Experiment / equipment
suggestion:
The gas discharge tube
This
a sealed glass tube filled with low-pressure air. At both ends there are
electrodes so one can apply a high DC-voltage over the length of the tube.
One
will observe a series of glowing rings successively further apart from one
another as one looks towards the anode.
The glowing appears because of collisions between the accelerating electrons and the atoms in the air. These collisions excite the atoms, which in turn de-excite by emitting photons. The reason we see this in the series of rings, is twofold:
1)
Firstly
the electrons need a certain amount of energy to ionize the atoms. This means
they will have to travel a certain distance along the tube to gather enough
energy.
2)
When
the electrons collide with the atoms however, they will lose energy and thus be
returned to an energy-state to low to excite the atoms. They will then have to
accelerate some distance before further ionization takes place.
This
experiment can be used both as a demonstration of accelerating electrons in an
electric field as well as a good demonstration of Bohr’s model of the atom.
Especially the last point is an important one as there are so few good
demonstrations of this idea.
Using
physical tables one can perform simple calculations of both energy and
kinematics.
Depending
on the national curricula one would use this in either in a first or a second
level physics course