JJ Thomson (1 Viewer)

[theodore0307]

What was the significance of JJ Thomson's experiment?

 

[Rhinoz8142]

His experiment showed the in measuring the charge to mass ration of the cathode ray particles the electrons..

he built a modified cathode ray tube with a built in electric plates and magnetic plates.The experiment was done in 2 stages

1) because of the varying magnetic field and electric field the opposing fields canceled out which makes the beam to be become undeflected this allowed thomson to calculate the velocity of the beam..

2) by applying the same strength magnetic field (ONLY) and determing the radius of the circle path travelled by the charged particles in the magnetic field.

He then combined the 2 results and obtained the magnitude of the charge to mass ratio for the charged particles

 

[Fizzy_Cyst]

Make sure at each of those 2 stages you using equations

i.e.,

stage 1: |Fe| = |Fb|

stage 2: Fc = Fb

then bring it all together

and really EXPLICITLY relate this to the signifiance i.e., it has mass -- therefore must be a particle and also relate it to its q/m ratio being ~1800x higher than Hydrogen, suggesting that it is 1/1800th the size of the H-atom, suggesting a sub-atomic particle --> leading to change in the model of the atom

 

[Rhinoz8142]

Make sure at each of those 2 stages you using equations

i.e.,

stage 1: |Fe| = |Fb|

stage 2: Fc = Fb

then bring it all together

and really EXPLICITLY relate this to the signifiance i.e., it has mass -- therefore must be a particle and also relate it to its q/m ratio being ~1800x higher than Hydrogen, suggesting that it is 1/1800th the size of the H-atom, suggesting a sub-atomic particle --> leading to change in the model of the atom

Holy shit.. I didnt know that JJ Thompson experiments had equations, it not in my textbook.

 

[theodore0307]

Holy shit.. I didnt know that JJ Thompson experiments had equations, it not in my textbook.

Charge to mass ratio

 

[itsalberttrinh]

In stage one Thomson attempts to calculate the velocity of the cathode rays by turning on the magnetic field and electric field such that the cathode ray passed through undeflected.
|Fe| = Fb
Eq = qvB (note theta = 90, sin90 =1)
v = E/B (where E and B are known values)

Stage two he set out the measure the radius of the curvature by switching on the magnetic field only (since the cathode rays entered the magnetic field perpendicularly they moved in a circular fashion). Through direct observation he measured the radius. Since the cathode rays moved in a circular motion he equated magnetic force (Fb) and centripetal force (Fc) to calculate the charge to mass ratio.
Fb = Fc
qvB = mv^2/r
q/m = v/rB (where v,r and B are known values)

He found that the charge to mass ratio was a constant 1.76x10^11 C/kg regardless of the cathode material used. The charge to mass ratio was such a large number (1800x greater than that of Hydrogen), that he concluded that the large value was due to the fact the cathode rays were made up of particles with such a small mass. There were not atoms but rather constituents of atoms, which he called 'electrons'.

 

[Quantum College]

The significant things about his experiment was that:

1. It ended the debate about whether cathode rays were streams of charged particles or electromagnetic radiation. They were particles because they had a charge to mass ratio. Anything with mass is a particle.

2. It led to the discovery of the first subatomic particle, the electron. I think rutherford discovered the proton shortly afterwards. Correct me if I am wrong. This discovery led to many great and wonderful things in physics that we should all be very grateful for.