There are 3 batteries in a series circuit, each containing 9V, and the total current is 1.5 A. Along the circuit is 4 resistors. The first is 3 ohms, the second is 6 ohms, the third is unknown, and the fourth is 5 ohms. What is the voltage drop across the unknown resistor

The voltage drop across the unknown resistor is 6 volts.

Explanation:

Given;

the current flowing in the series circuit, I = 1.5 A

number of batteries in the series circuit, = 3

voltage in each battery = 9V

total voltage in the circuit, 3 x 9V = 27 V

number of resistor in the circuit, = 4

R₁ = 3 ohms

R₂ = 6 ohms

R₃ = ?

R₄ = 5 ohms

Determine the total resistance in the circuit by applying Ohms law;

V = IR

R = V / I

R = 27 / 1.5

R = 18 ohms

Determine the unknown resistance;

R₁ + R₂ + R₃ + R₄ = 18 ohms

3 + 6 + R₃  + 5 = 18

R₃ = 18 - 14

R₃  = 4 ohms

The voltage drop across the unknown resistor (R₃  = 4 ohms);

V₃ = IR₃

V₃ = 1.5 x 4

V₃ = 6 volts.

Therefore, the voltage drop across the unknown resistor is 6 volts.