# Physics Question Help

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#1
Hello, need some help with these Multiple choice questions.
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#2
For Q7:
I used Wiens Displacement
so
Wavelength x Temp = const
Wavelength1 x 750 = wavelength2 X 960
so
Wavelength2 = 750W1 / 960 = 0.78W1 = Option A but apparently answer is Option B?
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#3
For Q10:
Option A looks exactly like the Suns spectrum but has been slightly red-shifted but the answer is Option B?
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#4
For Q14:
I used the equation:
PV = 1/3Nmc^2
We're told PV is constant after a 1/3 of the gas is removed. so I thought

1/3 * 1/3 Nmc^2 = 1/3 Nm 3x10^5
Cancelling down this gives me the answer
C^2 = 9x10^5 which is Option D but the answer is Option C?
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#5
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1 month ago
#6
(Original post by Afrrcyn)
For Q7:
I used Wiens Displacement
so
Wavelength x Temp = const
Wavelength1 x 750 = wavelength2 X 960
so
Wavelength2 = 750W1 / 960 = 0.78W1 = Option A but apparently answer is Option B?
For Wien's displacement law, the absolute temperature is used, so you need to convert to kelvin
Last edited by golgiapparatus31; 1 month ago
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1 month ago
#7
7. B
10. A
14. D
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1 month ago
#8
Ignore me, I'm just bumping
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#9
(Original post by golgiapparatus31)
For Wien's displacement law, the absolute temperature is used, so you need to convert to kelvin

Note that pV is constant, but not T. so use pV = nRT to work out the new temperature, then use pV = 1/3 Nmc^2
Thanks! I'm very silly Physicist!

(Original post by ItsStarLordMan)
7. B
10. A
14. D
OHhhh, now I get it!
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1 month ago
#10
(Original post by Afrrcyn)
OHhhh, now I get it!
Sarcasm, right?
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#11
(Original post by ItsStarLordMan)
Sarcasm, right?
Maaaaaaybe.
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1 month ago
#12
(Original post by Afrrcyn)
Maaaaaaybe.
Well, I think 10 is A. Could be wrong, but I'm pretty sure
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#13
(Original post by ItsStarLordMan)
Well, I think 10 is A. Could be wrong, but I'm pretty sure
I mean that's what I put but apparently it's not! Hoping someone can explain it to me before Monday
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1 month ago
#14
(Original post by Afrrcyn)
I mean that's what I put but apparently it's not! Hoping someone can explain it to me before Monday
No way is that wrong!
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#15
(Original post by golgiapparatus31)
For Wien's displacement law, the absolute temperature is used, so you need to convert to kelvin

Note that pV is constant, but not T. so use pV = nRT to work out the new temperature, then use pV = 1/3 Nmc^2
Why would I use pV = 1/3 Nmc^2, if T isn't even in that equation, would I not use 1/2mc^2 = 3/2 kT?

(Original post by ItsStarLordMan)
No way is that wrong!
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1 month ago
#16
(Original post by Afrrcyn)
I mean that's what I put but apparently it's not! Hoping someone can explain it to me before Monday
You have to convert temperature to kelvin.
(Original post by Afrrcyn)
Why would I use pV = 1/3 Nmc^2, if T isn't even in that equation, would I not use 1/2mc^2 = 3/2 kT?

Sorry

[Alternative method below]

Spoiler:
Show
You have
So, N is inversely proportional to T.
So the new temperature is 3/2 times the old one.
Then as you 1/2 mc^2 = 3/2 kT
So c^2 is multiplied by a factor of 3/2.
Last edited by golgiapparatus31; 1 month ago
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1 month ago
#17
(Original post by Afrrcyn)
For Q14:
I used the equation:
PV = 1/3Nmc^2
We're told PV is constant after a 1/3 of the gas is removed. so I thought

1/3 * 1/3 Nmc^2 = 1/3 Nm 3x10^5
Cancelling down this gives me the answer
C^2 = 9x10^5 which is Option D but the answer is Option C?
1/3 of the gas is removed, so 2/3 N molecules are left. Using this, we get the answer
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#18
(Original post by golgiapparatus31)
1/3 of the gas is removed, so 2/3 N molecules are left. Using this, we get the answer
Ohhh, yes, yes. I was using 1/3 and not 2/3! Any idea for Q10? Thanks for the help!
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1 month ago
#19
(Original post by Afrrcyn)
Ohhh, yes, yes. I was using 1/3 and not 2/3! Any idea for Q10? Thanks for the help!
As you say, there is a red-shift of the wavelengths.

But the change for each one is not the same, so the first 2 lines cannot remain the same distance apart, so it can't be A. We find that it is B, because it can't be C or D
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#20
(Original post by golgiapparatus31)
As you say, there is a red-shift of the wavelengths.

But the change for each one is not the same, so the first 2 lines cannot remain the same distance apart, so it can't be A. We find that it is B, because it can't be C or D
Wait. it can't be the same, it that because of dWavelength/wavelength = Velocty / c, so each wavelength would change by different amounts, bigger the wavelength, the bigger the change?
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