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Figure 1 shows a gas discharge tube devised by William Crookes in one of his investigations - AQA - A-Level Physics - Question 1 - 2017 - Paper 7
Question 1
Figure 1 shows a gas discharge tube devised by William Crookes in one of his investigations. When a large potential difference is applied between the cathode and an... show full transcript
Worked Solution & Example Answer:Figure 1 shows a gas discharge tube devised by William Crookes in one of his investigations - AQA - A-Level Physics - Question 1 - 2017 - Paper 7
Step 1
Explain how this experiment led Crookes to conclude that cathode rays are particles.
Answer
Crookes observed that the paddle wheel in the gas discharge tube rotated when a potential difference was applied. This indicated that there was a force acting on the wheel. Since the wheel rotated towards the anode, it can be concluded that cathode rays, which move from the cathode to the anode, exerted a force on the paddle. This evidence suggests that cathode rays behave as particles, as they impart energy and momentum to the paddle wheel.
Step 2
Explain how cathode rays are produced in a gas discharge tube.
Answer
In a gas discharge tube, a high voltage is applied across the cathode and anode, causing electrons to be pulled from the cathode. When these electrons gain sufficient energy, they ionize gas atoms in the tube, freeing more electrons. This creates a chain reaction, where multiple electrons are released, resulting in a beam of cathode rays traveling towards the anode.
Step 3
Suggest the effect that this absorption may have on the motion of the paddle wheel.
Answer
The absorption of air molecules by the walls of the tube would lead to a decrease in the number of air molecules that could collide with the cathode rays. This may cause the paddle wheel to rotate less efficiently or possibly slow down due to fewer collisions providing momentum to the wheel. The reason for this is that with fewer air molecules present, there is less resistance, resulting in less energy being transferred from the cathode rays to the paddle wheel.