Thank you.

1.(d) for this the backward force would increase and become equal to forward force!
4.(b)ii) Microwaves (its a fact u have to learn it)
4.(b)ii) television satellite sends microwaves to the satellite above earth and microwaves move though vaccum above the and through air to reach it. Then satellite send signals back to earth which antenna's receives

 

[jhaime]

1.(d) for this the backward force would increase and become equal to forward force!
4.(b)ii) Microwaves (its a fact u have to learn it)
4.(b)ii) television satellite sends microwaves to the satellite above earth and microwaves move though vaccum above the and through air to reach it. Then satellite send signals back to earth which antenna's receives

How about the answer for 4 (b) iii? Suggest one advantage of using satellite to send television signals.
My answer is it can transmit television program from one country to another at the same time. Do you think this will gain 1 mark?

Thank you.

 

[raynalist]

Hiii,please can you explain me what is limiting factor in photosynthesis.

 

[raynalist]

suggested ans:The zone of communication is very large and total global coverage is possible

 

[Zuhsid]

Hiii,please can you explain me what is limiting factor in photosynthesis.

Its is a specific factor that inhibits or prevents the rate of photosynthesis from increasing further, but this can change over time and condition..... Only a change to the limiting factor will increase or decrease the rate of photosynthesis.
Possible factors are: Sunlight, Temperature, Carbon dioxide (these 3 are thye main ones, on which the questions are based almost all of the times... Though other minor factors may be.... availability of water and availability of minerals etc......
So for an example, if all other factors are in very high concentration/amount, but there is only a small amount of sunlight, so it becomes the limiting factor, as due to it the rate would not increase further, like now the excess CO2, water or minerals or chlorophyll will remain unused..... so only if the amount of the limiting factor is changed, will the rate of photosynthesis change accordingly!!!

Hope this helps!!

 

[raynalist]

Its is a specific factor that inhibits or prevents the rate of photosynthesis from increasing further, but this can change over time and condition..... Only a change to the limiting factor will increase or decrease the rate of photosynthesis.
Possible factors are: Sunlight, Temperature, Carbon dioxide (these 3 are thye main ones, on which the questions are based almost all of the times... Though other minor factors may be.... availability of water and availability of minerals etc......
So for an example, if all other factors are in very high concentration/amount, but there is only a small amount of sunlight, so it becomes the limiting factor, as due to it the rate would not increase further, like now the excess CO2, water or minerals or chlorophyll will remain unused..... so only if the amount of the limiting factor is changed, will the rate of photosynthesis change accordingly!!!

Hope this helps!!

t