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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 and that these particles caused the movement of the paddle.
Answer
The paddle wheel within the gas discharge tube rotates when a potential difference is applied between the cathode and anode. This indicates that there are moving particles exerting a force on the paddle. Therefore, Crookes concluded that cathode rays must consist of particles that are capable of transferring energy and momentum to the paddle, causing it to rotate. Since the movement is consistent with the behavior of mass, it supports the idea that cathode rays are indeed particles.
Step 2
Explain how cathode rays are produced in a gas discharge tube.
Answer
Cathode rays are formed when a high voltage is applied across a gas discharge tube, causing electrons to be pulled out of the cathode. The positive ions produced near the cathode are attracted back to it, which helps in the generation of new electrons that escape from the atoms. This process creates a stream of electrons that travel toward the anode, thus forming cathode rays.
Step 3
Suggest the effect that this absorption may have on the motion of the paddle wheel. Give a reason for your answer.
Answer
The absorption of air molecules by the walls of the tube would likely result in a reduction of air resistance within the tube. Consequently, fewer air molecules would collide with the moving electrodes, allowing the paddle wheel to rotate more freely and possibly at a higher speed. Since there are fewer obstacles to impede the motion of the paddle wheel, it can rotate more efficiently.