PHYSICS · Single correct · A Level · Physics · Fields and their consequences · Fields and their consequences
Problem
A circuit contains a battery, a fixed resistor, and a small light-emitting diode (LED) wired in series. An initially uncharged capacitor is wired in parallel with the LED. Immediately after the switch is closed, the LED remains dark, but several seconds later it glows steadily. Which of the following best explains this observation?
(a) Initially the uncharged capacitor acts like an open circuit, so no current can flow in that branch; after it is charged, current finally reaches the LED and it glows.
(b) The battery’s output voltage rises slowly after the switch is closed, reaching the LED’s forward voltage only after several seconds.
(c) The uncharged capacitor initially offers a low-resistance bypass around the LED, diverting the current; after the capacitor charges, that path is effectively open and the current instead passes through the LED.
(d) After a short delay the capacitor discharges through the LED, supplying the energy that makes it glow.