OCR GCSE Physics A (J249) — Practical skills
Every time you bake a cake, using a digital kitchen scale rather than guessing the weight makes the difference between a perfect sponge and a disaster. In physics, selecting the correct instrument ensures accurate and reliable data. You must choose apparatus with an appropriate resolution to match the scale of what you are measuring.
Resolution is the smallest change in the quantity being measured that gives a perceptible change in the reading on the instrument. Precision refers to how close repeated measurements are to each other, while accuracy is how close a measurement is to the true value.
| Quantity | Apparatus Selected | Typical Resolution | Technique to Minimise Error |
|---|---|---|---|
| Length | Metre ruler / Micrometer screw gauge / Vernier calipers | Ruler: 1 mm <br> Micrometer: 0.01 mm | Use a micrometer for very thin objects like wires. Use a fiducial marker (e.g., a pin) to mark clear start/end points. |
| Mass | Digital balance | 0.1 g or 0.01 g | Tare (zero) the balance before placing the object to remove systematic errors. Always dry wet objects before weighing. |
| Time | Stopwatch / Light gates | Stopwatch: 0.01 s | Time 10 or 20 oscillations and calculate a mean to reduce the impact of human reaction time. Use light gates for high-speed motion. |
| Volume | Measuring cylinder / Displacement can | 1 ml or 2 ml | Read the scale at eye level. For irregular solids, use a displacement can and measure the displaced water. |
| Temperature | Thermometer (Analogue or Digital) | 1°C, 0.5°C, or 0.1°C | Stir liquids thoroughly to ensure a uniform temperature before taking the reading. |
| Area |
| 1 cm² grid paper |
| 1 cm² per square |
| Trace irregular objects on the grid. Count all squares that are >50% covered. |
A clock that is always exactly five minutes fast will make you consistently early, demonstrating how a small flaw in an instrument shifts every single result. This type of flaw is called a zero error, which is a systematic error where a measuring instrument displays a non-zero value when the true quantity is zero. To correct this, you must apply a mathematical correction to your final readings.
A parallax error is another common systematic error caused by viewing a scale from an angle rather than straight on. To avoid this, always read scales with your eye line directly perpendicular (90°) to the measurement marking. When measuring liquids in a cylinder, always read from the bottom of the curved surface, known as the meniscus.
For timing experiments, typical human reaction time is between 0.2 s and 0.5 s. This delay supersedes the 0.01 s resolution of a stopwatch, meaning high-resolution stopwatches can give a false sense of precision for very short events.
A student uses vernier calipers to measure the thickness of a metal cylinder. Before placing the cylinder in the jaws, the calipers read . The final reading with the cylinder is . Calculate the true thickness.
Step 1: Identify the zero error and the measured reading.
Step 2: Subtract the zero error from the measured reading.
Step 3: Calculate the final answer.
If you throw a pebble into a still pond, the expanding rings of water behave exactly like the invisible light and sound waves travelling around us. We can study these properties using a ripple tank, which is a shallow glass tank filled with a shallow layer of water (less than 1 cm deep).
A motor-driven oscillating paddle (dipper) creates straight waves across the water surface. A lamp placed above the tank casts shadows of the wavefronts onto a white screen below, where light patches represent wave troughs and dark patches represent wave crests.
To measure the wavelength (), use a ruler to measure the total distance across 10 complete wavefronts on the screen, then divide that distance by 10. To measure the frequency (), count how many complete waves pass a fixed point in 10 seconds and divide by 10.
Alternatively, frequency can be read directly from the signal generator powering the paddle. To make measuring wavelength easier, a stroboscope can be used to flash light at the exact same frequency as the waves, making the pattern appear "frozen" on the screen.
When a guitarist plucks a string, it vibrates so fast it becomes a blur, but at specific frequencies, clear, stable shapes appear along the wire. These stable shapes are called stationary waves, and they form when an incoming wave reflects off a fixed boundary and interferes with itself.
To observe this, attach a string to a vibration generator (driven by a signal generator) at one end, pass it over a wooden bridge and pulley at the other, and hang masses to provide constant tension. Turn on the signal generator and adjust the frequency until clear "loops" appear on the string.
The points on the string that do not move are called nodes, and the points of maximum vibration are called antinodes. Crucially, the length of a single loop represents only half a wavelength (). To find the full wavelength, you must measure the length of one loop using a metre ruler and multiply it by 2.
Thunder and lightning happen simultaneously, but we calculate the speed of the storm by timing the delay because sound travels much slower than light. For any wave, whether in a ripple tank or on a string, the wave speed can be calculated if you know its frequency and wavelength.
