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

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Original post by frankiejayx
Yeah that one is okay, its all the thermal ones i dont like!


Which thermal ones exactly? I can't think of many experiments apart from the nuclear diameter ones
Original post by JJBinn
Which thermal ones exactly? I can't think of many experiments apart from the nuclear diameter ones


Theres the three gas laws i dont know if we need to know those experiments
Original post by frankiejayx
Theres the three gas laws i dont know if we need to know those experiments


I don't think we do, or at least I hope not haha. They aren't specified in the CGP revision guide and I've noticed that most of the past paper questions are on stuff in there
Original post by MangoFreak
There probably won't be a question on nuclear waste storage/treatment right? I can't stand that stuff. Hardly physics.


We all hope not. The extent of my knowledge on it is that the waste is stored in cooling tanks until it reaches a safe temperature and then stored underground in trenches until it's activity reaches a safe level.
Original post by JJBinn
Which thermal ones exactly? I can't think of many experiments apart from the nuclear diameter ones


I think there is an experiment in the textbook to find the specific heat capacity of a substance, it might be a good idea to check it out quite a simple one tbh
Original post by Boop.
I think there is an experiment in the textbook to find the specific heat capacity of a substance, it might be a good idea to check it out quite a simple one tbh


Is that the inversion tube one?
Guys, I also have a feeling that a six marker could be on commenting on the assumptions made before deriving the equation of the kinetic theory of gases. The textbook does comment on two of the assumption but no further. Here are some comments relating to the assumptions made:

1) The volume of each molecule is negligible compared to the volume of the gas.
• Comment: that is to say the molecules of the gas are very far apart because in liquid state the molecules are sufficiently close to each other for the attractive forces between molecules to be important.

2) The time of each collision between molecules or each molecule with the container walls is negligible compared with the time between successive collisions.
• Comment: ( i still don't really know why this assumption is made. I would be very happy if any one could let us know.)

3) The molecules do not attract each other.
• Comment: if the particles attracted each other they would not exert force on the walls of the container as they would all tend to clump together in the middle of the container.

4) the particles move in continual random motion.
• Comment: the forces exerted by the particles is evenly distributed over the entire container surface that's why particles are assumed to have random motion in 3 dimensions.

5) The collisions of particles with each other and with the container surface are perfectly elastic, so that no kinetic energy is lost.
• Comment: the total kinetic energy of particles is the internal energy of the gas, therefore kinetic energy is assumed to be conserved so that the total energy of the gas in the total kinetic energy of the molecules.


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How is everyone feeling for this exam? Im starting to understand alot of the content now which is great haha
Do they ask 6 markers on both parts to the unit 5 exam?
Original post by SuperMushroom
How is everyone feeling for this exam? Im starting to understand alot of the content now which is great haha


Not feeling very great but ok. How about you?


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Original post by SuperMushroom
Do they ask 6 markers on both parts to the unit 5 exam?


Yeah, they do


Posted from TSR Mobile
Quick question!

If you have a question which involves rearranging equations, i.e. v=Hd to d=v/H or something more complicated involving combining multiple equations and even substituting them into one another, can you simply show the final rearranged equation in symbols and then use the values to get your answer? Or will you lose method marks?
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Avatar for Mai.H
Mai.H
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There's only been one thermal 6 marker in the past do you think that makes a thermal 6 marker more likely? Not that it matters too much!
Technique for estimating the age of the universe with Hubble's constant?
Original post by Mehrdad jafari
Guys, I also have a feeling that a six marker could be on commenting on the assumptions made before deriving the equation of the kinetic theory of gases. The textbook does comment on two of the assumption but no further. Here are some comments relating to the assumptions made:

1) The volume of each molecule is negligible compared to the volume of the gas.
• Comment: that is to say the molecules of the gas are very far apart because in liquid state the molecules are sufficiently close to each other for the attractive forces between molecules to be important.

2) The time of each collision between molecules or each molecule with the container walls is negligible compared with the time between successive collisions.
• Comment: ( i still don't really know why this assumption is made. I would be very happy if any one could let us know.)

3) The molecules do not attract each other.
• Comment: if the particles attracted each other they would not exert force on the walls of the container as they would all tend to clump together in the middle of the container.

