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# Particle track interpretation: charge and momentum watch

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1. A particle entering from the bottom of the diagram decays into three particles at A. One of the particles leaves no track, but subsequently decays into two particles at B.

There is a magnetic field directed into the plane of the paper. What can you conclude about the charge and momentum of particles C and D?

Ans of book: Particles C and D must have equal and opposite charge...field. Applying Fleming's left hand rule, particle C must be negatively charged.
Particles C and D must have the same momentum, as r=p/Bq, and both tracks have the same radius of curvature.

Problems: 1. C is deviated to the left, and field is into paper, so according to fleming's left hand rule C must be positively charged. What is wrong with this reasoning, and how do the book arrive at the answer that C is positively charged?

2. Both C and D have same momentum. It always happens when two particles are formed, isn't it? But D's track is longer, what does that mean?
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A particle entering from the bottom of the diagram decays into three particles at A. One of the particles leaves no track, but subsequently decays into two particles at B.

There is a magnetic field directed into the plane of the paper. What can you conclude about the charge and momentum of particles C and D?

Ans of book: Particles C and D must have equal and opposite charge...field. Applying Fleming's left hand rule, particle C must be negatively charged.
Particles C and D must have the same momentum, as r=p/Bq, and both tracks have the same radius of curvature.

Problems: 1. C is deviated to the left, and field is into paper, so according to fleming's left hand rule C must be positively charged. What is wrong with this reasoning, and how do the book arrive at the answer that C is positively charged?

2. Both C and D have same momentum. It always happens when two particles are formed, isn't it? But D's track is longer, what does that mean?
The answer in the book must be wrong. Positive charges will be deflected to the left if the field is into the page.
The length of the track is determined by how quickly the particle loses its energy by collision with the atoms in the chamber. For example, an alpha particle and electron with the same momentum would not have the same track length because the alpha is more ionising, makes more collisions, and loses its energy more quickly than the electron.
A large slow particle will tend to have a shorter track that a small faster one.

3. (Original post by Stonebridge)
The answer in the book must be wrong. Positive charges will be deflected to the left if the field is into the page.
The length of the track is determined by how quickly the particle loses its energy by collision with the atoms in the chamber. For example, an alpha particle and electron with the same momentum would not have the same track length because the alpha is more ionising, makes more collisions, and loses its energy more quickly than the electron.
A large slow particle will tend to have a shorter track that a small faster one.

thnx, it's just frustrating!

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Updated: January 23, 2010
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