# Charged Particles in Magnetic Fields

This is part of the HSC Physics course under the topic Charged Particles, Conductors and Electric and Magnetic Fields.

### HSC Physics Syllabus

• analyse the interaction between charged particles and uniform magnetic fields, including: (ACSPH083)

acceleration, perpendicular to the field, of charged particles
the force on the charge F=qv_(_|_)B = qvBsintheta

### Charged Particles Interacting with Magnetic Fields

This video analyses the interaction between moving charged particles and uniform magnetic fields.

### Force on Charged Particles in Magnetic Field

moving charged particle experiences a force within an external magnetic field. The magnitude of the force is expressed by:

F_m=qvBsintheta

When the angle between a charged particle’s motion and the magnetic field is 90º, the equation can be simplified to:

F_m=qv_(_|_)B

The direction of magnetic force acting on the charged particle is determined by the right-hand palm rule. The diagram below shows the rule for a positively charged particle. For a negative particle, the thumb must point in the opposite direction to the particle's motion. ### Uniform Circular Motion in Magnetic Fields Since the magnetic force always acts perpendicularly to the charged particle's velocity, it supplies a centripetal force. The particle is thus brought into uniform circular motion.

F_m=F_c

qvB=(mv^2)/r

qB=(mv)/r

Practice Question 1

An electron is fired into a magnetic field, as shown in the diagram below. (a) On the diagram, sketch the path followed by the electron once it enters the magnetic field. (1 mark)

(b) If the magnetic field has a strength of 0.05 T and the velocity of the electron is 300 m/s, calculate the force experienced by the electron once it enters the magnetic field. (1 mark)

(c) An additional electric field can be applied to negate the magnetic force acting on that electron such that the particle can pass through both fields undeflected. Calculate the strength of the electric field that is required to achieve this and outline how the charged plates must be orientated. (2 marks)

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