4) the particles move in continual random motion.
• Comment: the forces exerted by the particles is evenly distributed over the entire container surface that's why particles are assumed to have random motion in 3 dimensions.

5) The collisions of particles with each other and with the container surface are perfectly elastic, so that no kinetic energy is lost.
• Comment: the total kinetic energy of particles is the internal energy of the gas, therefore kinetic energy is assumed to be conserved so that the total energy of the gas in the total kinetic energy of the molecules.


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For why the time elapsed for each collision is negligible compared to the time between collisions. This is due to what you have used in your equation. In the derivation of the kinetic equations you say that delta t is the time taken for the molecule to travel from one side of the container to the other and back. Hence dt is not the time duration of each collision, but an average time which requires that the actual time of collision is small compared to the time between. I don't think they will ask that in the exam. It's to do with your assumption in the motion of the particle itself
Original post by Protoxylic
For why the time elapsed for each collision is negligible compared to the time between collisions. This is due to what you have used in your equation. In the derivation of the kinetic equations you say that delta t is the time taken for the molecule to travel from one side of the container to the other and back. Hence dt is not the time duration of each collision, but an average time which requires that the actual time of collision is small compared to the time between. I don't think they will ask that in the exam. It's to do with your assumption in the motion of the particle itself


"Hence delta t is not the time duration of each collision, but an average time which requires that the actual time of collision is small compared to the time between."
Would you explain that a bit more. I remember Amanzz said something similar to this but if dt is not the time of each collision then the assumption is somewhat wrong.


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Original post by Mehrdad jafari
"Hence delta t is not the time duration of each collision, but an average time which requires that the actual time of collision is small compared to the time between."
Would you explain that a bit more. I remember Amanzz said something similar to this but if dt is not the time of each collision then the assumption is somewhat wrong.


Posted from TSR Mobile


dt is the time between collisions. So this is the overall time it takes to change the momentum of the particle from it's initial position (wall A) by 2mux in the x direction. So that when time dt is elapsed, the particle is at the same position as it was, but now with the opposite momentum. This requires that the time of the collision is very small in comparison to the length of time between the start and end points of which you're describing the change in momentum/force. Otherwise if the collision takes place over a larger time interval, then it HAS to be true that the speed is not constant (which is the case in reality) and this dt cannot be calculated using the magnitude of vx as it isn't constant, it changes with time as there is a force acting on it at the boundary.
(edited 8 years ago)
Original post by IWantSomeMushu
Quick question!

If you have a question which involves rearranging equations, i.e. v=Hd to d=v/H or something more complicated involving combining multiple equations and even substituting them into one another, can you simply show the final rearranged equation in symbols and then use the values to get your answer? Or will you lose method marks?


I'd say depends on the question - a low mark question on a simple rearrangement of a 3 term equation, going straight to your final rearrangement (eg the v=Hd could be written as d =v/H) shouldn't lose you any marks as far as I know. A complicated substituting things in etc, I'd definitely show the steps.

Original post by Mai.H
There's only been one thermal 6 marker in the past do you think that makes a thermal 6 marker more likely? Not that it matters too much!


Not necessarily, thermal is a pretty small topic and except for experiments (which they don't seem so keen on asking about at A2 as they were at AS) there's not so many possible six markers really whereas nuclear has lots. However 1 in 5 so far and the last in 2011 does make it seem like it's got a good chance of popping up this year!
Original post by Protoxylic
For why the time elapsed for each collision is negligible compared to the time between collisions. This is due to what you have used in your equation. In the derivation of the kinetic equations you say that delta t is the time taken for the molecule to travel from one side of the container to the other and back. Hence dt is not the time duration of each collision, but an average time which requires that the actual time of collision is small compared to the time between. I don't think they will ask that in the exam. It's to do with your assumption in the motion of the particle itself


Dude you seem so clever! , always see you in these threads! I take it you want to study chemistry or something at university?


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Original post by SuperMushroom
Dude you seem so clever! , always see you in these threads! I take it you want to study chemistry or something at university?


Posted from TSR Mobile


Offer holder for NatSci at Cambridge :colondollar:. My passion is in Chemistry, however. But (grades permitting) I get to continue with Physics, Chem, Maths and take up Material Sciences at Cambridge :tongue:

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