Explanation about how a charged particle acts when it goes into magnetic field

This article gives knowledge about how a charged particle moves when it enters into a magnetic field. In this article, three cases of motion of a charged particle in magnetic field is explained with diagrams and mathematical explanation. The definition for pitch of helix is also given in this article.

Motion of a charged particle in a magnetic field:

Suppose a charged particle of charge 'q' travels with velocity v in magnetic field B, then that particle experiences a force F is given by
Equation for force
For the above equation we consider the three cases
1. Suppose if the charged particle is at rest in a magnetic field, i.e., v=0, then the charged paticle doesn't experience any force.
2. If the charged particle is moved along the direction of the magnetic field, then also the particle doesn't experience any force because velocity 'v' and magnetic field 'B' are parallel.
3. If the charged particle is moving perpendicular to the magnetic field, then the charged particle experiences maximum force.
Now we have to consider the motion of the charged particle in a magnetic field as shown below figure.
Diagram for charged particle
At that instant, consider the case of a charged particle goes in a magnetic field at a right angle to the magnetic field. As the charged particle is moved perpendicular to the magnetic field, then the force acted upon the charged particle is perpendicular to the direction of motion of particle and direction of field such that there is no work done by the magnetic field on the charged particle which shows that the particle does not get in any kinetic energy. As a result, the velocity of the charged particle remains unchanged. But the direction of the charged particle is changed. It comes about in practical. Here three parameters v, B and F remain constant. At that instant we treat F as Fmax or Fm. This force provides necessary centripetal force for circular path of charged particle.
Now we get Fmax or Fm=qvB-------2
Let the acceleration come from the above force is 'a', then
equation for force
From equation 3, we get
Then according to the angular velocity definition, angular velocity w=qB/m.
and frequency
The frequency here we found that is indenpendent of v. This frequency can be known as cyclotron frequency.
Some particles are faster and some particles are slower. Therefore, faster particles can move in bigger circle, on other hand slower particles can move in smaller circle.
Now we consider a case, in which a charged particle makes angle with direction of magnetic field as shown in the below figure.
Diagram for different cases of charged particle when it enters into magnetic field
Then we can analyse that velocity can be resolved in two components as Vx and Vy. Vx is taken along B and Vy is taken along normal to B. For figure(1), we can know that the angle between Vx and B is zero such that Vx is uneffected by the field. Therefore, the particle will move in the direction of magnetic field with constant velocity. Where as the component Vy is perpendicular to the magnetic field B, in this case the particle takes circular path in Y-Z plane. The radius of the circle is given as
Altogether, we can result for a particle which is moving by making an angle with magnetic field B as helix. This is the superposition of two motions in which one is in the direction of magnetic field with constant velocity and second is circular motion which is perpendicular to magnetic field B. We can find the pitch of the helix as the distance between any two successive circular paths. Therefore
Pitch p=velocity*time to describe a circular path
The component of Lorentz force in the direction always becomes zero because
Equation for fx
As Vx remains constant, therefore
Equation for fY
The angle between Vy and B becomes 90 degrees because Fy is maximum.

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Guest Author: Priyadarshi01 Jul 2014

Thanks for this article. Will this motion be valid if the velocity of the particle is variable?

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