2 · IGCSE Physics
Current–voltage graphs
Change the voltage across a fixed resistor, a filament lamp or a diode, build its I–V graph point by point and watch how the resistance R = V/I behaves.
R = V / I
V =
I =
R = V/I =
Try this
- With the resistor, record points at 2.00 V and 4.00 V. Does doubling V double I? Check that R stays at 10.0 Ω.
- Sweep the filament lamp and compare R at 1.00 V with R at 6.00 V. Why does the graph bend towards the V axis?
- With the diode, find the forward voltage at which I first exceeds 10 mA, then set V = −2.00 V and read the resistance.
Exam tip: calculate resistance from the coordinates of one point using R = V/I, not from the gradient of a curved graph – for a lamp the tangent gradient is not equal to 1/R.