**Example 1**

There is a current of 1.2 mA in an electric circuit. Calculate the charge transferred by this current in 40 s.

**Solution.**

**Q = I x t**

Q = charge in coulombs (C)

I = current in amperes (A)

t = time in seconds (s)

Q = I x t = (1.2mA) x (40s) = (1.2 x 10^{-3 }A) x (40s) = **4.8 x 10 ^{-2} C **

**Example 2**

A student connects an electric fire to the mains supply. The power rating of the fire is 2.5kW.

(a) 1kWh of electricity costs 20p. Calculate the cost of keeping the fire on for 12 minutes.

(b) The electric fire is set to its maximum setting and connected to the 230V main supply. Calculate the current in the electric fire.

Solution.

(a) (20p) x (2.5kW) x (12/60h) = **10p**

(b)

**P = V x I**

P = power in watts (W)

V = voltage in volts (V)

I = current in amperes (A)

2.5 x 10^{3} W = 230V x I

I = (2.5 x 10^{3} W) / 230V = **11A**

**Example 3**

A technician is testing a filament lamp from a car. He connects the lamp to a test circuit with a 1.5V d.c. power supply.

(a) The current in the circuit is 0.18A. Calculate the resistance of the filament in the lamp.

(b) The lamp is designed to be used with a car with a 12V battery. When it is connected to the 12V battery, there is a current of 800 mA in the lamp. Calculate the power of the lamp.

**Solution.**

(a)

**V = I x R**

V = voltage in volts (V)

I = current in amperes (A)

R = resistance in ohms (Ω)

1.5V = 0.18A x R

R = 1.5V / 0.18A = **8.3Ω**

(b)

P = 12V x 800mA = 12V x 0.800A = **9.6W**