Experiment: Comparing ohmic and non-ohmic conductors

Aim

To compare the current–voltage characteristics of an ohmic conductor (a constantan wire) and a non-ohmic component (a filament bulb).

Variables

• Manipulated: Potential difference across the component, V
• Responding: Current through the component, I
• Constant: The component under test (and, for the ohmic wire, its temperature)

Apparatus & materials

• Constantan wire (fixed resistor)
• Filament bulb
• Rheostat
• Ammeter
• Voltmeter
• Dry cells or power supply
• Switch
• Connecting wires

Procedure

1. Connect the constantan wire in series with the ammeter, rheostat, switch and battery, and connect the voltmeter across the wire.
2. Adjust the rheostat until the voltmeter reads V = 0.5 V and record the current I.
3. Increase V in equal steps, recording I at each step; switch off between readings to stop the wire heating up.
4. Replace the wire with the filament bulb and repeat the readings over the same range.
5. Record V and I for each component.

Tabulating results

For each component record the potential difference V in V and the current I in A. You may add a column for the ratio V / I to compare the resistance.

The graph

Plot I (y-axis) against V (x-axis) for both components. The ohmic wire gives a straight line through the origin (constant resistance); the filament bulb gives a curve that bends over.

Analysis

For the ohmic conductor V / I is constant, so it obeys Ohm's law. For the filament bulb the resistance R = V / I increases as V increases, because the filament gets hotter; it is non-ohmic.

Electricity · Graph skills

Sample results and what they show

Example data for the two components over the same range of potential difference:

• Constantan wire: V = 0.5 V, I = 0.10 A; V = 1.0 V, I = 0.20 A; V = 1.5 V, I = 0.30 A; V = 2.0 V, I = 0.40 A; V = 2.5 V, I = 0.50 A
• Filament bulb: V = 0.5 V, I = 0.20 A; V = 1.0 V, I = 0.34 A; V = 1.5 V, I = 0.44 A; V = 2.0 V, I = 0.52 A; V = 2.5 V, I = 0.58 A

For the constantan wire the ratio V / I is the same each time (0.5 / 0.10 = 5.0 Ω, 2.5 / 0.50 = 5.0 Ω), so its resistance is constant. For the filament bulb V / I rises (0.5 / 0.20 = 2.5 Ω up to 2.5 / 0.58 ≈ 4.3 Ω), so its resistance increases.

The difference is because the bulb’s filament gets hotter as the current increases, while the constantan wire is kept cool.

Reading the graph and finding the answer

Plot current I (y-axis) against potential difference V (x-axis) for both components on the same axes.

The constantan wire gives a straight line through the origin: constant gradient means constant resistance, so it obeys Ohm’s law. The gradient of this line is 1 / R. Using a large triangle from (0.5 V, 0.10 A) to (2.5 V, 0.50 A):

gradient = ΔI ÷ ΔV = (0.50 − 0.10) A ÷ (2.5 − 0.5) V = 0.40 A ÷ 2.0 V = 0.20 A V⁻¹, so R = 1 ÷ 0.20 A V⁻¹ = 5.0 Ω.

The filament bulb gives a curve that bends towards the V-axis: as V rises the line gets less steep, showing the resistance is increasing. Because its I–V line is not straight, the bulb is a non-ohmic conductor.

Marks examiners look for

For the practical marks: switch off between readings so the constantan wire does not heat up and change its resistance, check both meters for zero error before starting, and connect the ammeter in series with the component and the voltmeter in parallel across it.

For the Paper 3 science-process-skill marks, name the variables: the manipulated variable is the potential difference V, the responding variable is the current I, and the fixed variable is the component under test (and, for the wire, its temperature). Tabulate V in V and I in A, with a column for V / I in Ω so the constant-versus-rising pattern is visible.

Draw both I–V lines on even scales. The conclusion should contrast the two: the constantan wire obeys Ohm’s law because V / I is constant, while the filament bulb is non-ohmic because its resistance rises as it heats up.

Always give resistance values with the unit Ω.

Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)