electrochemistry
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Drop a battery into a flashlight and chemistry inside it quietly pushes electrons out one terminal, through the bulb, and back in the other. Run the reverse — plug something in and force current through a liquid — and you can plate a spoon with silver or split water into its gases. Electrochemistry is the study of this two-way street between chemical change and electricity.
More precisely, electrochemistry deals with reactions in which electrons are transferred between substances, and with how that electron flow can be made to travel through a wire as an electric current. When a reaction releases energy, it can drive a current (a battery); when you supply a current, it can drive a reaction that would not happen on its own (electrolysis). The handover always takes place at an electrode, the boundary where a metal or carbon surface meets a solution.
This matters because so much of modern life runs on it: every battery and fuel cell, the refining of aluminium and chlorine, electroplating, sensors that measure pH or blood sugar, and the slow rusting of iron are all electrochemistry. The field links the invisible bookkeeping of electrons to a quantity you can measure with a meter — voltage and current.
A lemon, a copper coin, and a zinc nail make a working cell: the metals react with the lemon's acid, and electrons pushed off the zinc travel through a wire to the copper — enough to flicker a tiny LED.
A lemon battery: chemistry on each metal turned into a current through the wire.
Electrons move through the wire, but inside the solution it is ions (charged atoms) that carry the charge — not loose electrons. Both kinds of motion together complete the circuit.