thomson's charge to mass ratio (1 Viewer)

[lil_kimmy]

can someone explain his experiment to me? i read the jacaranda text book but my teacher said it was wrong so now i'm completely confused!

 

[mojako]

it wasnt really wrong
strictly speaking it is a modified version of what Thomson did but almost every answer Ive seen do it that way so its ok I guess

Thomson actually didnt remove the electric field.. he let the rays go on... to a place where only the magnetic field exists

the one in Jacaranda assumes that the rays travelled horizontal in the first place without any need to use electric & magnetic field

 

[helper]

He did it with two set ups at different times. The Jac is one of them. Explained
http://hsc.csu.edu.au/physics/core/implementation/9_4_1/941net.html#net11
The other your teacher is thinking of had the mag and electric fields at different positions.

 

[speersy]

Just a quick explanation

In the cathode ray tube thomson balanced out the magnetic and electric fields so that the cathode rays were not deflected. By equating the expressions for these forces he was able to calculate the velocity of the electrons.

He then swithched off the electric field and measured the radius of the particles deflection. He combined the centripetal force and magnetic force to arrive at the charge to mass ratio

NB It would be better if u used equations in your answer

 

[lil_kimmy]

thank you! by the way.. what were the equations?

 

[gordo]

qvb = mv^2/r
qvp = Eq

solve them simulataneously etc

 

[sub]

equate qvB = qE.
=> v = B/E
from magnetic field producing a circle: (mv^2)/r = qvB
rearrange and simplify both sides and u get q/m = (rB^2)/E (all of which are known constants)
r is found emipirically (from the circle formed by the magnetic field)

 

[sub]

gordo, whats the p for? in qvp

 

[speersy]

Just combine the Magnetic force equation (it has a B and sin(theta)) with F=qe to get the velocity then the centripetal force with the same magnetic one to get the charge to mass ratio (q/m)

 

[smallcattle]

gordo, whats the p for? in qvp

where is the p?? its qvB

 

[gordo]

YEH
LOOK HOW CLOSE THE p is to the b on the keyboard
derrrrrrrrrrrrrr
obviously it was a typo

:shifty eyes:

 

[sub]

where is the p?? its qvB

in gordo's second line of working...