Physics: Post your doubts here!

[periyasamy]

Guys ,very confused with the internal energy part.Just want to ask something to clear it up.
1)1st of all when work is done on system is it +ve or -ve?
2)example

ice > water -------->steam

Guys,this conversion of ice to water to steam ?Is it all work done on system or by system.
Thanks a lot

 

[periyasamy]

Another question here guys,
i have a question that i got the answers from this forum long time ago.But i dont get 1 of the part in the working.
why in the 3 b part 2.there is a value of 8676 joule,isnt it suppose to be 8700,the answer for the 1st part.
Even the answer show the same thing.Help guys.Thanks a lot....

 

[BreakingBad]

why cant charles law v1/t1=v2/t2 pressure is kept constant. also the pv=nRT why cant we use any of these equation from both equations i get 120K and 140K i dont get it why is this wrong.

The question says that the volume is increased from 1.8 to 2.1. This is done by moving the piston and not by heating the cylinder because the question mentioned 'insulated' cylinder. Therefore, it doesn't make sense for the temperature to RISE (as it seems to do when using pV = nRT or v1/t1 =v2/t2) when the volume is increased. For that to happen, heat must be supplied and q=mc(dT) where 'c' is missing.

Guys ,very confused with the internal energy part.Just want to ask something to clear it up.
1)1st of all when work is done on system is it +ve or -ve?
2)example

ice > water -------->steam

Guys,this conversion of ice to water to steam ?Is it all work done on system or by system.
Thanks a lot

1) Work done on the system is negative.
Work done by the system is positive.
When you use the equation U = q - W

2)
Take a cylinder with insulated walls with a piston on one end. Let's assume you are pushing the piston. The gas is the system. You are the surroundings doing work on that system. Therefore, the work done on the gas is the work done by you (surroundings) and is positive. The work done by the gas in order to reduce it's volume is the work done by the system and is negative.

I didn't exactly understand the ice to water to steam part. It depends on the conditions. Try to elaborate.

Another question here guys,
i have a question that i got the answers from this forum long time ago.But i dont get 1 of the part in the working.
why in the 3 b part 2.there is a value of 8676 joule,isnt it suppose to be 8700,the answer for the 1st part.
Even the answer show the same thing.Help guys.Thanks a lot....

The answer of the first part IS 8676 J.
mc(dT) + mL

m =24/1000 kg
c = 2.1 * 10^3
dT = 15
L = 3.3 * 10^5

Substitute values and you'll get 8676, which will then be used in the second part.

 

[periyasamy]

Thank u breaking bad....Nvr mind for the ice part,i got ady.Thanks boss!!!

 

[sara kamal]

http://papers.xtremepapers.com/CIE/...and AS Level/Physics (9702)/9702_w06_qp_4.pdf

please explain Q4 part B....i m getting answer as 6 x 10^22 but its 6 x 10^24..

 

[BreakingBad]

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w06_qp_4.pdf

please explain Q4 part B....i m getting answer as 6 x 10^22 but its 6 x 10^24..

Change in PE = GMm [1/(R+h1) - 1/(R+h2)]
Change in KE = 1/2m (v1^2 - v2^2)

Equate equations. m cancels. Substitute values of G, (R+h1), (R+h2), v1 and v2 and you'll get the correct answer.

 

[biscuitbiscuit]

someone please explain ;

http://papers.xtremepapers.com/CIE/...and AS Level/Physics (9702)/9702_w02_qp_2.pdf
'Q7 b (i)'

 

[BreakingBad]

someone please explain ;

http://papers.xtremepapers.com/CIE/Cambridge International A and AS Level/Physics (9702)/9702_w02_qp_2.pdf
'Q7 b (i)'

E = Pt = V^2t/R Where V (pd) and (t) are constant in both the cases therefore:

E1/E2 = R2/R1
E1 = 1.1 * 10^5
R1 = 45
R2 = 15

Substitute the values into the equation. Answer is 3.67 * 10^4

 

[biscuitbiscuit]

E = Pt = V^2t/R Where V (pd) and (t) are constant in both the cases therefore:

E1/E2 = R2/R1
E1 = 1.1 * 10^5
R1 = 45
R2 = 15

Substitute the values into the equation. Answer is 3.67 * 10^4

Can u explain again, how did you get the formula 'E1/E2 = R2/R1'

