Transcript How to calculate e/m

How to calculate the momentum
of an electron ?
Measuring the radius of curvature in a
magnetic field …
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
If electrons are entering a magnetic field,
they are forced to go on a circle.
They go on spirals if they are also
loosing energy
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
The Lorentz-law helps to describe this
behaviour :

 
FL  qv  B
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001

B
goes into the page !
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
The Lorentz-force is the centripetal force,
which pulls the electrons to the centre :
 
ev B 
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
2
mv
r
Look at this sketch…
…click it !
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
L  (r  D)  r
2
07/17/2001
2
Dora Wagner & Miguel Vieira
HST 2001
2
How to measure the radius ?
L D
r 
2D
2
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
2
r=?
If we know the radius r …
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
B= ?
… and if we know the strength of the
magnetic field B…
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
 
ev B 
2
mv
r
… we are able to calculate the momentum :

 
eBr  m v  p
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
Try it !
What to measure,
what to know
Scale 1 : 6.4
For more information
click here
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
In the Fermilab
where this picture
was taken, the
magnetic field
was :
B = 3 Tesla
The charge of the
observed electron
is :
e = 1.6x10-19
Coulomb
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
Are you able to use the Pythagoras theorem to
calculate the radius r from L and D ?
BACK
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001
A photon
traveled
between the
two marked
points. The
traveled
distance was, in
the real
experiment,
11.5 cm!
If you want to know more,click-me
07/17/2001
Dora Wagner & Miguel Vieira
HST 2001