Charge doesn't flow on its own — it needs a push, called potential difference, the same way water needs a height difference to flow.
Just as water needs a height difference to flow, charge needs a potential difference to flow — a cell maintains that difference.
Charges don't flow through a wire by themselves, just like water doesn't flow through a perfectly horizontal tube. Connect one end of that tube to a raised tank, though, and the pressure difference pushes water out the other end. For charges in a wire, there's no gravity involved — instead, electrons move only when there's a difference of electric pressure, called the potential difference, along the conductor.
This potential difference is usually produced by a cell or battery: the chemical reaction inside it generates a potential difference across its terminals, even before any current is drawn. Once connected to a circuit, this potential difference sets charges moving and produces current — and the cell keeps expending its stored chemical energy to maintain that current.
Formally, the potential difference V between two points is the work done per unit charge moved between them: V = W/Q. The SI unit is the volt (V) — one volt means one joule of work is done moving one coulomb of charge between the two points (1 V = 1 J/C).
A voltmeter measures potential difference. Unlike the ammeter, it's always connected in parallel across the two points being measured — it sits alongside the component, not inline with the current path.
Key exam points
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Potential Difference - Electricity | CBSE Class 10 Physics Chapter 12 | NCERT · Gurukul by Oswal