Magnetic Deflection and sensitivity:-

We know that when an electron is moving in magnetic field,the force exerted on an electron is constant and is perpendicular to the velocity and the magnetic field and thus travels in a circular path,having radius R=mv/Bq metres,where v=velocity of electron in m/s ,B=magnetic field intensity in Wb/m 2.

• This principle is used in cathode ray tubes to deflect an electron beam,which is called magnetic deflection system.


• Consider the rectangle ABCD representing the cross section of a uniform magnetic field.


• The direction of magnetic field is coming out of the paper,having an intensity B.


• The magnetic field is restricted in this region and zero outside this region.
• The electron moves in a straight line path with velocity v ox =root of (2eV a/m)
• When a particle like an electron enters this magnetic field,it experiences a constant force and is perpendicular to both,direction of motion and magnetic field.
• Thus the electron travels in a circular path.
• We know that the force of an electron in magnetic field is f m=Bev and as the electron is moving in a circular path it experiences centripetal force f=mv 2/R.
• In general these forces are equal,on equating we get R=mv/eB.
• Now after leaving the magnetic field at point M,the electron travels along straight line path MP' till it reaches the screen at P'.
• This line is a tangent to the circular path of an electron at M.
• If it is extended backwards it intersects x-axis at O',which is the centre of the magnetic field.This point is called as apparent origin.
• At an instant let the angle subtended by the arc at the centre of the circle be ô.
• Thus tanô=D/L,here D=magnetic deflection distance on the screen and L=distance from apparent origin to the screen.
• For smaller angles ô=D/L,i.e D=Lô,D=Ll/R=LleB/mv=lLeB/m*root of(m/2eV a).
• On simplification we get D=lLB*root of(e/2mV a).
• Magnetic deflection sensitivity:-
• It is denoted by S and is defined as deflection on screen in metres per tesla of magnetic flux density or ratio of magnetic deflection by magnetic flux density.
• Therefore,S=Ll*root of(e/2mV a).
• Thus it can be seen that by increasing V a,it does not affect S.
• Uses:-
• For smaller display area ,shorter tubes are enough with electromagnetic scheme of deflection.
• The more brightness and resolution is obtained with magnetic deflection system.
• The magnetic deflection system is preferred in T.V and radar while electrostatic deflection is preferred for general purpose oscilloscope.