Physics 9702_s06_qp 2

[XPFMember]

Assalamoalaikum!!

Can anybody help me with Q:7 part (a) and (b)...i dont get the logic behind it!

here's the link to the paper June:2006 Paper:2

 

[Tweety-Angie]

Lamp A is in series with lamp B and C, whereas B and C are in parallel with each other. Resistance of each lamp is to be considered 15ohm.
1st: When all switches are left open, there is no flow of electricity, so reading of resistance is infinity.
2nd: When S1 is closed, and S2 and S3 are open, there is current flow only through S1, so resistance reading is shown by lamp A only which is 15ohm.
3rd: When S2 is closed, and remaining two are open, the current flows through lamp A and B. So, total resistance shown here is: R1+R2, i.e. 30ohm as they are connected in series.
4th: When S2 and S3 is closed, the current flows through all the lamps. So, total resistance here should be: R1+ (1/R2 + 1/R3), i.e. 15.15ohm. But is is given only 15ohm, which is incorrect. So, we come to know lamp C is faulty.
But even I don't know what the fault is. If you know the 2nd ans let me know too..

 

[DragonCub]

The fault is that Lamp C is short-circuited.
When S1 open, S2 S3 closed, Lamp B&C are in parallel, and the BC part is in series with Lamp A. So the total R of the whole circuit shoule equal to: R of A + R of BC part. The actual reading shows total R = 15 Ohm, and the R of A is already 15 Ohm, this means R of BC part is zero - one of the two lamps must be short-circuited to give a zero-resistance. So the faulted one, Lamp C, is short-circuited.

 

[DragonCub]

For Q7(b) I think that at the beginning we do not know which lamp is faulted.
So there is a risk for using the power supply - if the faulted one is Lamp A and the fault is short-circuit, when only S1 is closed the whole circuit becomes shorted can this wuold cause the damage of power supply.

 

[Tweety-Angie]

Hmm..so the lamp is called short-circuted! I didn't know that, tenku

 

[XPFMember]

Assalamoalaikum!!

The fault is that Lamp C is short-circuited.
When S1 open, S2 S3 closed, Lamp B&C are in parallel, and the BC part is in series with Lamp A. So the total R of the whole circuit shoule equal to: R of A + R of BC part. The actual reading shows total R = 15 Ohm, and the R of A is already 15 Ohm, this means R of BC part is zero - one of the two lamps must be short-circuited to give a zero-resistance. So the faulted one, Lamp C, is short-circuited.

well ...when C is faulted only then y isnt the resistance of B taken into consideration...?? i mean shudnt the combined resistance be 30 ohms

and one more thing...i dont understand what actuallt is a short circuit??

 

[Tweety-Angie]

Lamp C is faulted so it doesnot show any resistance! See above!

 

[XPFMember]

now but then just as in the previous case ...so shudnt in this case also....it will be as if A and B are in series since C is faulted so wont make any effect??

 

[Tweety-Angie]

I guess it won't! Current won't flow through that circuit, it flows via A and B, no current flows through C.

 

[XPFMember]

yeah so when it flows through B..combined res. will be A + B ??]
sorry bt its just so confusing

and also can u tell me for diff. grating dsinθ = nλ when do we use θ as 90 ?? sumtimes we use it as 90 when angles not there :?

 

[Tweety-Angie]

yeah resistance will be A+B as they are in series combination!

Sorry, I hvn't yet read diffraction grating..

 

[XPFMember]

so how come is it 15 ohms then??

 

[Tweety-Angie]

Hmm...I got stuck lol...
Aahh...okei, then may be the current flows through lamp C also, so that total resistance will be like R1+ (1/R2 + 1/R3).
and as R3 is not completely working I mean since there is some fault, so the total resistance 15 ohm is shown, but actually there should have been 15.13 ohm according to the calculation of lamp C was not faulted. It is the trick which we require to know how one of the lamp is faulted!

 

[Tweety-Angie]

And Shortcircuit is a faulty connection in an electrical circuit in which the current flows through a shorter route..according to the book. But I didn't understand, how come here be a short-circuit!!

 

[XPFMember]

well...here got an explanation!!
when C is shorted....current will not flow through B....cuz in a short circuit as u said the current flows through the shorter route...so it will flow from C whose res. is zero!!

hence we can conclude lamp C is shorted

but i'm a bit confused in part (b)....can sumeone plz help in that!!

 

[Tweety-Angie]

Ooopsy..can't help u out wid that..sorry

 

[XPFMember]

btw did u get the first part

 

[Tweety-Angie]

hehhe I really didn't get that, how come u say that there is no current flow in B and flows through C???

 

[XPFMember]

u said na when there's a short circuit the current takes the shortest possible route!!

 

[Tweety-Angie]

yeah. so? how is it related here?? hehe