Welcome to Physics Experimental Skills!
Hello and welcome! In Unit AS 1: Experimental Techniques, you will build hands-on laboratory skills across biology, chemistry, and physics. While physics skills might seem daunting with all the numbers, meters, and graphs, do not worry! Once you learn a few simple rules, taking precise measurements, calculating uncertainties, and plotting perfect graphs will become second nature.
In life and health sciences, physics skills are crucial. Whether calibrating an electrocardiogram (ECG) monitor, measuring the exact diameter of a blood vessel or syringe needle, or testing optical lenses for eyesight correction, precision and accuracy save lives.
Unit AS 1 Portfolio Fact: This unit is internally assessed coursework (portfolio-based) and moderated by CCEA. It makes up 33.34% of your AS Award (and 13.34% of your full A Level). Getting your experimental physics techniques right is a massive boost toward your final grade!
1. Precision Instruments: Measuring Length and Thickness
A standard school ruler can measure down to the nearest millimeter (\(\pm 1\text{ mm}\)). But what if you need to measure the thickness of a microscope glass slide or the diameter of a fine wire? You need specialist instruments with much higher resolution (the smallest change an instrument can detect).
A. The Micrometer Screw Gauge
• Resolution: \(\pm 0.01\text{ mm}\)
• Best used for: Very small external thicknesses and diameters (such as wire gauge, glass cover slips, or thin metal sheets).
• How it works: As you turn the thimble, the spindle moves. Always use the ratchet knob at the end to tighten it gently until it clicks. This prevents you from over-tightening and damaging the object or the instrument.
Watch out for Zero Error:
Before placing your object in the micrometer, close the jaws completely using the ratchet. The zero line on the rotating thimble should align perfectly with the datum line on the main sleeve.
• If it reads a value above zero when fully closed, it has a positive zero error (you must subtract this value from every reading).
• If it reads below zero, it has a negative zero error (you must add this correction to your reading).
B. Vernier Callipers
• Resolution: Typically \(\pm 0.1\text{ mm}\) or \(\pm 0.02\text{ mm}\) (depending on the precision scale).
• Best used for: Measuring the internal diameter of a tube (using the upper jaws), the external diameter of a cylinder (using the lower jaws), or the depth of a small container (using the depth probe at the tail).
Key Takeaway for Length Instruments: Always record the instrument's zero error first, tighten gently, and record all measurements to the exact decimal places corresponding to the tool's resolution (e.g., \(0.35\text{ mm}\) on a micrometer, not just \(0.3\text{ mm}\)).
2. Core Laboratory Measurements: Circuits, Timing, Temperature & Optics
A. Electrical Measurements
Understanding how to wire a Direct Current (DC) circuit properly is a vital lab skill:
• Ammeters: Measure electric current (\(I\)) in amperes (\(\text{A}\)) or milliamperes (\(\text{mA}\)). They have very low resistance and must always be placed in series (in the same loop) with the component.
• Voltmeters: Measure potential difference / voltage (\(V\)) in volts (\(\text{V}\)). They have very high resistance and must always be placed in parallel (across the component).
• Digital Multimeters: A versatile tool that can measure current, voltage, or resistance (\(R\)). Always check that your leads are plugged into the correct ports and the dial is switched to the appropriate range (e.g., DC volts vs AC volts).
B. Timing Oscillations and Periodic Events
When timing a repetitive event (such as a pendulum swing or a vibrating mass on a spring), human reaction time introduces a small delay when pressing the stopwatch (typically \(0.2\text{ s}\) to \(0.3\text{ s}\)).
The Physics Technique:
1. Set up a fiducial marker (a reference pin or mark placed right at the equilibrium / centre position) to give a sharp, clear point where the oscillation passes.
2. Time multiple complete oscillations—usually \(N = 10\) or \(N = 20\) swings together.
3. Divide your total measured time by \(N\) to calculate the time period for one single oscillation (\(T\)).
Why do this? The human reaction time error is spread across all \(10\) or \(20\) cycles, dramatically reducing the percentage uncertainty of your final calculated period!
C. Temperature Measurements
• Immersion Depth: Make sure the bulb of a liquid-in-glass thermometer or the tip of a digital temperature probe is adequately submerged in the liquid without touching the base or sides of the glass beaker.
• Thermal Equilibrium: Always wait until the reading stabilizes before recording the temperature.
D. Ray Tracing and Optics
Optical investigations involve convex/concave lenses, plane mirrors, optical pins, and illuminated target objects.
• When using optical pins for ray tracing, space the pins far apart (at least \(5\text{ cm}\)) to ensure accurate sightlines.
• When determining focal lengths, adjust the screen or lens position until the projected image of an illuminated object is crisp and in sharp focus.
Key Takeaway: Good experimental technique is about minimizing errors before they happen—connect meters correctly, use fiducial markers for timing, and wait for thermal equilibrium.
3. Errors and Uncertainties
In science, no measurement is ever 100% exact. Understanding the difference between systematic errors and random errors will help you evaluate your experimental data like a true scientist.
A. Systematic vs. Random Errors
• Systematic Errors: These shift all your measurements in one direction by a consistent, fixed amount. Examples include an uncalibrated balance, a zero error on a micrometer, or a persistent parallax error (reading a scale or meniscus from an angle rather than straight on at eye level).
