Biology; Chemistry; Physics: Post your doubts here!

[Saad Mughal]

btw can u tell me the difference b/w electromotive force and potential difference

Electromotive force is the work done per unit charge in driving charge around a complete circuit (in other words it is the energy converted from all other forms to electrical energy); it is measured by connecting the voltmeter in series with the circuit.
Potential difference is the work done per unit charge in driving charge through a COMPONENT (such as a resistor) (in other words, it is the energy converted from electrical energy to all other forms); it is measured by connecting the voltmeter in parallel (or across) the component.
That's all you need to know according to the syllabus. Hope that helps.

 

[Saad Mughal]

i had a problem with the same question but I didn't really get it!can u please explain it in words mayb :-/

The number of moles is the same on both sides, i.e. 1:1 ratio on both sides since all the carbon reacts to form carbon dioxide where all the hydrogen reacts to form water, using this we can calculate the volume of the gases on the left using the formula Volume = No.of Moles * Molar Gas Volume (24dm^3 or 24000cm^3).

 

[Shahzaib Zindani]

Nov 2004, Paper 2 Question 3 part (a). How do I draw this gene?
http://papers.xtremepapers.com/CIE/Cambridge International O Level/Biology (5090)/5090_w04_qp_2.pdf
http://papers.xtremepapers.com/CIE/Cambridge International O Level/Biology (5090)/5090_w04_ms_2.pdf

 

[asma tareen]

Electromotive force is the work done per unit charge in driving charge around a complete circuit (in other words it is the energy converted from all other forms to electrical energy); it is measured by connecting the voltmeter in series with the circuit.
Potential difference is the work done per unit charge in driving charge through a COMPONENT (such as a resistor) (in other words, it is the energy converted from electrical energy to all other forms); it is measured by connecting the voltmeter in parallel (or across) the component.
That's all you need to know according to the syllabus. Hope that helps.

yea it did thnx fr the help

 

[Saad Mughal]

 

[nidz]

I found this in the Chem syllabus.
explain how photosynthesis can provide a renewable energy resource

do they mean fermentation by that? Im confused.

 

[nidz]

Is there weightage of different papers in Physics, Biology and Chemistry or not. there's nothing in the syllabus

Yeah there is. The weightage is written in the syllabus. Normally paper 2 of all three have around 50% weightage. You can check out the syllabus for the rest.

 

[nidz]

Nov 2004, Paper 2 Question 3 part (a). How do I draw this gene?
http://papers.xtremepapers.com/CIE/Cambridge International O Level/Biology (5090)/5090_w04_qp_2.pdf
http://papers.xtremepapers.com/CIE/Cambridge International O Level/Biology (5090)/5090_w04_ms_2.pdf

You just have to randomly draw them. For eg, the first gene circle shaded, second gene also shaded, but third not shaded.
As it actually wants us to show meiosis, which is random, u can just choose whichever you want but the figure should be the same

 

[usama321]

I found this in the Chem syllabus.
explain how photosynthesis can provide a renewable energy resource

do they mean fermentation by that? Im confused.

well i don't study bio. But still what little i know is that plants take in CO2 from the environment. then through sunlight, water and the chlorophyll, they turn the CO2 into glucose, oxygen and ENERGY. Now, this oxygen is used up by humans and we release CO2 in the respiratory process. Overall a cycle is formed of CO2 and Oxygen, and the plants constantly get energy from the CO2.

My answer might not be good enough as i do not study biology. Please someone correct me if i am wrong

 

[Amnaa]

what is the grading system in 2013 may/june exams paper..

 

[Usama Jawad]

what is the grading system in 2013 may/june exams paper..

These are the exact words from this website http://ask.cie.org.uk/system/selfse...&USERTYPE=1&CMD=VIEW_ARTICLE&ARTICLE_ID=33226
The process of grading in Cambridge O Level involves deciding the minimum mark necessary for each grade. We do not have predetermined thresholds (or quotas of candidates). After each examination series we decide the minimum marks for each threshold (grade boundaries) for each individual component. We carry out this process in order to ensure that the demands on the candidates are the same for every subject and every examination series. We are therefore unable to provide a general list of percentages required for each grade.

