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# Brownian Motion. watch

1. possibly the smoke particles could be accelerating more slowly because theyre bigger?

I think it could mean they're "slower"
2. Nope, that explanation is rejected by mark scheme. And we only get the 1st mark for suggesting that the movement is slower. =/
3. It's one mark for the suggestion and one for the explanation.
The movement of the smoke is due to an exchange of momentum between the air molecules and the smoke molecules.
If the smoke molecules are "bigger" they are presumably also more massive. If they have larger mass then their change in velocity will be less because their mass is greater. (For the same change in momentum due to collision)
4. (Original post by Stonebridge)
It's one mark for the suggestion and one for the explanation.
The movement of the smoke is due to an exchange of momentum between the air molecules and the smoke molecules.
If the smoke molecules are "bigger" they are presumably also more massive. If they have larger mass then their change in velocity will be less because their mass is greater. (For the same change in momentum due to collision)
Yeah, I gave the same explanation but didn't get mark for THIS 'momentum' explanation.
What mark scheme says:
randomness of collisions would be ‘averaged out’ B1
so less (haphazard) movement B1 [2]
(do not allow ‘more massive so less movement’)
I don't understand mark scheme's explanation.
5. I presume they are referring to the fact that, as the smoke particles get larger, they are impacted by more air molecules at any one time. For small smoke particles, they will be hit by a small number of air atoms that are in random motion, and will move randomly in response. For large smoke particles, as more air molecules hit the smoke, from all sides and in all directions, the recoil motion will indeed become "evened out". There is more chance, for example, of being hit simultaneously on opposite sides, thus cancelling out the impacts.
I should have spotted this. There is a subtle difference between the smoke particles just getting more massive (which they do) and also getting larger.
If they were to just get more massive, the Brownian motion would be less, in the sense that the changes in velocity would be less, but they would still be just as random as they would be hit by the same small number of air molecules.
It's the increase in size that they want you to consider. Larger smoke particle = more collisions with air molecules and more mass.
I would argue that, if the mark scheme will not accept "more massive" and wants only consideration of size, it should have made it clear in the question that the smoke particles are larger but of similar mass. A small point, I agree, but probably worth including.
6. If you have a larger particle it will be more affected by more collisions (more things will collide with it) , these collisions will cancel out because for each collision forcing it right, there will be one forcing it left effectively and you will get less amplitude to the oscillations. I think I read that in my OCR book the other day.

Hope this helps
7. (Original post by Stonebridge)
I presume they are referring to the fact that, as the smoke particles get larger, they are impacted by more air molecules at any one time. For small smoke particles, they will be hit by a small number of air atoms that are in random motion, and will move randomly in response. For large smoke particles, as more air molecules hit the smoke, from all sides and in all directions, the recoil motion will indeed become &quot;evened out&quot;. There is more chance, for example, of being hit simultaneously on opposite sides, thus cancelling out the impacts.
I should have spotted this. There is a subtle difference between the smoke particles just getting more massive (which they do) and also getting larger.
If they were to just get more massive, the Brownian motion would be less, in the sense that the changes in velocity would be less, but they would still be just as random as they would be hit by the same small number of air molecules.
It's the increase in size that they want you to consider. Larger smoke particle = more collisions with air molecules and more mass.
I would argue that, if the mark scheme will not accept &quot;more massive&quot; and wants only consideration of size, it should have made it clear in the question that the smoke particles are larger but of similar mass. A small point, I agree, but probably worth including.
Yeah, they should've have told this in the question. You're absolutely right - thanks a lot for the explanation. And btw if the question mentioned that the particle size isn't changed, but the mass is, then could we apply the 'momentum' explanation?

If you have a larger particle it will be more affected by more collisions (more things will collide with it) , these collisions will cancel out because for each collision forcing it right, there will be one forcing it left effectively and you will get less amplitude to the oscillations. I think I read that in my OCR book the other day.

Hope this helps
Stonebridge explained it already. But thanks anyway!
8. (Original post by Zishi)

Stonebridge explained it already. But thanks anyway!
Yeah just seen his explanation now, bit more in depth than mine but nvm.
9. (Original post by Zishi)
Yeah, they should've have told this in the question. You're absolutely right - thanks a lot for the explanation. And btw if the question mentioned that the particle size isn't changed, but the mass is, then could we apply the 'momentum' explanation?
Yes of course, but with the condition below*
Most text books state that the effect is increased by
*increasing the temperature of the fluid
*decreasing the size of the suspended particles

The argument is that statistically, if the particle is small and the molecules are moving randomly, there is more chance of a particle receiving a bigger impact on one side rather than the opposite. In other words, the smaller the particle, the more likely the "statistical imbalance" and the more it will jump about.

*The effect of mass (on its own) is such that the larger the mass of the smoke particles, the lower will be their average speed.
You can also argue this from a kinetic theory viewpoint. As the fluid and the smoke particles are in thermal equilibrium, they will have the same mean kinetic energy. This means that if the mass of the particles is increased, their mean speed must be decreased.
Of course, if you consider a single collision from a momentum point of view, you can argue that the change in velocity of the larger mass will be less than that for a smaller one.

Hope that's cleared it up.
10. (Original post by Stonebridge)
Yes of course, but with the condition below*
Most text books state that the effect is increased by
*increasing the temperature of the fluid
*decreasing the size of the suspended particles

The argument is that statistically, if the particle is small and the molecules are moving randomly, there is more chance of a particle receiving a bigger impact on one side rather than the opposite. In other words, the smaller the particle, the more likely the &quot;statistical imbalance&quot; and the more it will jump about.

*The effect of mass (on its own) is such that the larger the mass of the smoke particles, the lower will be their average speed.
You can also argue this from a kinetic theory viewpoint. As the fluid and the smoke particles are in thermal equilibrium, they will have the same mean kinetic energy. This means that if the mass of the particles is increased, their mean speed must be decreased.
Of course, if you consider a single collision from a momentum point of view, you can argue that the change in velocity of the larger mass will be less than that for a smaller one.

Hope that's cleared it up.
Yippie. That really helped! Thanks.

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