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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 5

Topic 4.2.4 · Resistance

Defining and measuring resistance

R = V / I with R in ohms (Ω), V in volts and I in amperes.

To measure resistance: connect an ammeter in series and a voltmeter in parallel with the component. Calculate R from V and I. Repeat for several values if asked.

A resistor has a p.d. of 12 V when the current is 0.50 A. Calculate its resistance.

Worked example: resistance is 12 volts over 0.50 amperes equals 24 ohms
R = V / I = 24 Ω.
Circuit to find resistance: ammeter in series with a wire or resistor, voltmeter in parallel, variable supply or variable resistor to change the current.
Diagram 1: Circuit to find resistance: ammeter in series with a wire or resistor, voltmeter in parallel, variable supply or variable resistor to change the current.

Wires: length and area

For a metallic wire, qualitatively: longer wire → greater resistance; thicker wire (larger cross-sectional area) → smaller resistance.

Quantitatively: resistance is directly proportional to length and inversely proportional to cross-sectional area.

Current–voltage graphs

Sketch and explain I–V graphs:

Constant resistor
Straight line through the origin. R is constant (V proportional to I).
Filament lamp
Curve: current increases less quickly at higher V because the filament heats, so R increases.
Diode
Very small current one way (reverse); current grows when forward p.d. is large enough.
Current–voltage graphs: straight line through the origin for a constant resistor; a curve for a filament lamp; a diode conducting in one direction only.
Diagram 2: Current–voltage graphs: straight line through the origin for a constant resistor; a curve for a filament lamp; a diode conducting in one direction only.

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