VISUAL PHYSICS ONLINE

8.1 FROM THE UNIVERSE TO THE ATOM

THE ATOM

P81 004

The diagram shows four discharge tubes and the patterns of striations observed in them.

(A1) Explain the patterns.

(A2) Account for the difference in the striation patterns.

(A3) List the tubes in order of decreasing pressure.

(B)

Explain: Cathode rays have a constant charge to mass ratio, but canal rays have a variable charge to mass ratio.

View solution below only after you have completed answering the question. The solution is not in a form that you would answer in an examination. The answers are often in more detail to help improve your appreciation and understanding of the physics.

Solution

All the tubes contain the same gas, but the gas pressure is difference inside each tube.

Tube A (~ 1 mmHg): cathode glow (left) / Faraday’s dark space (centre) / striations (right)

Tube B (~40 mmHg): violet streamers

Tube C (~<0.1 mmHg): glass shows fluorescence due to impact of energetic electrons

Tube D (~10 mmHg): positive column

(A1, A2) See link: spectral tubes

(A3) The tubes have different pressures inside them.

Highest to lowest pressure B D A C

(B)

Cathode rays are a stream of electrons. All electrons have identical charge and identical mass. So, the charge to mass ratio is the same for all electrons.

Canal rays are a stream of positively charged ions. The positively charged ions can have different masses and different charges. So, canal rays can have a variable charge to mass ratio.