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A2 Daily A Level Physics question

2025-08-23 OCR A A Level Physics 6.1.1(a) 6.1.3(a)(i)

Two identical capacitors, one initially at 6.0 V and the other uncharged, have their negative plates joined. At t = 0 their positive plates are linked through a resistor R. Which statement must be true?

  1. A Just after connection the current through R is 6 V/R from the charged to the uncharged capacitor; both settle to 3 V; the rate of change is fastest at t = 0 and then slows. (correct)
  2. B Just after connection the current through R is 3 V/R and increases toward 6 V/R as the voltages equalise; the final voltage on each is 3 V.
  3. C The two voltages move linearly toward 3 V with equal drops per second; the equalisation is complete after a time equal to RC.
  4. D Both capacitors finally reach 6 V; the current halves every RC from the moment of connection.

Answer

The correct answer is A.

Correct: A — Just after connection the current through R is 6 V/R from the charged to the uncharged capacitor; both settle to 3 V; the rate of change is fastest at t = 0 and then slows. The resistor initially sees the full 6 V difference, so the current is largest at t = 0; charge then redistributes until both identical capacitors share at 3 V, with an exponential-like slowing of the change. B confuses terminal and instantaneous values (uses 3 V/R initially and claims the current grows), which is opposite to RC transients where the current falls from its initial maximum. C treats the change as linear and assigns a duration RC; in reality the approach to 3 V is non-linear (halving in equal time steps), and the relevant timescale for this two-capacitor sharing is shorter than RC. D gives the wrong final voltage (they share to 3 V, not 6 V) and also misuses RC as the interval for halving, which is not correct for this arrangement.

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