Question

A battery of e.m.f. 3.0 V supplies current through a circuit in which the resistance can be changed. A high resistance voltmeter is connected across the battery. When the current is 1.5 A, the voltmeter reads 2.7 V. Find the internal resistance of the battery.

Answer 1

all capital denotions are for external factors while smaller are for internal
V = IR
R = 27/15
EMF = V + v
or , I(R+r) = EMF
or, (3 / 1.5 ) - 9/5 = r
therefore r = 0.2ohm

Answer 2

Use the Formula
V=I x R
R = V/I
R = 2.7 / 1.5
R = 1.8 ohm

i = e. m. f. / R+r
Or,
R+r = e. m. f. / i
1.8 + r = 3.0/ 1.5
1.8 +r = 2
r = 2 - 1.8
r = 0.2 ohm