Unit 3 Practical Quick Reference
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Edexcel A-level Physics | AS Core Practicals | WPH13
Instrument Precision
| Instrument | Precision | Used for |
|---|---|---|
| Metre rule | 1 mm | Length, pendulum length, wire length |
| Vernier calliper | 0.1 mm | Tube diameter, small lengths |
| Micrometer | 0.01 mm | Wire diameter, thin metal wire |
| Stopwatch | 0.01 s (but human reaction time ~0.2 s) | Longer time intervals |
| Light gate | Very high | Short time intervals, instantaneous speed |
⚠️ Wire diameter must be measured with a micrometer — Vernier calliper is not precise enough.
10 Core Practicals
Practical 1: Determination of Acceleration of Free Fall \(g\)
Formula: \(s = \frac{1}{2}gt^2\)
Key operations:
- Electromagnet releases ball and starts timer simultaneously — eliminates reaction time error
- Vary drop height \(h\); plot \(h\) vs \(t^2\) graph; gradient \(= g/2\)
Sources of error:
- Residual magnetism causes time delay on release
- Air currents
- If using pendulum: keep amplitude \(< 10°\); time 20 complete oscillations then divide
Practical 2: Determination of Young Modulus
Formula: \(E = \dfrac{FL}{A\Delta L}\)
Key operations:
- Use the longest possible wire — increases extension \(\Delta L\), reduces percentage uncertainty
- Measure wire diameter with micrometer at 3 different positions and average
- Plot \(F\) vs \(\Delta L\) graph; gradient \(= EA/L\)
Safety: Safety goggles must be worn — snapping wire can cause injury.
Practical 3: Determination of Resistivity of a Metal Wire
Formula: \(R = \rho\dfrac{l}{A}\)
Key operations:
- Vary wire length \(l\) and measure corresponding resistance \(R\)
- Plot \(R\) vs \(l\) graph; gradient \(= \rho/A\)
- Switch off immediately after each reading — prevents wire heating and resistance change
Practical 4: Determination of E.M.F. and Internal Resistance
Formula: \(V = \varepsilon – Ir\) (i.e. \(V = -rI + \varepsilon\) — linear form for graph)
Key operations:
- Use a variable resistor (rheostat) to vary current \(I\)
- Plot \(V\) vs \(I\) graph:
– y-intercept \(= \varepsilon\) (e.m.f.)
– magnitude of gradient \(= r\) (internal resistance)
Practical 5: Investigation of NTC Thermistor Characteristics
Observation: Temperature increases → resistance decreases (non-linear)
Key operations:
- Heat using a water bath; record resistance at different temperatures
- Plot \(R\) vs \(T\) (curve), or \(\ln R\) vs \(1/T\) (straight line)
Practical 6: Determination of Viscosity of a Liquid
Formula: \(F_{drag} = 6\pi\eta rv\) (Stokes’ Law)
Key operations:
- At terminal velocity: weight = upthrust + drag force
- Use light gates at different depths to confirm terminal velocity has been reached
Practical 7: Determination of Refractive Index
Formula: \(n = \dfrac{\sin i}{\sin r}\)
Key operations:
- Use pins to mark the incident and refracted ray paths
- Vary angle of incidence \(i\) from 10° to 70°; record angle of refraction \(r\)
- Plot \(\sin i\) vs \(\sin r\) graph; gradient \(= n\)
Practical 8: Investigation of Stationary Waves on a String
Formula: \(f = \dfrac{1}{2L}\sqrt{\dfrac{T}{\mu}}\), fundamental: \(\lambda = 2L\)
Key operations:
- Adjust frequency until first harmonic (one antinode) appears
- Vary string length or tension; record resonant frequencies
Practical 9: Finding the Centre of Gravity — Plumb Line Method
Hang the irregular lamina from one point; draw a vertical line through the pivot
Hang from a different point; draw another vertical line
Intersection of the two lines = centre of gravity
Practical 10: General Error Analysis
See: 01 Uncertainty Quick Reference
General Graph Rules
Table Format
Correct: Length l / m Resistance R / Ω
Incorrect: Length l (m) Resistance R (Ω)Header format: quantity symbol / unit
Graph Drawing Rules
- Axis label: quantity / unit (e.g.