Crucial rule: Systematic errors cannot be eliminated or reduced by repeating trials and calculating an average.
• Random Errors: These cause unpredictable, scattered fluctuations around the true value due to uncontrollable environmental changes (such as slight room temperature shifts, draughts, or human reaction variations).
Crucial rule: Random errors can be reduced by taking repeated measurements and calculating a mean value.
B. Calculating Absolute and Percentage Uncertainty
1. Absolute Uncertainty:
• For a single measurement, absolute uncertainty is usually the resolution (smallest readable division) of the instrument.
• For repeated readings of a value, calculate the spread of the data:
\(\text{Absolute Uncertainty} = \pm \frac{\text{Range}}{2} = \pm \frac{\text{Maximum Value} - \text{Minimum Value}}{2}\)
2. Percentage Uncertainty:
This tells you how significant the uncertainty is compared to the size of the measurement itself:
\(\text{Percentage Uncertainty} = \left( \frac{\text{Absolute Uncertainty}}{\text{Measured Value}} \right) \times 100\%\)
Example: If you measure a voltage of \(5.0\text{ V}\) with an uncertainty of \(\pm 0.1\text{ V}\):
\(\text{Percentage Uncertainty} = \left( \frac{0.1\text{ V}}{5.0\text{ V}} \right) \times 100\% = 2.0\%\)
C. Combining Uncertainties in Calculations
When you combine different measurements to calculate a final result, you must combine their uncertainties using these three golden rules:
Rule 1: Addition or Subtraction (\(y = a + b\) or \(y = a - b\))
Always ADD the ABSOLUTE uncertainties.
\(\Delta y = \Delta a + \Delta b\)
Example: If initial temperature \(T_1 = (20.0 \pm 0.5)^\circ\text{C}\) and final temperature \(T_2 = (35.0 \pm 0.5)^\circ\text{C}\), then the temperature rise \(\Delta T = 15.0^\circ\text{C}\) and total absolute uncertainty \(= 0.5 + 0.5 = \pm 1.0^\circ\text{C}\).
Rule 2: Multiplication or Division (\(y = a \times b\) or \(y = \frac{a}{b}\))
Always ADD the PERCENTAGE uncertainties.
\(\% \Delta y = \% \Delta a + \% \Delta b\)
Rule 3: Powers (\(y = a^n\))
MULTIPLY the percentage uncertainty by the power \(n\).
\(\% \Delta y = n \times (\% \Delta a)\)
Example: Calculating the area of a circle where \(A = \pi r^2\). If the radius \(r\) has a percentage uncertainty of \(3\%\), the area \(A\) has an uncertainty of \(2 \times 3\% = 6\%\).
Key Takeaway: Add absolute uncertainties for \(+\) and \(-\). Add percentage uncertainties for \(\times\) and \(\div\). Multiply percentage uncertainty by the power for exponents.
4. CCEA Portfolio Standards: Tables and Graphs
When presenting your experimental findings in your AS 1 portfolio, examiners look for strict, professional conventions.
A. Data Tables
• Column Headers: Must always follow the standard format: Quantity / Unit (for example: \(V\text{ / V}\), \(I\text{ / mA}\), or \(L\text{ / mm}\)).
• Consistency: All raw readings in a column must be recorded to the same number of decimal places, matching the resolution of the measuring tool used.
B. Graph Plotting Rules
• Scale: Your plotted points must occupy at least 50% of the grid area in both the horizontal (\(x\)) and vertical (\(y\)) directions. Avoid awkward scale increments like multiples of 3 or 7; always use clean steps like 1:1, 1:2, 1:5, or 1:10 multiples.
• Plotting Points: Mark data points neatly and accurately using a sharp pencil as a small crisp cross (\(\times\)) or a circled dot (\(\odot\)).
• Line of Best Fit: Draw a single, smooth, continuous straight line (with a clear ruler) or smooth curve. Aim for an even balance of points on either side of the line, ignoring clear anomalous results.
• Gradient Triangles: When calculating the gradient (\(\text{Gradient} = \frac{\Delta y}{\Delta x}\)), construct a large gradient triangle that spans more than 50% of the drawn line of best fit. Never calculate the gradient using two raw recorded data points unless they happen to lie directly on your line!
5. Quick Review & Common Examiner Pitfalls
Avoid these common mistakes in your AS 1 portfolio:
• Forgetting Zero Errors: Always check and record whether your micrometer or meter has a zero error before taking measurements.
• Inconsistent Decimal Places: Never write a mean value like \(4.33333\text{ cm}\) when your ruler only measures to \(0.1\text{ cm}\). Match significant figures to your data's precision.
• Tiny Gradient Triangles: Drawing tiny triangles to find a gradient leads to rounding errors and loses easy marks. Make your triangle huge (covering \(> 50\%\) of the line).
• Parallax Error: Always take liquid readings and meter needle readings directly at eye level perpendicular to the scale.
Memory Aid for Uncertainty Combinations:
• Add / Subtract \(\rightarrow\) Add Absolute uncertainties (Triple A rule!)
• Multiply / Divide \(\rightarrow\) Add Percentages