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The AQA A-Level Physics specification includes several required practicals related to electricity. This lesson covers the three main electricity practicals: determining EMF and internal resistance, measuring resistivity, and investigating I-V characteristics. For each practical, we cover the method, apparatus, analysis, and common sources of error.
To determine the EMF (ε) and internal resistance (r) of a cell by measuring terminal p.d. and current for different external resistances.
The cell is connected in series with the ammeter and the variable resistor. The voltmeter is connected in parallel across the cell (measuring terminal p.d.). A switch is included so current only flows when readings are taken (to avoid draining the cell and to minimise heating effects).
Plot a graph of V (y-axis) against I (x-axis).
From V = ε − Ir:
A student obtains the following data:
| I (A) | V (V) |
|---|---|
| 0.10 | 1.43 |
| 0.20 | 1.37 |
| 0.30 | 1.30 |
| 0.40 | 1.23 |
| 0.50 | 1.17 |
| 0.60 | 1.10 |
From the graph (V vs I), the line of best fit gives:
Verification: At I = 0.30 A: V = 1.50 − 0.67 × 0.30 = 1.50 − 0.20 = 1.30 V ✓
| Source of error | Effect | Improvement |
|---|---|---|
| Cell draining during experiment | ε decreases, readings inconsistent | Open switch between readings; work quickly |
| Heating of components | Resistance changes with temperature | Allow cooling time; use low currents |
| Contact resistance | Adds to measured internal resistance | Use clean, tight connections |
| Voltmeter draws current | Measured V slightly low; calculated ε slightly low | Use a high-resistance voltmeter (digital) |
| Parallax error (analogue meters) | Random error in readings | Read at eye level, perpendicular to scale |
Exam Tip: When asked to describe this experiment, always state what you measure, what you plot, and how you obtain ε and r from the graph. A common exam error is to describe the method but forget the analysis.
To determine the resistivity of a metal wire by measuring its resistance at different lengths.
The wire under test is stretched alongside a metre ruler. One crocodile clip is fixed at one end. The other is moved to different positions along the wire to vary the length. An ammeter is in series with the wire and power supply. A voltmeter is connected across the section of wire being tested (between the two crocodile clips).
Plot a graph of R (y-axis) against L (x-axis).
From R = ρL/A:
A student measures the diameter of a nichrome wire at five positions:
| Reading | Diameter (mm) |
|---|---|
| 1 | 0.26 |
| 2 | 0.27 |
| 3 | 0.26 |
| 4 | 0.25 |
| 5 | 0.26 |
Mean diameter = (0.26 + 0.27 + 0.26 + 0.25 + 0.26)/5 = 1.30/5 = 0.260 mm = 2.60 × 10⁻⁴ m
Radius = 1.30 × 10⁻⁴ m
Area = π × (1.30 × 10⁻⁴)² = π × 1.69 × 10⁻⁸ = 5.31 × 10⁻⁸ m²
The student's R vs L graph has a gradient of 19.8 Ω m⁻¹.
ρ = gradient × A = 19.8 × 5.31 × 10⁻⁸ = 1.05 × 10⁻⁶ Ω m
The accepted value for nichrome is approximately 1.10 × 10⁻⁶ Ω m, so this result is within 5% — good agreement.
The range in diameter readings is 0.27 − 0.25 = 0.02 mm.
Half-range = 0.01 mm.
Percentage uncertainty in d = (0.01/0.26) × 100 = 3.8%
Since A = πd²/4, the percentage uncertainty in A is 2 × 3.8% = 7.7% (because d is squared).
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