Internal resistance from emf and voltage drop

Steps:

• Calculate terminal voltage: V = I × R = 0.26 A × 5 Ω = 1.3 V
• Apply emf formula: ε = V + I r, where ε = 1.5 V
• Solve for r: r = (ε - V) / I = (1.5 V - 1.3 V) / 0.26 A
• Compute: r = 0.2 V / 0.26 A = 0.77 Ω

Why D is correct:

• Matches the formula ε = V + I r, where internal resistance causes the 0.2 V drop across itself at 0.26 A current.

Why the others are wrong:

• A: Underestimates drop; assumes negligible internal resistance.
• B: Equals the drop voltage (0.2 V) instead of dividing by current.
• C: Approximates total resistance (R + r) without solving properly.

Final answer: D