SPH3U Static Charge, Current and Electron Flow: Notes and Practice

Everything electrical starts with charge. There are two kinds, positive and negative, and the rule is the one everyone already knows: like charges repel, opposite charges attract. Charge is measured in coulombs (C), and one coulomb is an enormous number of electrons — about $6.24 \times 1 0^{18}$ of them.

Fig 1Rubbing transfers electrons. Neither object creates charge — it just moves.

Only electrons move

In a solid, the protons are locked in the nuclei and cannot go anywhere. Every static-electricity effect is electrons being transferred. An object becomes negative by gaining electrons and positive by losing them — which means a positively charged object has fewer electrons than normal, not extra protons.

Method	How it works	Result
Friction	Rubbing two materials transfers electrons from one to the other	One positive, one negative
Contact	A charged object touches a neutral one and shares its charge	Both end up with the same sign
Induction	A charged object is brought near, repelling or attracting electrons within	Charge separates without touching

Conductors — metals, salt water, graphite — let electrons move freely, because their outer electrons are not bound to individual atoms. Insulators — plastic, glass, rubber, dry air — hold their electrons tightly. This is why a metal rod discharges through your hand instantly while a plastic one keeps its charge for hours.

Grounding and lightning

Connecting a charged object to the Earth lets charge flow away into a reservoir so large that the object ends up neutral. Lightning is the same process at enormous scale: charge separates within a storm cloud until the air's insulating strength fails, and tens of coulombs move in a few milliseconds.

Current: charge in motion

Current is the rate at which charge flows past a point. Static electricity is charge sitting still; current electricity is charge moving, and it is what does useful work.

I = \frac{q}{Δ t}

Current in amperes, charge in coulombs, time in seconds. One ampere is one coulomb per second.

The ampere (A) is one of the seven SI base units, and the definition is worth reading carefully: $1 A = 1 C/s$ . A current of $2 A$ means two coulombs — about $1.25 \times 1 0^{19}$ electrons — pass a point every second.

Conventional current versus electron flow

Fig 2Two arrows, opposite directions, describing the same circuit.

An eighteenth-century guess we never corrected

Benjamin Franklin assumed current was a flow of positive charge from positive to negative. He had a fifty-fifty chance and got it wrong — in a metal it is electrons, which are negative, moving from negative to positive. By the time this was discovered the convention was everywhere, so conventional current still points from positive to negative. Every formula, diagram and circuit symbol in this course uses it.

AConventional current

Positive terminal → negative terminal
Imagines positive charge moving
Used in every equation and diagram
The default unless a question says otherwise

BElectron flow

Negative terminal → positive terminal
What actually happens in a metal
Mentioned when a question asks about the physical reality
Exactly opposite to conventional current

Electrons are astonishingly slow

The drift velocity of electrons in a copper wire is under a millimetre per second — slower than walking. A light comes on instantly not because electrons race to it, but because the electric field propagates through the wire at close to the speed of light, nudging electrons already present all along its length into motion at once. The wire is already full of electrons before you flip the switch.

Circuits and measurement

Fig 3The standard symbols. Learn them once and every schematic becomes readable.

Instrument	Measures	Connected	Ideal resistance
Ammeter	Current (A)	In series — the current must pass through it	Zero, so it does not impede the current
Voltmeter	Potential difference (V)	In parallel — across the component	Infinite, so it draws no current

Meters connected wrongly

An ammeter has nearly zero resistance. Connect it in parallel across a battery and you have created a short circuit — a huge current, a blown fuse, and possibly a ruined meter. A voltmeter in series does the opposite: its near-infinite resistance stops the current almost entirely, and the circuit simply does not work.

Static Charge, Current and Electron Flow

Key things to remember

Current: charge in motion

Conventional current versus electron flow

Circuits and measurement

Worked examples

Where marks get lost

Flashcards 11

Practice