During the flow of electric current in a closed circuit, motion of electrons is continuously opposed by the resistance of the circuit. Hence, in order to maintain current in the circuit, cell or battery has to expend its energy. Now, question arises that where does this energy go ? Normally, some part of source energy is utilized in doing some useful work (like rotating the blades in a fan) and remaining energy is transformed into heat. If the circuit is purely resistive (i.e. source of energy is connected to only resistors) then source energy gets dissipated entirely in the form of heat (for example in electric kettle, electric heater, electric iron etc.) This is known as "heating effect of electric current.