The experimental set up is enclosed in a highly evacuated glass vessel to avoid the deflection of silver atoms by gas. Each silver atom behaves like a magnet due to its magnetic moment resulting from spin angular momentum of its 5s1 electron. A beam of Silver atoms is passed in an inhomogeneous magnetic field. The silver atoms gets deflected in the inhomogeneous magnetic field due to the unequal forces experienced by poles of each 5s1 electron of an atom and is traced in a photographic plate. The presence of two curves on photographic film confirms that electron have spins either in clockwise or anticlockwise direction. The magnitude of the spacing between the curves on photographic film agrees well with experimental result. Thus, this experiment proves that space and spin of an electron is quantised and validates vector atom model.

1. Stern Gerlac Experiment
Course Facilitator: Dr. G. Little Flower
Department of Physics
Maris Stella college
Vijayawada

2. Outlines
• Angular momentum
• Inhomogeneous magnetic field
• Quantization of Spin
• Expression for space quantization.

3. Pre-requisite knowledge to understand Stern-Gerlach experiment
Angular Momentum:
 All rotating bodies will have angular momentum
 An electron has only two possible values of spins: s = +1/2 and s = -1/2
 So, angular momentum of an electron is quantized
Stern-Gerlach experiment involves magnetic fields
Properties of magnets:
 Opposite magnetic poles attract and similar poles repel each other.
no net force net force in inhomogeneous field
http://www.thephysicsmill.com/2015/02/22/the-stern-gerlach-experiment/

4. Orbiting or spinning electrons behaves like tiny magnets
(a) Expected (b) Observed
Inferences:
 A magnet can be oriented any way.
 But an electron can only have two orientations.
This is the principle of Stern Gerlac experiment.
A simple video:
https://www.youtube.com/watch?v=rg4Fnag4V-E

5. Verification of space quantization and electron spin by Stern-Gerlach experiment
 A beam of Silver atoms is passed in an inhomogeneous magnetic field.
 Intensity of the magnetic field increases as moved from centre towards the upper knife edge pole and
decreases as moved below towards the lower pole.
 A photographic plate records the configuration of the beam after its passage through the field.
 The whole arrangement is enclosed in a highly evacuated glass vessel to avoid the deflection
of silver atoms by gas.

6. (a) Field off: thin line pattern (b) Field on: Two Component pattern (c) Un-quantised pattern
Fig.(b) verifies the existence of electron spin and the postulate of space quantization.

7. Expression for the separation between two traces:
dB
dY
 Let the inhomogeneous magnetic field B be applied along Y axis.
 The magnetic field is increasing along Positive Y axis. The field gradient is is positive.
 The field strength at one pole is B, then the field at the other pole will be ( )
dB
B Cos
dY
 l .
l

silver atom is an atomic magnet with magnetic moment M, pole strength p and length
be inclined at an angle with the magnetic field direction.

9. The theoretical as well as experimental results are in excellent agreement. This strongly supports the
postulate of space quantization and the existence of electron spin.

10. Thank You!