Unit 6 Practical Quick Reference
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Edexcel A-level Physics | A2 Core Practicals | WPH16
Unit 6 builds on Unit 3 skills but demands more rigorous data processing and uncertainty analysis.
See also: 01 Uncertainty Quick Reference for graph and uncertainty format rules.
6 Core Practicals
Practical 1: Charging and Discharging Capacitors
Aim: Determine time constant \(RC\) or capacitance \(C\)
Linearisation:
\[\ln V = -\frac{1}{RC} \cdot t + \ln V_0\]
Plot \(\ln V\) against \(t\):
- Gradient \(= -1/RC\) (negative)
- y-intercept \(= \ln V_0\)
Key operations:
- Use a high-resistance voltmeter — prevents meter discharging capacitor
- If discharge is too fast: use a data logger with voltage sensor
Note: Axis label must be written as \(\ln(V/\text{V})\)
Practical 2: Investigating Simple Harmonic Motion
Pendulum aim: Determine \(g\)
\[T^2 = \frac{4\pi^2}{g} \cdot l\]
Plot \(T^2\) against \(l\); gradient \(= 4\pi^2/g\)
Spring-mass aim: Determine spring constant \(k\)
\[T^2 = \frac{4\pi^2}{k} \cdot m\]
Plot \(T^2\) against \(m\); gradient \(= 4\pi^2/k\)
Key operations:
- Time 20 complete oscillations; divide by 20 — reduces percentage uncertainty
- Set fiducial marker at equilibrium position — speed is maximum there, timing is most accurate
- Keep pendulum amplitude \(< 10°\) to satisfy small-angle approximation for SHM
Practical 3: Force on a Current-Carrying Conductor in a Magnetic Field
Aim: Determine magnetic flux density \(B\) (\(F = BIl\))
Method:
Place magnet on electronic balance; tare to zero
By Newton’s third law: force on wire = change in balance reading × \(g\)
Plot \(F\) against \(I\); gradient \(= Bl\)
Note: Switch off circuit immediately after each reading — prevents resistance change due to heating.
Practical 4: Investigating Gas Laws
Verifying Boyle’s Law (\(pV = \text{constant}\), constant temperature):
- Vary the volume \(V\) of gas in a syringe; record pressure \(p\)
- Plot \(p\) against \(1/V\) — should give a straight line through the origin
Key operations:
- After compressing the gas, wait before taking readings — allow temperature to return to room temperature
- Compress slowly — rapid compression increases gas temperature
Measuring specific heat capacity:
\[E = VIt = mc\Delta\theta\]
- Source of error: heat loss to surroundings
- Improvement: add insulation, use a lid, stir thoroughly for uniform temperature
Practical 5: Thermal Properties of Matter
Specific heat capacity:
- Use electrical heater; record temperature vs time
- \(c = \dfrac{VIt}{m\Delta\theta}\)
Specific latent heat:
- Temperature is constant during change of state
- \(L = \dfrac{VIt}{m}\)
Key precautions:
Specific Heat Capacity:
- Use lagging and lid to reduce heat loss to surroundings
- Add drops of oil in holes to ensure good thermal contact (for solid blocks)
- Wait after switching off until max temperature is reached (thermal equilibrium)
Specific Latent Heat of Fusion:
- Use crushed ice for larger surface area and uniform heating
- Ensure ice is at 0°C so energy only goes to changing state
- Use an unheated control apparatus to correct for mass melted due to room temperature
Specific Latent Heat of Vaporisation:
- Wait for steady boiling before starting timing
- Use lid to reduce convection heat loss, with a hole to prevent pressure build-up
Practical 6: Radioactivity and Nuclear Decay
Aim: Determine half-life; verify the random nature of decay
Essential first step: Measure background radiation (average over 10 minutes)
\[\text{corrected count rate} = \text{measured count rate} – \text{background count rate}\]
Linearisation:
\[\ln A = -\lambda t + \ln A_0\]
Plot \(\ln A\) against \(t\); gradient \(= -\lambda\); half-life \(= \ln 2 / \lambda\)
Safety precautions (must state at least 3):
Use long-handled tongs — never handle source directly
Return source to lead-lined container when not in use
Never point source at any person
Minimise time of exposure
WPH16 Planning Questions
Typical Question Format
“Design an experiment to verify the relationship between X and Y using a straight-line graph”
Key Points to Cover
Variables: independent, dependent, control variables
Linearisation: state which quantities to plot to give a straight line
Gradient/intercept: how to extract the target quantity from the graph
Repeats: mention repeating measurements and averaging
Error bars: how to draw error bars on the graph
Common Linearisation Templates
| Relationship | Graph to plot | Gradient |
|---|---|---|
| \(y = kx^2\) | \(y\) vs \(x^2\) | \(k\) |
| \(y = ax^n\) | \(\lg y\) vs \(\lg x\) | \(n\) |
| \(V = V_0 e^{-t/RC}\) | \(\ln V\) vs \(t\) | \(-1/RC\) |
| \(T^2 = (4\pi^2/g) \cdot l\) | \(T^2\) vs \(l\) | \(4\pi^2/g\) |
| \(A = A_0 e^{-\lambda t}\) | \(\ln A\) vs \(t\) | \(-\lambda\) |
WPH16 Q2: Data Analysis Guide
Typical Structure
WPH16 Q2 provides experimental data and asks you to:
Complete the table (calculate derived quantities, logarithms, propagate uncertainties)
Draw linearised graph with error bars
Calculate target quantity from gradient/intercept
Write conclusion comparing with theoretical value
Step-by-Step Guide
Step 1: Complete the table
- Calculate derived quantities (e.g. \(\ln V\), \(T^2\), \(1/V\))
- \(\ln\) values: 3 decimal places
- Uncertainty in \(x^2\): \(\dfrac{\Delta(x^2)}{x^2} = 2\dfrac{\Delta x}{x}\)
Step 2: Calculate error bars
\[\text{upper bar} = \ln(V + \Delta V) – \ln V \qquad \text{lower bar} = \ln V – \ln(V – \Delta V)\]
These are asymmetric — calculate each side separately.
Step 3: Draw the graph
- Error bars on every point — compulsory
- Best-fit line must pass through all error bars
- Anomalous point: circle it; do not include in best-fit line; but still plot it
Step 4: Read gradient and intercept
- Use two points on the line (not data points); use as large a triangle as possible
- Uncertainty in gradient: draw steepest and shallowest lines through all error bars, then:
\[\Delta k = \frac{k_{max} – k_{min}}{2}\]
Step 5: Write conclusion
| Situation | Model conclusion |
|---|---|
| Straight line through origin | “The graph is a straight line through the origin, confirming X ∝ Y.” |
| Straight line, not through origin | “X and Y are linearly related but not directly proportional.” |
| Curved graph | “The graph is not linear, inconsistent with the proposed equation.” |
Handling Anomalous Points
- Circle it on the graph
- State it is excluded from the best-fit line
- Do NOT omit it — it must still be plotted
Note on Error Bars in Past Papers
Although full error bar graphs are rarely drawn in timed exams, the underlying concepts are examined in these ways:
Calculation focus: Questions typically ask for percentage uncertainty or gradient uncertainty directly.
Key concepts to master:
– Error bars represent the absolute uncertainty of each data point
– Anomalous point: the best-fit line does not pass through its error bars
– Gradient uncertainty: \(\text{uncertainty} = \text{best gradient} – \text{worst acceptable gradient}\)
– To find maximum possible value of a quantity: “Draw a worst acceptable line through the error bars.”
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