 

[BreakingBad]

Can u explain again, how did you get the formula 'E1/E2 = R2/R1'

E = Pt = V^2t/R, since V and t are constant we can write this equation as:
E = k/R
So,
E is directly proportional to 1/R
So, can can conclude:
E1/E2 = R2/R1

 

[sara kamal]

Change in PE = GMm [1/(R+h1) - 1/(R+h2)]
Change in KE = 1/2m (v1^2 - v2^2)

Equate equations. m cancels. Substitute values of G, (R+h1), (R+h2), v1 and v2 and you'll get the correct answer.

i did this actually ,can u show me steps please..?

 

[BreakingBad]

i did this actually ,can u show me steps please..?

GMm [1/(R+h1) - 1/(R+h2)] = 1/2m (v1^2 - v2^2)
m cancels.
GM [1/(R+h1) - 1/(R+h2)] = 1/2 (v1^2 - v2^2)
Make M subject:
M = (v1^2 - v2^2)/2G[1/(R+h1) - 1/(R+h2)]
M = (5370^2 - 5090^2)/2*6.67*10^11 [1/(6.38+19.9)*10^6 - 1/(6.38+22.7)*10^6]
M = 2928800/13.34*10^-11 [3.66385*10^-9]
M = 2928800/4.88758*10^-19
M = 5.99*10^24 = 6*10^24

 

[sara kamal]

but

GMm [1/(R+h1) - 1/(R+h2)] = 1/2m (v1^2 - v2^2)
m cancels.
GM [1/(R+h1) - 1/(R+h2)] = 1/2 (v1^2 - v2^2)
Make M subject:
M = (v1^2 - v2^2)/2G[1/(R+h1) - 1/(R+h2)]
M = (5370^2 - 5090^2)/2*6.67*10^11 [1/(6.38+19.9)*10^6 - 1/(6.38+22.7)*10^6]
M = 2928800/13.34*10^-11 [3.66385*10^-9]
M = 2928800/4.88758*10^-19
M = 5.99*10^24 = 6*10^24

But why are we subtracting like 1/R,why not just directly R+h1 - R+h2?

sorry for trouble!

 

[BreakingBad]

but



But why are we subtracting like 1/R,why not just directly R+h1 - R+h2?

sorry for trouble!

The formula for P.E. = -GMm/r
r is the distance and it is in the denominator. In this case, r1 is R+h1 and r2 is R+h2.
Change in PE = final PE - Initial PE
= -GMm/r2 - (-GMm/r1)
= GMm/r1 - GMm/r2
Take GMm common:
GMm(1/r1 - 1/r2)

Oh and you cannot simply multiply the distances r1 and r2 on the right side during calculation without taking LCM as there is a - sign between them.

 

[sara kamal]

The formula for P.E. = -GMm/r
r is the distance and it is in the denominator. In this case, r1 is R+h1 and r2 is R+h2.
Change in PE = final PE - Initial PE
= -GMm/r2 - (-GMm/r1)
= GMm/r1 - GMm/r2
Take GMm common:
GMm(1/r1 - 1/r2)

Oh and you cannot simply multiply the distances r1 and r2 on the right side during calculation without taking LCM as there is a - sign between them.

Thank you sooo much !!

 

[hacker]

plz solve this !

 

[sara kamal]

http://papers.xtremepapers.com/CIE/...and AS Level/Physics (9702)/9702_s09_qp_4.pdf

please explain Q11 b (3)

 

[BreakingBad]

plz solve this !

Intensity is directly proportional to Amplitude squared
Intensity is inversely proportional to distance squared.
Therefore:
A^2 = k/x^2
When x=r, A=8.
64 = k/r^2
k = 64r^2

When x = 2r, A=?
A^2 = 64r^2/(2r)^2
A^2 = 64/4
A = 4 um

 

[Galactic]

Why do magnets attract ferromagnetic materials only? Why not other materials as well?

 

[Hassan Ali Abid]

Why do magnets attract ferromagnetic materials only? Why not other materials as well?

Other materials dont have the proper alignment of north and south pole and the materials whose alignment cant be changed by the magnet are not attracted by the magnet.