Where:
A student sets up a stationary wave on an elastic cord. The signal generator displays a frequency of 45 Hz. The student measures the total length of 3 vibrating loops to be 1.2 m. Calculate the speed of the wave.
Step 1: Find the length of one single loop.
Step 2: Calculate the full wavelength ().
Step 3: Substitute the values into the wave equation.
Step 4: Calculate the final answer with units.
Students frequently assume the length of one loop on a vibrating string is the full wavelength, but one loop is actually only half a wavelength (). Always multiply the length of one loop by 2.
When describing a ripple tank experiment in a 6-mark question, explicitly state that you measure the distance across 10 wavefronts and divide by 10 to find the mean, as examiners award marks for techniques that reduce percentage uncertainty.
If an exam question asks for the most appropriate instrument to measure the diameter of a very thin wire, always select a micrometer screw gauge over a metre ruler, as its 0.01 mm resolution is necessary for such small dimensions.
Remember to convert centimetres (cm) or millimetres (mm) into metres (m) before using the wave speed equation, as OCR requires standard SI units.
Resolution
The smallest change in the quantity being measured that gives a perceptible change in the reading of the instrument.
Precision
How close repeated measurements are to each other, indicating the spread of the data.
Accuracy
How close a measurement is to the true or accepted value.
Light gates
Digital timing devices that use an infrared beam to eliminate human reaction time when measuring high-speed motion.
Zero error
A systematic error where a measuring instrument indicates a non-zero value when the quantity being measured is actually zero.
Parallax error
A systematic error caused by viewing a measurement scale from an angle rather than directly perpendicular to it.
Meniscus
The curved upper surface of a liquid in a tube. For water, the measurement must be taken from the bottom of the curve at eye level.
Fiducial marker
A fixed point of reference, such as a pin or mark, used to ensure measurements are started or ended at the exact same position every time.
Ripple tank
A shallow glass tank used to observe the properties of water waves by casting shadows of wavefronts onto a screen.
Wavefront
A line representing all points on a wave that are in the same phase, such as a line joining all the crests together.
Wavelength (λ)
The shortest distance between the same point on two consecutive waves (e.g., from crest to crest), measured in metres.
Frequency (f)
The number of complete waves passing a fixed point per second, measured in Hertz (Hz).
Signal generator
An electronic device that produces an oscillating signal at a set frequency, used to drive vibration generators.
Stroboscope
A device that flashes light at a controllable frequency, used to make moving waves appear stationary.
Stationary waves
A wave pattern that remains in a constant position, characterized by alternating nodes and antinodes, formed by interference.
Nodes
Points on a stationary wave where there is zero amplitude (no movement).
Antinodes
Points on a stationary wave where there is maximum amplitude.
Put your knowledge into practice — try past paper questions for Physics A
Resolution
The smallest change in the quantity being measured that gives a perceptible change in the reading of the instrument.
Precision
How close repeated measurements are to each other, indicating the spread of the data.
Accuracy
How close a measurement is to the true or accepted value.
Light gates
Digital timing devices that use an infrared beam to eliminate human reaction time when measuring high-speed motion.
Zero error
A systematic error where a measuring instrument indicates a non-zero value when the quantity being measured is actually zero.
Parallax error
A systematic error caused by viewing a measurement scale from an angle rather than directly perpendicular to it.
Meniscus
The curved upper surface of a liquid in a tube. For water, the measurement must be taken from the bottom of the curve at eye level.
Fiducial marker
A fixed point of reference, such as a pin or mark, used to ensure measurements are started or ended at the exact same position every time.
Ripple tank
A shallow glass tank used to observe the properties of water waves by casting shadows of wavefronts onto a screen.
Wavefront
A line representing all points on a wave that are in the same phase, such as a line joining all the crests together.
Wavelength (λ)
The shortest distance between the same point on two consecutive waves (e.g., from crest to crest), measured in metres.
Frequency (f)
The number of complete waves passing a fixed point per second, measured in Hertz (Hz).
Signal generator
An electronic device that produces an oscillating signal at a set frequency, used to drive vibration generators.
Stroboscope
A device that flashes light at a controllable frequency, used to make moving waves appear stationary.
Stationary waves
A wave pattern that remains in a constant position, characterized by alternating nodes and antinodes, formed by interference.
Nodes
Points on a stationary wave where there is zero amplitude (no movement).
Antinodes
Points on a stationary wave where there is maximum amplitude.
