Electric field strength depends on potential difference and plate separation

Steps:

• Recall the formula for electric field strength between parallel plates: E = V/d, where V is potential difference and d is plate separation.
• The setup includes a resistor, but for steady-state DC, V across plates equals supply voltage if no charging current flows significantly.
• Evaluate each change: increasing d directly reduces E if V is constant; increasing V increases E; resistor change affects charging but not steady E; plate area affects capacitance but not steady E.
• Conclude that only increasing d decreases E.

Why A is correct:

• From E = V/d, increasing d while V remains constant decreases E, as field strength is inversely proportional to separation (Coulomb's law for plates).

Why the others are wrong:

• B: Increasing V directly increases E via E = V/d.
• C: Increasing resistance slows charging but steady-state V and E remain unchanged.
• D: Increasing plate area increases capacitance but steady-state E = V/d is unaffected.

Final answer: A