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Atomic and Nuclear Physics
 
Aim: To measure the Charge to Mass Ratio of Electrons
To perform this experiment, you must have a cathode ray tube something like that shown in the diagram below.
If your c.r.t. is not similar to the one shown, read the manufacturer’s user guide.
 
If necessary, see the following sections before starting:
Electric field strength
Motion of charged particles in magnetic fields
Measuring the charge to mass ratio of particles
Cathode ray tube.
 
Method
The method proposed here is similar to that used by J.J. Thomson in 1897.
Electrons in an evacuated tube are sent towards a region of space where there are electric and magnetic fields at 90° to each other.
If the field strengths have a particular ratio then charged particles, moving with a certain velocity, can pass through them undeflected.
 
The filament is heated to high temperature by the low voltage supply thus causing it to emit electron.
The high voltage across the anode and filament (cathode) produces an electron beam.
The beam is directed towards a fluorescent screen.
Notice that, in the setup shown above, the electric field deflecting the beam is derived from the same high voltage source as is used to accelerate the electrons towards the screen.
This means that all we have to do is vary the current flowing through the two Helmholtz coils in order to produce an undeflected beam.
 
To calculate the magnitude of the electric field strength, E we use
where V is the potential difference across the deflecting plates and d is the distance between them.
 
To calculate the magnetic flux density, B near the centre of two Helmholtz coils (in a vacuum), we use
where μo is the permeability of free space, N is the number of turns on one of the coils, r is the radius of the coils and I is the current flowing through them.
 
N, I, r, V and d can all be measured with reasonable precision so what is the greatest source of error likely to be in this experiment?
It will probably be easier to answer this when you have done the experiment...
 
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