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Potential Difference: Energy Carried per Coulomb

Potential difference is energy transferred per coulomb, measured in volts. Why a voltmeter goes in parallel, and how the volts share out in a series circuit.

Potential difference

Potential difference is the energy transferred per coulomb of charge as it moves between two points in a circuit, measured in volts.

يُسمّى أيضاً: Voltage, p.d.
أين يقابله الطلاب: Grade 8 science, and again at GCSE and IGCSE in every use of V = I × R and in every series and parallel circuit question.

الإجابة باختصار
Potential difference is the energy transferred per unit of charge between two points in a circuit, measured in volts, where one volt is one joule per coulomb. A lamp with 3 V across it transfers 3 joules of energy for every coulomb of charge that passes through it. It is always a difference between two points.

مثال

V = E ÷ Q = 6 J ÷ 2 C = 3 V

If 2 coulombs of charge pass through a component and 6 joules of energy are transferred there, the potential difference across it is 3 volts. Read the unit as what it is: 3 joules per coulomb. That reading also tells you what a 12 V battery is promising — 12 joules of energy handed to every coulomb it sends round the circuit.

The name is the definition

You can speak of the current through a lamp, because charge really does flow through it. You cannot speak of the potential difference at a lamp in the same way. There is a p.d. across it — between one end and the other — and a single point in a circuit has no potential difference any more than a single place has a distance.

This is why the two meters are connected so differently. A voltmeter has to touch two points, so it goes in parallel with the component, bridging it. An ammeter has to have the charge pass through it, so the circuit is broken and the meter inserted in series.

Voltmeter: in parallel, across the component, and built with very high resistance so it draws almost no current
Ammeter: in series, in the path of the charge, and built with very low resistance so it barely reduces the current it is measuring

How the volts divide

In a series circuit the potential differences across the components add up to the potential difference of the supply, because the charge passes through each component in turn and gives up some of its energy at each. Two identical lamps on a 6 V supply take 3 V each. If one lamp has twice the resistance of the other, it takes twice the share: 4 V and 2 V, still totalling 6 V.

In a parallel circuit every branch is connected directly across the supply, so every branch gets the full 6 V. That is the reason household lighting is wired in parallel — each lamp runs at its proper brightness, and switching one off leaves the rest unchanged.

Potential difference is not current

Current is the rate of flow of charge, measured in amperes, where one ampere is one coulomb per second. Potential difference is the energy each coulomb carries. One counts how much charge goes past per second; the other measures what each unit of it delivers. Power needs both, which is why P = V × I.

The mental model that causes trouble is the one where voltage flows. Nothing flows but charge. A cell does not push volts into a circuit; it maintains a potential difference across it, and the resistance of the circuit then determines how much current that potential difference drives.

أسئلة شائعة

Is voltage the same as potential difference?

In school physics, yes — voltage is the everyday name and potential difference is the formal one, and both are measured in volts. Written work should use potential difference, because the longer name contains the reminder that keeps the topic straight: it is a difference, measured between two points.

Why must a voltmeter go in parallel?

Because it is comparing two points, and it can only do that if it is connected to both of them. Placing it in series would break the very circuit it is measuring, and since a voltmeter has an extremely high resistance, the current would drop to almost nothing and the reading would be meaningless.

Why do the readings add up in series but not in parallel?

In series each coulomb passes through every component in turn, spending part of its energy at each one, so the shares must total what it started with. In parallel each coulomb goes through only one branch, and every branch is connected straight across the supply, so each sees the full potential difference.

Does a battery supply current or voltage?

Potential difference. A cell maintains a fixed p.d. across its terminals; how much current flows then depends on what you connect. The same 1.5 V cell drives a tiny current through a large resistance and a dangerous one through a short circuit, and the cell has done nothing different in either case.

آخر تحديث 21 أغسطس 2026