 

[Saad Mughal]

I found this in the Chem syllabus.
explain how photosynthesis can provide a renewable energy resource

do they mean fermentation by that? Im confused.

I don't know too much about this but; it is a renewable resource since sunlight and air are two things that will never end (hence renewable), it uses the phenomenon of converting Light energy (sun) to Electrochemical energy, this process produces much more energy than other known renewable sources and has no waste products. Research is being done into this, using the reactions similar to those of fermentation and photosynthesis, energy is potentially going to be created, this is going to be in much larger quantity than other solar panels, it will involve a number of catalysts that speed up and control the process. Basically, it is a solar panel (light - electrical energy) which uses the process of photosynthesis to produce high amount of energy that is cheap and readily available, it is known as artificial photosynthesis I think. It has the advantages of being a clean, quick and cheap method of producing energy which uses the natural resources with no byproducts, however it will take lots of years to get this process running and it will be very expensive to start off the project! Hope that helps.

 

[Saad Mughal]

well i don't study bio. But still what little i know is that plants take in CO2 from the environment. then through sunlight, water and the chlorophyll, they turn the CO2 into glucose, oxygen and ENERGY. Now, this oxygen is used up by humans and we release CO2 in the respiratory process. Overall a cycle is formed of CO2 and Oxygen, and the plants constantly get energy from the CO2.

My answer might not be good enough as i do not study biology. Please someone correct me if i am wrong

Your correct in what you said but the question is that how can photosynthesis be used as a renewable resource, that means that we need to describe how the process of photosynthesis, its products, reactants can be used to produce energy for our use.

 

[Khadija_1234]

please help

 

[Khadija_1234]

And can anyone explain the part b of this question?

 

[Saad Mughal]

Since the ratio is 1:1,
Taking atomic and molecular mass,
24g Mg + Excess Air ----> 24+16 = 40g MgO
Hence,
24g Mg ----> 40g MgO
Simplifying,
Mg ----> 5/3 g MgO

Given, 12 kg Magnesium reacts,
Therefore,
12kg Mg ----> 5/3(12)kg MgO
12kg Mg ----> 20kg MgO

Applying Percentage Yield = Theoretical Value/Experimental Value * 100
75/100 = Mass of MgO/20
Mass of MgO = 20* 0.75 = 15kg.

 

[ijlalm]

If the electrodes are made up of different metals such as magnesium and copper,a voltage or emf would be detected by the voltmeter.The more reactive metal would dissolve in the solution and change into ions,thereby producing electrons. As electrons are produced.that metal becomes the negative electrode: Mg(s)=Mg2+(ions) + 2 electrons. These electrons travel to the copper electrode and produce bubbles of Oxygen gas. So the movement of electrons is from the magnesium electrode from the wire to the copper electrode.

 

[ijlalm]

BTW I have a question.. Could anyone explain November 08 physics, paper 2, 7(b)...the question is quite arduous..
http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/

 

[Saad Mughal]

BTW I have a question.. Could anyone explain November 08 physics, paper 2, 7(b)...the question is quite arduous..
http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/

For the first row, it's quite obvious that all the current passes through filament F since the switch is open and no current passes through filament G. So, 4.6 and 0.
For the second row; we know that if 12V is applied across the filament, a 4.6A current passes through it, since the two filaments are connected in parallel hence the voltage across them will be same i.e. 12V, therefore, from the first part, a current of 4.6A passes through both the filaments and the current supplied by the battery is then (4.6+4.6)A=9.2A
Hope you get it.

 

[usama321]

Your correct in what you said but the question is that how can photosynthesis be used as a renewable resource, that means that we need to describe how the process of photosynthesis, its products, reactants can be used to produce energy for our use.

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thanks for clearing it up I did not know that​