Imaginary numbers, or complex numbers, can be written as a real number multiplied by the imaginary unit i.

For example, 3i is an imaginary number and its square is -9. These numbers are frequently used by engineers.

The electricity we use at home is so-called AC power, or alternating current. This means that the current direction in our circuits is switched constantly back and forth. This switching leads to a time delay between the voltage and the current profile in our circuits. This time delay is described with imaginary numbers. Being able to accurately model and calculate the AC current used in devices at home, helps engineers to build well-working and energy-efficient devices. This means that imaginary numbers were used to design many devices in our daily lives, for example the washing machine, the microwave, and the toaster.