Intelligent assessment and revision tools for OCR A Physics

An oscilloscope uses horizontal parallel plates to deflect an electron beam that travels midway between them with constant horizontal speed. The plate separation is doubled while, at the same time, the potential difference across the plates is doubled. The plate length, beam speed and screen distance are unchanged. What happens to the beam’s vertical deflection at the screen, and why?

• A It increases, because a larger potential difference creates a stronger downward force.
• B It decreases, because the wider gap weakens the field between the plates.
• C It becomes zero, because the effects of increasing potential difference and spacing cancel completely.
• D It stays the same, because the field strength between the plates is unchanged when both changes are made.