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Difficulty: HardConduction of Electricity Through Gases and Cathode Rays

In discharge tube experiments, electrical conduction in gases transitions through distinct physical regimes as the internal gas pressure is progressively reduced. Match each discharge phenomenon with its corresponding physical cause or operational pressure condition.

  • Formation of the luminous positive columnContinuous de-excitation and radiative recombination of gas atoms at pressures around 1.0 mmHg1.0\text{ mmHg}.
  • Appearance and expansion of the Crookes dark spaceIncrease in the mean free path of electrons, allowing them to accelerate past the cathode without frequent inelastic collisions at pressures around 0.01 mmHg0.01\text{ mmHg}.
  • Emission of high-velocity cathode raysBombardment of the metallic cathode by energetic positive ions at pressures below 0.01 mmHg0.01\text{ mmHg} releasing secondary electrons.
  • Complete cessation of electric current flowExtensive evacuation of gas molecules below 10−4 mmHg10^{-4}\text{ mmHg}, leaving insufficient gaseous ions to sustain collision ionization.

Answer

The correct pairings are: (1) Formation of the luminous positive column matches continuous de-excitation and radiative recombination of gas atoms at pressures around 1.0 mmHg1.0\text{ mmHg}; (2) Appearance and expansion of the Crookes dark space matches increase in the electron mean free path at pressures around 0.01 mmHg0.01\text{ mmHg}; (3) Emission of high-velocity cathode rays matches bombardment of the cathode by energetic positive ions releasing secondary electrons below 0.01 mmHg0.01\text{ mmHg}; and (4) Complete cessation of electric current flow matches extensive evacuation below 10−4 mmHg10^{-4}\text{ mmHg} leaving insufficient gas molecules for ionization.
Conduction through gases relies heavily on pressure. At moderate low pressure (1.0 mmHg1.0\text{ mmHg}), excited gas atoms emit light forming the positive column. Decreasing pressure to 0.01 mmHg0.01\text{ mmHg} increases the electron mean free path to produce the Crookes dark space and generates energetic cathode rays through ion bombardment. Extreme evacuation (<10−4 mmHg< 10^{-4}\text{ mmHg}) removes all gas charge carriers, halting electric conduction.

Step-by-Step Solution

1
Analyze the pressure regime of 1.0 mmHg1.0\text{ mmHg} in a discharge tube.
Identified the positive column as the main luminous region filling most of the tube due to atom excitation and light emission.
At this pressure, gas density is sufficient to undergo repeated inelastic collisions that excite atoms and produce visible glow.
2
Examine the physical origin of the Crookes dark space at 0.01 mmHg0.01\text{ mmHg}.
Understood that lower gas density increases electron mean free path.
Electrons near the cathode travel a longer distance before hitting gas particles, creating a dark gap where collisions do not occur.
3
Determine how cathode rays are emitted at very low pressures.
Linked cathode ray emission to positive ion impact on the cathode surface.
High electric fields accelerate residual positive ions to strike the cathode, causing secondary electron emission.
4
Evaluate the extreme vacuum limit below 10−4 mmHg10^{-4}\text{ mmHg}.
Concluded that current stops when gas particles are virtually absent.
Gases conduct electricity via ion and electron production from collision ionization; eliminating gas molecules prevents charge transport.

Key Concept

Pressure-dependent regimes of gas conduction and cathode ray generation
Estimated Time:2m 0s
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Topics

Conduction of Electricity Through Gases and Cathode Rays

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