This topic accounts for approximately 16% of your exam marks.
stable
Very High
Stable16%
Ohm's Law calculations and I-V characteristic graphs are among the most reliably tested question types.
What resistance is
Resistance is the opposition a component offers to the flow of electric current through it
The greater the resistance, the harder it is for charge to flow, and so the smaller the current for a given voltage
Resistance is measured in ohms, symbol Ω (the Greek capital omega)
A component has a resistance of 1 Ω when a voltage of 1 V across it drives a current of exactly 1 A through it (so 1 Ω = 1 V/A)
What sets the resistance of a component
Material: good conductors like copper and silver have very low resistance; insulators like rubber and dry wood have enormous resistance
Length: a long wire has more resistance than a short one of the same material and thickness, because every extra metre adds to the obstruction
Cross-sectional area: a thick wire has less resistance than a thin one, because there is more room for charge to flow
Temperature: in most metals, resistance rises as temperature rises; the metal ions vibrate harder and collide with the drifting electrons more often
Even the connecting wires themselves have a small resistance, but in exam circuits they are taken to be zero unless the question states otherwise
Exam tip
Why a filament lamp's I–V graph is curved
What comes up: "Explain why the current–voltage graph for a filament lamp is not a straight line through the origin" (2 marks).
Write (two marks): (1) The current through the filament heats it up, raising its temperature. (2) As temperature increases, the resistance of the filament increases, so the current increases more slowly for each extra volt — giving a curve with a decreasing gradient.
Watch out: Saying the graph is curved "because the lamp does not obey Ohm's law" alone earns no marks; the mark scheme requires you to link the curve to the temperature-dependent resistance. Simply stating "resistance increases" without saying why (temperature rises) also risks losing the first mark.
Ohm's law: V = I × R
The defining equation linking voltage, current and resistance is:
V = I × R
where:
V = voltage across the component (V)
I = current through the component (A)
R = resistance of the component (Ω)
Rearrangements:
I = V / R (current is voltage divided by resistance)
R = V / I (resistance is voltage divided by current, which is the experimental way to measure R)
A component is said to be ohmic if its resistance is constant across the working range (V is then directly proportional to I). Many resistors and lengths of metal wire at a steady temperature behave this way; filament lamps and diodes do not, and so are non-ohmic (covered fully in topic 06)
Worked example
Finding resistance from voltage and current
A resistor has a voltage of 9.0 V across it and a current of 0.30 A through it. Calculate its resistance.
Solution:
Write the equation: V = I × R, so R = V / I
Substitute: R = 9.0 / 0.30
R = 30 Ω
Idealisations on circuit diagrams
In exam questions, unless stated otherwise:
the wires have zero resistance
the internal of the cell or battery is zero
an has zero resistance and so does not alter the current it is reading
a has infinite resistance and so draws no current away from the component it is reading
These idealisations make the algebra clean; in real life, every one of them is a small approximation rather than an exact truth