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# Bubble/cloud chamber tracks watch

1. I just don't seem to be able to understand how to work out whether it's a track of a positron or an electron or whatever. I know we use Fleming's left hand rule, and I know that the current used is conventional (therefore the electron will move opposite to the direction of the current), but I have NO clue how to work it out when I'm presented with a question.

Is there an easy way? Or can someone explain it? (I always seem to forget the method I used before.. )
2. So is it Fleming's LHR that seems to be confusing you the most?
3. (Original post by trm90)
So is it Fleming's LHR that seems to be confusing you the most?
yes.
4. Just use the left hand rule. The FIRST FINGER is the direction of the magnetic FIELD (either into or out of the page), the thuMb is the Motion (in this case it is the centripetal force, i.e. it acts perpendicular to the motion of the particle in the direction of the centre of the curve) and your seCond finger will now show the Current.

Current goes in the same direction as the particle, it's positively charged.
Current goes in the opposite direction as the particle, it's negatively charged.
5. (Original post by aper_son)
Just use the left hand rule. The FIRST FINGER is the direction of the magnetic FIELD (either into or out of the page), the thuMb is the Motion (in this case it is the centripetal force, i.e. it acts perpendicular to the motion of the particle in the direction of the centre of the curve) and your seCond finger will now show the Current.

Current goes in the same direction as the particle, it's positively charged.
Current goes in the opposite direction as the particle, it's negatively charged.
see, I know that, but what confuses is me is the direction of the force. How do you know? (when they give you those bubble/cloud chamber pictures)
6. Find where the particle curves. The direction of the resultant force is what causes the particle to curve - it's the centripetal force.

Your thumb should point perpendicular to the motion of the particle, in the direction of the centre of the circle. If the particle is curving and not forming a complete circle, you need to imagine where the centre of the circle would be.

Try not to break your wrist
7. (Original post by aper_son)
Find where the particle curves. The direction of the resultant force is what causes the particle to curve - it's the centripetal force.

Your thumb should point perpendicular to the motion of the particle, in the direction of the centre of the circle. If the particle is curving and not forming a complete circle, you need to imagine where the centre of the circle would be.

Try not to break your wrist
All right, so now I know that where the force is. How do I know whether the particle would be following the current finger or going the other way?

If the finger is pointing away from the spiral's centre, does that mean the particle's travelling in the other direction (hence being an electron)? And vice-versa
8. Arrgh, you replied while I was editing my post!

The current is conventional i.e. positive. If your current finger points along the track, eventually toward the spiral centre then your particle, which is spiralling in, is positive. If it points the opposite direction, it would be negtive.

I know the particle tracks are pretty intimidating. You've just got to remember the rule and follow the path along
Attached Images

9. (Original post by aper_son)
Arrgh, you replied while I was editing my post!

The current is conventional i.e. positive. If your current finger points along the track, eventually toward the spiral centre then your particle, which is spiralling in, is positive. If it points the opposite direction, it would be negative.

I know the particle tracks are pretty intimidating. You've just got to remember the rule and follow the path along
Do you by any chance have the Edexcel A2 physics Miles Hudson book? If you do, the worked example on page 92 seems to go against what you've just told me! Why? Or am I interpreting it wrong?
10. Oh...um...err...
I have Letts Revise A2 Physics, a CGP book and Hodder Physics for Edexcel.
Let me do a bit of research into this...this is worrying me when the exam is only next week.
11. (Original post by aper_son)
Oh...um...err...
I have Letts Revise A2 Physics, a CGP book and Hodder Physics for Edexcel.
Let me do a bit of research into this...this is worrying me when the exam is only next week.
..I just used my brain (they say it's good for understanding things), and I think I might know why. The radius curvature of the particle is proportional to it's velocity, right? That would mean the slower it went, the smaller the radius - hence spiral inwards? {that's what it says in the book, and after thinking about it, it sort of makes sense}
12. (Original post by unamed)
..I just used my brain (they say it's good for understanding things), and I think I might know why. The radius curvature of the particle is proportional to it's velocity, right? That would mean the slower it went, the smaller the radius - hence spiral inwards? {that's what it says in the book, and after thinking about it, it sort of makes sense}
Yeah, that's right.

It's explained by the formula rQ=p/B (r = radius, Q= charge on particle, p = momentum of particle, B = magnetic flux density). As the particle ionizes the particles in the cloud/bubble chamber, it loses kinetic energy, thus the velocity decreases, thus the momentum decreases, thus the radius decreases (assuming Q, B and the mass of the particle are all constant).

What you have to do is look at the particle at one instant. The particle is, as you said, spiralling inwards. Thus you know which direction it is going in along the circle (clockwise or anticlockwise). If your current finger is pointing along the track spiralling inwards, the conventional current is inwards and thus the particle is +ve. If your current finger is pointing along the track spiralling outwards, you know that the particle has a -ve charge.

Hope this helps. It's quite hard to get your head around but once you do it's fine.
13. (Original post by unamed)
Do you by any chance have the Edexcel A2 physics Miles Hudson book? If you do, the worked example on page 92 seems to go against what you've just told me! Why? Or am I interpreting it wrong?
isnt the red track electron and yellow track psoitron??

or am i wrong??
14. (Original post by aper_son)
Yeah, that's right.

It's explained by the formula rQ=p/B (r = radius, Q= charge on particle, p = momentum of particle, B = magnetic flux density). As the particle ionizes the particles in the cloud/bubble chamber, it loses kinetic energy, thus the velocity decreases, thus the momentum decreases, thus the radius decreases (assuming Q, B and the mass of the particle are all constant).

What you have to do is look at the particle at one instant. The particle is, as you said, spiralling inwards. Thus you know which direction it is going in along the circle (clockwise or anticlockwise). If your current finger is pointing along the track spiralling inwards, the conventional current is inwards and thus the particle is +ve. If your current finger is pointing along the track spiralling outwards, you know that the particle has a -ve charge.

Hope this helps. It's quite hard to get your head around but once you do it's fine.
Thank you, I think I understand now.
15. (Original post by kosy91)
isnt the red track electron and yellow track psoitron??

or am i wrong??
I'm talking about the worked example opposite, for some reason, they say Track A's a positiron, even though it's pointing out of the spiral? Contrary to what aper_son says, do you know why?

Oh and the red track is the electron and the other the positron!
16. (Original post by unamed)
I'm talking about the worked example opposite, for some reason, they say Track A's a positiron, even though it's pointing out of the spiral? Contrary to what aper_son says, do you know why?
B-field is into the paper and perpendicular
the force is towards the centre(centripetal)
the flow of current is perpendicular to the motion as expected
but due to centripetal force, the current is going to be along the spiral bend and move towards the centre.
so it is a positron

i know i have explained badly, but its soo difficult ti put it into words....
if arrow of current was in opposite diection, then it would have been an electron ad the spiral is bending inwards
17. (Original post by unamed)
Thank you, I think I understand now.
18. (Original post by kosy91)
B-field is into the paper and perpendicular
the force is towards the centre(centripetal)
the flow of current is perpendicular to the motion as expected
but due to centripetal force, the current is going to be along the spiral bend and move towards the centre.
so it is a positron

i know i have explained badly, but its soo difficult ti put it into words....
if arrow of current was in opposite diection, then it would have been an electron ad the spiral is bending inwards
I think I get it now! Thank you...
19. (Original post by aper_son)
You shall receive some reppage soon.!

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