A2 Daily A Level Physics question
In a lab, two parallel metal plates are used to create a uniform electric field. With a 1.0 cm gap, a potential difference of 200 V gives the required field to suspend a tiny charged bead. The spacer is changed so the gap becomes 2.5 cm. To produce the same field strength, what potential difference is needed now?
Answer
The correct answer is D.
Correct: D — 500 V. A assumes the potential difference should decrease when the plate separation increases, incorrectly treating V as inversely proportional to separation. B assumes the field strength does not depend on plate separation, so keeps V the same. C roughly doubles V as if the gap only doubled, ignoring that it increased by a factor of 2.5. D scales V in direct proportion to the separation (2.5× larger gap requires 2.5× the potential difference: 200 V × 2.5 = 500 V) to keep the field strength the same.