Direction of movement and sign of the charged particle

In summary, the particle experiences upwards force. But how to determine the direction of motion? I think there are two possibilities: if the particle is positive, it moves from Q to P and if it is negative it moves from P to Q.
  • #1
songoku
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Homework Statement
The diagram (please see below) shows the track of a charged particle in a magnetic field. The field is at right angles to the plane of the paper and its direction is out of the plane of the paper. AB is a thin sheet of lead that the particle passes through. What is the direction of movement of the particle and the sign of the charge of the particle?
Relevant Equations
Fleming left hand rule
1682755974178.png


From the picture, the particle experiences upwards force. But how to determine the direction of motion? I think there are two possibilities: if the particle is positive, it moves from Q to P and if it is negative it moves from P to Q.

Thanks
 
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  • #2
Do you notice something in the shape of the trajectory?
 
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  • #3
BvU said:
Do you notice something in the shape of the trajectory?
Yes, that also confuses me. I don't understand why the trajectory is not an arc of a circle. It curves more at Q part so I am thinking the radius of the trajectory is smaller at Q and maybe the speed is also lower but I don't know what causes the change in speed.

Thanks
 
  • #4
Regarding the change in the shape of the curve as it passes through the lead: maybe you have not been taught what happens to a charged particle in this sort of situation? It has a small chance of hitting a nucleus and scattering at a wide angle. It will tend to lose some energy to scattering off atomic electrons. If it loses enough energy it will tend to get stuck in the lead. If it only loses a little it will just slow down. A slower charged particle will move in a smaller radius. You can read more about that here.

https://openpress.usask.ca/physics155/chapter/8-3-motion-of-a-charged-particle-in-a-magnetic-field/

As to the sign of the charge: Remember Fleming's left hand rule, as you have cited in your problem statement. You have the direction of the force and the direction of the magnetic field. And you know the path of the charge. So if the charge is positive it is going this way, and if it's negative it is going that way. If you have difficulty getting your hand to twist round the right way, make yourself a little paper thingy that has three "prongs" pointed the correct way to represent your left hand. Color them to make it easy to see which is which prong is force, which is current, and which is magnetic field. Then twist it around so the force and magnetic field are correct. Then see which way the current is supposed to be going.
 
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  • #5
songoku said:
the radius of the trajectory is smaller at Q and maybe the speed is also lower but I don't know what causes the change in speed
Not maybe lower but definitely lower. Regardless of the mechanism, a change that lowers speed is infinitely more likely than a speed boost ...

##\ ##
 
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  • #6
Thank you very much for the help and explanation BvU and Grelbr42
 

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