Charge, Current and Potential Difference

Electric current is the rate of flow of charge, $I=\dfrac Qt$ , measured with an ammeter in series. Potential difference (and e.m.f.) is the energy transferred per unit charge, $V=\dfrac WQ$ , measured with a voltmeter in parallel. Rubbing insulators transfers electrons, giving them static charge; like charges repel and unlike charges attract.

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Section 1 of 14

Learning objectives

Explain charging by friction and the forces between charges.
Distinguish conductors and insulators.
Use $I=\dfrac Qt$ and connect ammeters correctly.
Use $V=\dfrac WQ$ for e.m.f. and p.d., and connect voltmeters correctly.

Pause and think

You rub a balloon on your hair and it sticks to the wall. Nothing was added to the balloon except rubbing. Where did its charge come from?

Section 2 of 14

Core idea

Start hereA balloon on the wall

Rub a balloon on your hair and it sticks to the wall. Electrons have moved from your hair to the balloon. Charge moving like this, and charge flowing steadily through wires, are the same stuff behaving differently.

The big idea

Electrons (−) move; protons (+) stay. Current $I=\frac Qt$ ; potential difference $V=\frac WQ$ (energy per coulomb).

1. Static electricity

Charging by friction: electrons move from cloth to rod

Only electrons move. The rod and cloth end up with equal and opposite charges.

Like charges repel; unlike attract.
Gaining electrons makes an object negative.
Electric field: region where a charge feels a force.

2. Current and voltage

A simple circuit with an ammeter and a voltmeter

The ammeter is part of the loop (the current flows through it). The voltmeter is connected across the component to compare energy either side.

Q=It\qquad W=QV

$Q$: charge (C).
$I$: current (A).
$V$: p.d. (V = J/C).
$W$: energy transferred (J).

Worked example2 A for 30 s at 12 V

Charge and energy.

Goal: $Q$ , $W$ .
Strategy: Two formulas.

$Q=2\times30=60\text{ C}$
Charge.
$W=60\times12=720\text{ J}$
Each coulomb carries 12 J.

Answer

60

720

✓ Check

P=VI=24

W × 30 s = 720 J ✓.

Ammeter in series; voltmeter in parallel.
Conventional current + to −; electrons − to +.

Common misconceptionPositive charge moving In solids only electrons move; a positive object has lost electrons.

Pause and think

A cloth gains electrons when rubbing a rod. Rod's charge?

Summary

$I=Q/t$ ; $V=W/Q$ .
Ammeter series, voltmeter parallel.

Coulomb: Unit of charge.
Potential difference: Energy per coulomb.

HistoryFranklin's kite. In

1752

Benjamin Franklin flew a kite in a thunderstorm to show lightning is electrical: charge travelled down the wet string to a key. He was lucky; others who repeated the experiment were killed. Franklin also chose the names "positive" and "negative".

Fun factElectrons are slow, signals are fast. In a typical wire, electrons drift at less than

1

mm per second, slower than a snail. A light comes on instantly because the whole wire is already full of electrons, and they all start moving at once.

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