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When a charged particle of mass m moves in a magnetic field of flux density B at a velocity v there is a force on it. If the field is at right angles to the motion of the particle this force is:

F = Bqv where q is the charge on the particle

This force is always at right angles to the motion and so the particle is deflected into a circle of radius R.


Magnetic force: Bqv = mv2/R

Example problem
A proton (mass 1.66 x 10-27 kg) with a charge to mass ratio (q/m) of 9.6x107 Ckg-1 moves in a circle in a magnetic field of flux density 1.2T at 4.5x107 ms-1 .
Calculate: (a) the radius of the circle
(b) the kinetic energy of the proton

(a) R = mv2/Bqv = mv/Bq = 4.5x107/(1.2x9.6x107) = 0.39 m
(b) kinetic energy = mv2 = 0.5x1.66x10-27x(4.5 x 107)2 = 1.68x10-12 J = 10.5 MeV


A particle of charge +q in an electric field of intensity E experiences a force Eq towards the negative plate .

If the plates are distance d apart and the p.d between them is V volts then E = V/d and the force is given by:

Electrostatic force: Force = Eq = Vq/d

Unlike a magnetic field, in an electric field a charged particle at rest will still feel a force.

Example problem
A proton passes though an accelerating gap in a particle accelerator. If the gap is 0.4 cm wide and the p.d between the two sides is 50 kV calculate:
(a) the force on the proton
(b) the acceleration of the proton

Force = qV/d = [1.6x10-19x50x103]/0.4x10-2 = 2x10-12 N
Acceleration = F/m = 1.2x1015 ms-2
© Keith Gibbs 2011