Biology; Chemistry; Physics: Post your doubts here!

[Awesome12]

here you will use the idea of combined resistance
now calculate the combined resistance in a parallel circuit
r1*r2/r1+r2
3*3/3+3=1.5 ohm
then at series circuit
R1+R2+R3.....
now 1.5+4.5=6 ohm
this is total resistance b/w X and Y
for this we have to close all switches so i don't think it is a B, it is A
anyways which year is it so i'll check the m.s

This is what the examiner report says

Question 29

This was a difficult question. A significant minority gave answer A. This answer assumes the resistance of
two 3.0 Ω resistors in parallel can be obtained by addition, which is not correct. Two resistors of resistance
3.0 Ω in parallel have a combined resistance of 1.5 Ω. The effect of switch S3 was widely recognised.

 

[Karim Nabil]

If S3 is closed the current will bypass 4.5 ohm resistor so resistance b/t X and Y will fall to 1.5 ohm.

can you be more specific?

 

[Karim Nabil]

I know that method. That is the method I used to get the answer but I was also getting A. I checked the marking scheme and it says B
It is oct/nov 2013 p11

yes i also got A as answer for this question, but till now i don't get it !

 

[Awesome12]

yes i also got A as answer for this question, but till now i don't get it !

I think there is something wrong here, but again how can the marking scheme be wrong?

 

[funky brat]

How is the answer B?

View attachment 40324

I got the answer A as well. I think there's something we don't know. Mark scheme can't be wrong but the explanation by the ER is very vague. Can somebody who knows explain?

 

[funky brat]

If S3 is closed the current will bypass 4.5 ohm resistor so resistance b/t X and Y will fall to 1.5 ohm.

What?

 

[Karim Nabil]

What?

hahahaha, yes that what i told him
nothing he wrote i understood

 

[Karim Nabil]

I think there is something wrong here, but again how can the marking scheme be wrong?

it sometimes happens
they aren't gods, they are humans!

 

[Dark Destination]

I think there is something wrong here, but again how can the marking scheme be wrong?

Switch S3 doesn't even matter. It's a useless switch. It won't hinder the passage of current.
By closing S3 the resistance will decrease further due to the resistance of the wire, i think.. That's the only logical explanation.

Even i don't completely understand why A can't be the answer, but B would be my first choice. Definitely.

Edit: I got it. Closing switch 3 would result in a total 1.5 Ohm resistance. Current won't flow through 4.5 Ohm resistor. They asked for 6 Ohm. B is the only answer.

 

[Awesome12]

Switch S3 doesn't even matter. It's a useless switch. It won't hinder the passage of current.
By closing S3 the resistance will decrease further due to the resistance of the wire, i think.. That's the only logical explanation.

Even i don't completely understand why A can't be the answer, but B would be my first choice. Definitely.

Edit: I got it. Closing switch 3 would result in a total 1.5 Ohm resistance. Current won't flow through 4.5 Ohm resistor. They asked for 6 Ohm. B is the only answer.

I don't get the edit part. How can closing the third switch not cause current to flow through it. Plus the total resistance across the parallel resistors is 1.5 ohm

 

[Dark Destination]

I don't get the edit part. How can closing the third switch not cause current to flow through it. Plus the total resistance across the parallel resistors is 1.5 ohm

The current won't go through the 4.5 ohm resistor that way
Electrons go the easy way. Lol, If you had to go a certain place, there were two roads, one with a lot of resistance, and one empty... you'd choose the empty road.
Electrons take the easy path without resistance, and flow through the wire. So the resistance would be 1.5 ohm between X and Y that way.
But if you don't close Switch 3, the resistance would be 6.

 

[Awesome12]

Oh i get it now

If s3 is close then this will happen

 

[Dark Destination]

If S3 is closed the current will bypass 4.5 ohm resistor so resistance b/t X and Y will fall to 1.5 ohm.

^ This guy gets it. (Y)

 

[Karim Nabil]

The current won't go through the 4.5 ohm resistor that way
Electrons go the easy way. Lol, If you had to go a certain place, there were two roads, one with a lot of resistance, and one empty... you'd choose the empty road.
Electrons take the easy path without resistance, and flow through the wire. So the resistance would be 1.5 ohm between X and Y that way.
But if you don't close Switch 3, the resistance would be 6.

the examiner who creates this question is a retard!

 

[Dark Destination]

Oh i get it now

If s3 is close then this will happen
View attachment 40332

Exactly.

 

[Awesome12]

Exactly.

That wan't that hard after all

 

[Dark Destination]

the examiner who creates this question is a retard!

Lol, dude. If all questions were easy, that would be no fun. xD

 

[Dark Destination]

That wan't that hard after all

It is, if you don't imagine the question properly.
As Einstein once said, ''Imagination is more important than knowledge''.

 

[Awesome12]

Lol, dude. If all questions were easy, that would be no fun. xD

Physics is that one subject which has this property; you solve the entire paper expecting full or an A*, and when it gets checked, you go like 'wth', cuz you end up with an A or B. But when you look at your errors, you go like, 'That was soo simple'

 

[Awesome12]

It is, if you don't imagine the question properly.
As Einstein once said, ''Imagination is more important than knowledge''.

Does putting up an Einstein avatar help you in this