COMEDK · Physics · 19. Current Electricity

A cell of emf E and internal resistance r is connected to two external resistances \(\mathrm{R_1}\) and \(\mathrm{R_2}\) and a perfect ammeter. The current in the circuit is measured in four different situations:
(a) without any external resistance in the circuit.
(b) with resistance \(\mathrm{R_1}\) only
(c) with \(\mathrm{R_1}\) and \(\mathrm{R_2}\) in series combination.
(d) with \(\mathrm{R_1}\) and \(\mathrm{R_2}\) in parallel combination.
The currents measured in the four cases in ascending order are

A a < d < b < c
B c < d < b < a
C a < b < d < c
D c < b < d < a

Verified Solution

Answer & Solution

Correct Answer

(D) c < b < d < a

Step-by-step Solution

Detailed explanation

The current \(I\) in a circuit with a cell of emf \(E\) and internal resistance \(r\) connected to an external resistance \(R_{ext}\) is given by \(I = \dfrac{E}{r + R_{ext}}\). The four cases for \(R_{ext}\) are: (a) \(R_{ext} = 0\), so \(I_a = \dfrac{E}{r}\). (b)…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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