How is heat produced by an electric current?

Explanation

A cell or a battery is a source of electrical energy.
The chemical reactions within a cell or battery generate potential difference between its two terminals.
This p.d. gets the electrons in motion, to flow through any circuit, 1.e., through a resistor or a system of resistors connected to the cell or battery.
So to maintain this flow, the source has to keep expending its energy.
But only a part of the source energy in maintaining the current may be consumed into useful work.
The rest of the source energy is converted into heat to raise the temperature of the electrical gadgets.