07/17/2001 Dora Wagner & Miguel Vieira HST 2001

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 moving in a magnetic field, they are

forced in a circle.

If they lose energy as they more they go in spirals

07/17/2001 Dora Wagner & Miguel Vieira HST 2001

The Lorentz-law helps to describe this behaviour :

BvqFL

07/17/2001 Dora Wagner & Miguel Vieira HST 2001

goes into the page !B

07/17/2001 Dora Wagner & Miguel Vieira HST 2001

The Lorentz-force is the centripetal force, which

pulls the electrons to the centre :

rvm

Bve2

07/17/2001 Dora Wagner & Miguel Vieira HST 2001

Look at this sketch……then click on it !

07/17/2001 Dora Wagner & Miguel Vieira HST 2001

222 rD)(rL

07/17/2001 Dora Wagner & Miguel Vieira HST 2001

How do you measure the radius ?

2DDLr22

07/17/2001 Dora Wagner & Miguel Vieira HST 2001

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

… we are able to calculate the momentum :

rvm

Bve2

pvmrBe

07/17/2001 Dora Wagner & Miguel Vieira HST 2001

Try it !

For more information click here

What to measure,

what to know

Scale 1 : 6.4

07/17/2001 Dora Wagner & Miguel Vieira HST 2001

In 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

A photon travelled between the two marked points which was 11.5 cm in the real experiment.

If you want to know more,click-me

07/17/2001 Dora Wagner & Miguel Vieira HST 2001

Can you use Pythagoras’s theorem to calculate the radius r from L and D ?BACK