A2 Daily A Level Physics question
In a vacuum tube, two large parallel plates 5.0 mm apart are connected to a 200 V supply, producing a uniform electric field. An electron is released from rest at the negative plate and accelerates to the positive plate. The technician doubles the plate separation to 10.0 mm but keeps the potential difference at 200 V. Which statement must be true for the electron’s motion compared with before?
Answer
The correct answer is D.
Correct: D — The arrival speed is unchanged, and the travel time doubles. A The field is halved when the gap is doubled, but the energy gain (and so arrival speed) depends on the potential difference, which is unchanged; time cannot stay the same because distance is larger and acceleration is smaller. B Keeping the same potential difference keeps the arrival speed the same, but the weaker field and larger distance make the travel time longer, not unchanged. C The arrival speed does not increase because the energy change depends only on the potential difference; the ~40% idea confuses distance with energy gain. D Correct because with the same potential difference, the final speed is unchanged (same energy gain), while doubling the distance halves the field and acceleration; since t ∝ √(s/a), doubling s and halving a makes t increase by a factor of 2.