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ResistanceCambridge IGCSE Physics: Revision notes

Section 1

What Is Resistance?

Resistance is a measure of how much a component opposes the flow of current through it.

  • Resistance (R) is calculated using R = V / I, where V is the potential difference across the component and I is the current through it.
  • Resistance is measured in ohms (Ω).
  • For a constant current, the p.d. across a conductor increases as its resistance increases — a higher-resistance component needs a bigger push (p.d.) to drive the same current through it.
Key termsresistance
Exam tip

Rearranging R = V/I gives V = IR — useful for finding the p.d. across a resistor if the current and resistance are known.

Section 2

Measuring Resistance

Resistance can be found experimentally using a voltmeter and an ammeter.

  1. Connect the ammeter in series with the component to measure the current, I.
  2. Connect the voltmeter in parallel across the component to measure the potential difference, V.
  3. Calculate the resistance using R = V / I.

Repeating this for several current values and taking an average improves accuracy.

Example

If a voltmeter reads 4 V across a resistor and the ammeter reads 2 A through it, the resistance is R = V/I = 4/2 = 2 Ω.

Section 3

Resistance, Length and Cross-Sectional Area

For a metallic conductor at constant temperature, resistance depends on its dimensions.

  • Resistance is directly proportional to length — a longer wire has more resistance.
  • Resistance is inversely proportional to cross-sectional area — a thicker wire has less resistance.

Qualitatively: doubling the length of a wire doubles its resistance; doubling its cross-sectional area halves its resistance.

Think of it like this

Think of a wire like a corridor for electrons — a longer corridor means more obstacles to bump into (more resistance), while a wider corridor lets more electrons pass side by side (less resistance).

Section 4

Current–Voltage Graphs

Different components have different current–voltage (I–V) characteristics.

  • A resistor of constant resistance gives a straight line through the origin — current is directly proportional to voltage.
  • A filament lamp gives a curve that flattens as voltage increases, because the filament heats up and its resistance increases.
  • A diode allows current to flow easily in one direction (forward bias) but almost no current in the reverse direction (reverse bias).
Key termsfilament lampdiode
Common mistake

Don't assume all I–V graphs are straight lines — only a component with truly constant resistance (an ohmic conductor) gives a straight line through the origin.

Must Know

  • Resistance R = V/I, measured in ohms (Ω); for constant current, higher resistance means higher p.d.
  • Resistance is found experimentally with a series ammeter and a parallel voltmeter.
  • Resistance is directly proportional to length and inversely proportional to cross-sectional area for a metallic conductor.
  • A resistor gives a straight-line I–V graph through the origin.
  • A filament lamp's I–V graph curves as it heats up and resistance rises.
  • A diode conducts easily in one direction only.

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