Electrical Quantities

Electric charge is a property of matter — electrons carry negative charge, protons positive. The unit of charge is the coulomb (C). One coulomb is roughly 6.24 × 10¹⁸ electrons.

Current is the rate of flow of charge:

$I = \frac{Q}{t}$

with I in amperes (A), Q in coulombs (C) and t in seconds (s). 1 A = 1 C/s.

When you flick a switch and turn on a lamp, billions of free electrons in the wire start to drift along the wire (slowly — at millimetres per second), pushed by the cell's electric field. The current you measure is the number of coulombs passing each cross-section every second.

A subtle point: conventional current is drawn flowing from + to − around the circuit, but the actual electrons flow the OTHER way (− to +). This is historical — the convention was set before electrons were discovered. MYP examiners may use either, so know both.

Voltage (also called potential difference, p.d.) tells you how much energy the cell or supply gives each coulomb of charge:

$V = \frac{E}{Q}$

with V in volts (V), E in joules (J), Q in coulombs (C). 1 V = 1 J/C. A 1.5 V cell gives 1.5 J of energy to every coulomb that passes through it.

Resistance measures how much a component opposes the current. The bigger R, the smaller the current for a given voltage:

$R = \frac{V}{I}$

with R in ohms (Ω). Wires, lamps and resistors all have some resistance.

Ohm's law:

$V = I \, R$

For a fixed-value resistor at constant temperature, this gives a straight-line I-V graph through the origin: doubling V doubles I.

The gradient of the I-V graph equals 1/R. If the line bends (e.g. a filament lamp at high current), the component is NON-ohmic — its resistance changes with temperature.

An ammeter measures current — it must have the current pass through it. So it sits IN SERIES with the component, in the same wire loop. A good ammeter has very low resistance so it doesn't reduce the current it's trying to measure.

A voltmeter measures the p.d. across a component — so it sits IN PARALLEL with that component, across its ends. A good voltmeter has very high resistance so almost no current is diverted through it.

Place them the other way around and you risk: (a) an ammeter in parallel short-circuits the supply (very large current — dangerous), or (b) a voltmeter in series gives basically zero current and a nonsense reading.