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8.1 FROM THE UNIVERSE TO THE ATOM

THE ATOM

P81 003

(A)

The debate as to whether cathode rays were charged particles or electromagnetic waves continued for many years. Which observation of cathode rays resolved this debate? Clearly distinguish the wave and particle properties.

(B)

A discharge is shown.

(B1) What type of pattern is shown?

(B2) What end is the cathode?

(B3) Explain the pattern observed?

(B4) If the gas was changed, does the pattern change significantly?

(B5) If the pressure was changed, does the pattern change significantly?

View solution below only after you have completed the answering the question.

Solution

(A)

Waves transfer energy without any material being transformed. At first cathode rays were thought to be waves like X-rays which has been recently discovered.

Cathode rays were deflected both by electric and magnetic fields indicating that cathode rays were a stream of negatively charged particles and J J Thompson measured the q_e/m_e ratio providing compelling evidence that cathode rays were a beam of negatively charged electrons.

(B)

(B1) Striations at the left, Faraday’s dark space in the centre and the cathode glow at the right-hand side of the discharge tube.

(B2) Right-hand side is the cathode – striations are nearer than anode

(B3) See link Electrical discharges

(B4) Pattern will not significantly change but molecules of different gases emit radiation of different frequencies in such transitions. Therefore, the glow of the positive column has a characteristic colour for each gas.

(B5) The pressure of the gas is mainly responsible for the observed patterns.