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# AQA Physics PHYA5 - Thursday 18th June 2015 [Exam Discussion Thread] watch

1. (Original post by toddle1)
Do we have to know how to derive the kinetic theory model? This equation:

pV=(1/3)Nm(crms)^2
Yeaj

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2. ahh ok, better get on it then, thanks
3. (Original post by toddle1)
ahh ok, better get on it then, thanks
No worries! I believe it's the only equation we need to know the derivation of in the whole unit.

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4. Have they ever asked a question on deriving the kinetic theory? I can't think of when they have... Probably means it's going to come up this time ;D
5. anyone doing applied physics, know any good sites or videos about the thermodynamics stuff?
6. (Original post by CD223)
Does this help?

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Thats awesome thank you so much
7. FP2 is over, only physics and M2 left to prepare for finally! Hopefully going to get a bit of physics reading done tonight and then a full day tomorrow, fingers crossed
8. (Original post by slaven123)
Have they ever asked a question on deriving the kinetic theory? I can't think of when they have... Probably means it's going to come up this time ;D
Yeah I'm reckoning it might. Just remember the main stages:

• Time between impacts with a specific wall
• Change of momentum
• Force = Change of momentum/time between impacts
• Pressure = Force/Area
• Mean square speed takes into account all three dimensions divided by N particles
• 1/3 Root mean square speed = velocity in one direction
• Equation falls out to be pV = 1/3 Nmc^2

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9. (Original post by ZedRigby)
Thats awesome thank you so much
No problem!

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10. Could I have help on 2cii please not really sure how to go about it? http://filestore.aqa.org.uk/subjects...1-QP-JUN12.PDF
Thanks

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11. Hi guys I don't know if any of you can help me. Its a question about the big derivation in kinetics. I don't understand why they divide by three to get between these two lines. (In the attachment) its when all the pressure expressions are added together. Is it because its a mean and there are three dimensions? Idk...

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Attached Images

12. (Original post by CD223)
Yeah I'm reckoning it might. Just remember the main stages:

• Time between impacts with a specific wall
• Change of momentum
• Force = Change of momentum/time between impacts
• Pressure = Force/Area
• Mean square speed takes into account all three dimensions divided by N particles
• 1/3 Root mean square speed = velocity in one direction
• Equation falls out to be pV = 1/3 Nmc^2

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That's great thanks

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13. (Original post by Jimmy20002012)
Could I have help on 2cii please not really sure how to go about it? http://filestore.aqa.org.uk/subjects...1-QP-JUN12.PDF
Thanks

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Does this help?

There are initially 3 x 10^22 atoms of U.

The half life of U is 4.5 x 10^9 years so there must be 1.5 x 10^22 atoms of U and 1.5 x 10^22 atoms of Pb after one half life. This is where they intersect on the graph.

After another half life, the U curve halves again. Conversely, the Pb curve doubles. They are the mirror of one another.

EDIT: lol, just seen its part two. Ignore me. I'll get on the case.

...

Right, you know that when there are twice as many U atoms as Pb atoms, the ratio is 2:1. This means that the number of U atoms is two thirds the total number.

So the number of U atoms is:

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14. Is the role of the coolant in a nuclear reactor just to cool the fuel down?
15. (Original post by frankiejayx)
Is the role of the coolant in a nuclear reactor just to cool the fuel down?
Yeah it cools the reactor down by absorbing heat produced and also means that it can evaporate into steam which drives a turbine and generates electricity.

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16. (Original post by CD223)
Yeah it cools the reactor down by absorbing heat produced and also means that it can evaporate into steam which drives a turbine and generates electricity.

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Ahhh right thanks!
17. (Original post by _Caz_)
Hi guys I don't know if any of you can help me. Its a question about the big derivation in kinetics. I don't understand why they divide by three to get between these two lines. (In the attachment) its when all the pressure expressions are added together. Is it because its a mean and there are three dimensions? Idk...

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Yeah, essentially, as U, V and W are equal perpendicular components of the velocity in 3D:

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18. (Original post by frankiejayx)
Ahhh right thanks!
No worries

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19. (Original post by CD223)
Yeah I'm reckoning it might. Just remember the main stages:

• Time between impacts with a specific wall
• Change of momentum
• Force = Change of momentum/time between impacts
• Pressure = Force/Area
• Mean square speed takes into account all three dimensions divided by N particles
• 1/3 Root mean square speed = velocity in one direction
• Equation falls out to be pV = 1/3 Nmc^2

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I can't think of how'd they'd go about asking about it... It would be worth like a good 10 marks if they just made you do it all at once from scratch.
20. Need a bit of motivation, anyone?

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