Resistance measures how hard a component makes the charge work to get through. A straight line on an I-V graph means a constant resistance; a curve tells a richer story about diodes, filaments and the material itself.
Resistance is defined by R = V / I. A component obeys Ohm law if the current is proportional to the voltage (a straight-line I-V graph through the origin) at constant temperature. The resistance of a uniform wire is R = ρL / A, where ρ is the resistivity of the material. A thermistor (NTC) and a light-dependent resistor (LDR) both fall in resistance, with rising temperature and rising light level respectively.
Sweep the voltage and plot the I-V characteristic for a metal conductor, a filament lamp and a diode, and see why only the first is a straight line through the origin.
The I-V sim treats resistance as a fixed number; here you see where that number comes from. Stretch the wire, change its cross-section, and swap the material, and watch R = ρL/A respond, longer and thinner and more resistive all push the resistance up.
One relation for the component, one for the material.
| Relation | Meaning | Note |
|---|---|---|
| R = V / I | definition of resistance | ohm (Ω) |
| Ohm law | I ∝ V at constant temperature | straight-line I-V graph |
| R = ρL / A | resistance of a uniform wire | ρ in Ω m |
Four quick checks tied to this lesson. Each correct answer earns XP and lights this skill on your star map.
The resistance of a component is defined by:
A component obeys Ohm law if, at constant temperature:
The resistivity of a material relates resistance to dimensions by:
As the temperature of an NTC thermistor rises, its resistance:
The shape of the characteristic reveals the component.
Resistance R = V/I is the value at a point, found from the coordinates, not the gradient, on a curved I-V graph. A filament lamp is not Ohmic: its resistance rises as it heats, so the graph curves. In R = ρL/A the resistance is proportional to length and inversely proportional to area, so a longer, thinner wire has more resistance.
Unlocks once the checks above are done. Worth more XP, written to AS Paper 1 and 2 standard.
A 6.0 V supply drives 0.50 A through a resistor. Its resistance is:
A wire has length 2.0 m, area 5.0 × 10⁻⁷ m² and resistivity 1.0 × 10⁻⁶ Ω m. Its resistance is:
A filament lamp is brighter at higher voltages. Its I-V characteristic curves because:
This skill is now lit gold on your star map. You have finished the lessons of Topic 9; the Paper 1 set awaits.