1. The problem statement, all variables and given/known data (a) In the circuit shown below E is a cell of source (internal) resistance r and the resistance of R is 4.0 Ω. With the switch S open, the high resistance voltmeter reads 10.0 V and with S closed the voltmeter reads 8.0 V. Show that r = 1.0 Ω. (b) If R were replaced by a cell of e. m. f. 4.0 V and source resistance 1.0 Ω with its negative terminal connected to B, what would be the reading of the voltmeter with S closed? Answer: (b) 7.0 V. 2. The attempt at a solution (a) E = I (r + R) → 10 = I (1 + 4) → I = 2 A. Then we find E = I r + V → E - V = I r → 10 - 8 = 2 r → r = 1 Ω. Looks correct. (b) Maybe something like 10 - 4 = 1 I3 + 1 I1, 10 - V = 1 I1?