t² / s²) - Plot data points with × or ⊕
- Best-fit line: straight; points scatter evenly either side; do not force through every point
- Gradient: use two points on the line as far apart as possible (large triangle)
Linearisation
| Original | Graph to plot | Gradient | Intercept |
|---|---|---|---|
| \(y = ax^2\) | \(y\) vs \(x^2\) | \(a\) | 0 |
| \(y = ax^n\) | \(\lg y\) vs \(\lg x\) | \(n\) | \(\lg a\) |
| \(T = 2\pi\sqrt{L/g}\) | \(T^2\) vs \(L\) | \(4\pi^2/g\) | 0 |
| \(V = \varepsilon – Ir\) | \(V\) vs \(I\) | \(-r\) | \(\varepsilon\) |
Experimental Design Framework (6-mark question)
For “design an experiment to measure X”, answer in this order:
Diagram — label all apparatus
Variables: independent, dependent, control variables
Procedure: how to change the independent variable; how to measure the dependent variable
Data analysis: state which graph to plot and what the gradient/intercept gives
Reducing uncertainty: repeat measurements and average; use more precise instruments
Safety: at least one safety precaution
Standard Sentence Bank
Sources of error:
- Parallax error when taking readings
- Human reaction time causes timing error
- Wire heats up during current flow, changing resistance
- Air currents affect pendulum motion
Improvements:
- Use a light gate instead of a stopwatch (eliminates reaction time error)
- Time 20 oscillations and divide by 20 (reduces percentage uncertainty)
- Switch off circuit immediately after each reading (prevents heating)
- Measure at multiple positions and average (reduces random error)
WPH13 Exam Format Guide
Mark Allocation
WPH13 = 40 marks, approximately 2 hours, two large practical questions.
| Section | Typical marks | Common cause of mark loss |
|---|---|---|
| Readings (with uncertainty) | 4–6 | Insufficient precision / missing units / no uncertainty |
| Data table | 2–3 | Wrong header format / inconsistent significant figures |
| Graph | 6–8 | No units on axes / data doesn’t fill graph / forced through every point |
| Gradient | 3–4 | Triangle too small / used data points not line points |
| Conclusion | 3–5 | Stated result without linking to equation |
Reading Standards
- Millimetre rule: read to 1 mm, estimate to 0.5 mm
- Vernier calliper: read to 0.1 mm
- Micrometer: read to 0.01 mm
- Stopwatch: read to 0.01 s (but human reaction time ~0.2 s)
- Every reading must include absolute uncertainty: \((34.5 \pm 0.5)\ \text{mm}\)
Table Format Rules
Correct: L / m T² / s² ln(V / V)
Incorrect: L(m) T²(s²) lnV- Decimal places must be consistent within a column
- Uncertainty in \(T^2\): \(\dfrac{\Delta T^2}{T^2} = 2\dfrac{\Delta T}{T}\)
Graph Rules
Uniform scale; label as “quantity / unit”
Data must occupy > 50% of grid
Best-fit line: points scatter evenly either side
Gradient: use points on the line (not data points); triangle must be large
Standard Improvement Phrases
| Problem | Model answer |
|---|---|
| Reduce timing error | Time 20 oscillations; divide by 20 to reduce percentage uncertainty |
| Reduce parallax | Read at eye level perpendicular to scale; use Vernier calliper or light gate |
| Reduce systematic error | Zero the instrument; use gradient method rather than single measurement |
| Detect terminal velocity | Use light gates at different depths until speed stops changing |
| Why repeat and average | Reduces random error and improves reliability |
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