Physics, Chemistry and Biology: Post your doubts here!

[diamondsky]

Could someone please help with this chemistry question?

First variant paper, question 9ci.
http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Chemistry (0620)/0620_s09_qp_3.pdf

This is the answer:
silicon is limiting reagent
0.07 moles of Si and 25/160 = 0.156 moles of Br2
because 0.14 (2 × 0.07) < 0.156
If 80 used to find moles of Br2 the mark 1 and 3 still available arguments based on masses can be used

Can you please explain "because 0.14 (2 × 0.07) < 0.156 "

Thank you

0.07 moles of Si can only react wid 0.14 moles of Br. so u still hav 0.016 moles of Br

 

[diamondsky]

q 3c) w 09 31 http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Chemistry (0620)/0620_w09_qp_31.pdf

 

[Nita Mahajan]

Hey can u guys advice about which chemistry topics are important to revise?? I have done all of them but can u tell which chapters are EXTREMELY important!???

 

[diamondsky]

badrobot14 n PlanetMaster help pls

 

[Era]

i have got some doubts in physics! i hope someone would help me out!
i asked my teacher that why do we NEED ac current in transformers! She said AC is must because current in ac changes direction so thus it will change the magnetic fiield! but my question is that WHY do we need to change the magnetic field?
and why the coil should be insulated?
and what helps transformers change its voltage?

 

[Amer1996]

Guys help me in physics paper 6 may/june 2009 question 5 part (b) http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Physics (0625)/0625_s09_qp_6.pdf

 

[examkiller]

Hello, I have few problems, cant know how to do it. physics paper 6 oct/nov 2003
question 1.d + 2 +4.B anybody know how to do those ?

And physics paper 6 oct/nov 2008 q 2.a how to draw the correct circuit and point C) its says about predicitions about P.D and then ii) of c .

Thnks

 

[Kareem Gamal Omar]

PHYSICS :
WHAT IS THE FREAKIN PARALLAX ERROR !!!!!!!!!!!

 

[smart_boy]

PHYSICS :
WHAT IS THE FREAKIN PARALLAX ERROR !!!!!!!!!!!

Parallax error is when you dont view sumthng like a thermometer or a burette by keeping the eye parallel and get a wrong reading because of the error caused by the human eye..

 

[smart_boy]

i have got some doubts in physics! i hope someone would help me out!
i asked my teacher that why do we NEED ac current in transformers! She said AC is must because current in ac changes direction so thus it will change the magnetic fiield! but my question is that WHY do we need to change the magnetic field?
and why the coil should be insulated?
and what helps transformers change its voltage?

Okay.. You need to understnd this concept calmly... U must be knowing about electromagnetic induction.. Whenevr u move a conducting wire inside a magnetic field, or when you move a bar magnet near a solenoid (coil of wire)... A voltage is induced in the wire.. that creates a current... So its important that sumthng shud b movin... Because there needs to be some movement energy that can be converted to electrical energy...
Transformerrs work on the principle of mutual induction... An ac current in the primary coil, creates a magnetic field that is continuously CHANGING, this CHANGING magnetic field is the cut by the wire of the secondary coil, in the transformer.... So again an ac voltage is induced and and an ac current flows there...
Therefore they cant work on dc cos there is no variation in the magnetic field so no magnetic field lines cut the secondary coil...
Now... if there are more number of turns in the secondary coil.. the same field will be able to cut more wire in the same time... Thats why, if secondary coil has more turns , then the voltage is increased... The opposite happens when the number of coils are less... And i dnt think its necessary fr the coil to b insulated...
Huh.!!! Typing is tedious..!!

 

[smart_boy]

there is also problem with Q.6(c) of nov 2009. can you plz briefly explain me all steps? http://www.xtremepapers.com/papers/CIE/Cambridge IGCSE/Chemistry (0620)/0620_w09_qp_32.pdf

First u need to find number of moles by dividing mass by molar mass... 12.16/152= 0.08
Then u need to see in the equation.. that half the moles of Fe2O3 will form.. Because ratio is 2:1.... so u write there.. 0.04
Calculatin one mole of Fe2O3 is simple... (56x2)+(16x3)= 160
Mass equals no of moles multiplied by molar mass 0.04 x 160 = 6.4
number of moles of gases formed = 0.04 + 0.04 = 0.08.... As mole ratio is 1:1...
volume of sulfur trioxide formed = 0.08 × 24 = 1.92 dm3... (24dm3 is molar gas volume)

 

[smart_boy]

In analog technology, a wave is recorded or used in its original form. And in digital technology, the analog wave is sampled at some interval, and then turned into numbers that are stored in the digital device....

Can anyone help me with this question...???

 

[Sahil_M]

Hey anyone gave physics paper 32?

 

[sparsha.mehta]

Hey anyone gave physics paper 32?

meeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

 

[Sahil_M]

Ok So I'm guessing u got the efficiency of the solar panels as 16.33?

 

[sparsha.mehta]

Ok So I'm guessing u got the efficiency of the solar panels as 16.33?

i swear i dont remmembr but u can ask Saahil Parikh(one of the xpc memmbers)

 

[Azhar.Euphoric]

Anyone did physics paper 33...????

hw much did u get for gradient and y-intercept..??

 

[Sahil_M]

i swear i dont remmembr but u can ask Saahil Parikh(one of the xpc memmbers)

Oh ok thanks anyways (Y)

 

[sparsha.mehta]

Oh ok thanks anyways (Y)

this is to solve ur confusion cozzzzzzzzzzz i forgot the ans....

 

[Sahil_M]

this is to solve ur confusion cozzzzzzzzzzz i forgot the ans....

I don't think saahil's online...guess I'll have to wait till tomorrow :/