Example

A torch lamp takes a current of 0.3 A from a 3 V battery. Calculate its resistance.

Answer

V = 3 V

I = 0.3 A

R = \frac{\text{V}}{\text{I}}

R = \frac{\text{3 V}}{\text{0.3 A}}

R = 10 \Omega

The resistance of the lamp is 10 \Omega

Question

Calculate the reading on the ammeter in the circuit shown below.

Circuit with 12V power supply, ammeter resistor with 2.7 x 10 to the power of 3 Ohms and a voltmeter connected in parrallel.

V = 12.0 V

R = 2.7 x 103 \Omega

I = \frac{\text{V}}{\text{R}}

I = \frac{\text{12.0 V}}{\text{2.7 x 10}^3\Omega}

I = 0.0044 A = 4.4 x 10-3 A = 4.4 mA

The reading on the ammeter is 4.4 mA.

Question

3 A flows through a 240 V lamp. What is the resistance of the lamp?

R = \frac{\text{V}}{\text{I}}

V = 240 V

I = 3 A

I = \frac{\text{240 V}}{\text{3 A}}

R = 80 \Omega

The resistance of the lamp is 80 \Omega