IB Physics SLTool 1 — Experimental TechniquesPaper 1, 2 & IAThe right tool for the job~8 min read
Measuring Variables
Good data starts with the right instrument, read the right way. Reach for a ruler when you need a micrometer, or squint at an analogue scale from the wrong angle, and your results are compromised before you’ve even started. This page pairs each quantity with the tool that measures it — and shows how to read each one precisely.
📘 What you need to know
You need to choose the most appropriate instrument for each variable, and read it accurately
Mass → digital balance (tare first); time → stopwatch; length → ruler, tape, or micrometer
Volume → measuring cylinder; temperature → thermometer or digital probe; force → force meter (spring balance)
Current → ammeter, connected in series; potential difference → voltmeter, connected in parallel
Analogue instruments have a needle and scale; digital ones give a numerical readout that’s usually more precise
Watch for zero errors (check the marker before you start) and parallax errors (read perpendicular to the scale)
Human reaction time matters most when timing very short intervals
Matching the Instrument to the Variable
The first skill is simply picking the right tool. Here’s the core set for IB physics, with the units and typical precision you’d quote.
Variable
Instrument
Typical unit
Note
Mass
Digital balance
kg (or g)
Tare (zero) before weighing; usually 2 d.p.
Time
Stopwatch / stop-clock
s
Limited by human reaction time for short intervals
Length
Ruler / tape / micrometer
m (mm, cm)
Ruler reads to the nearest mm; micrometer for tiny lengths
Volume
Measuring cylinder
ml (cm³)
1 ml = 1 cm³; read the bottom of the meniscus
Temperature
Thermometer / digital probe
°C
Probe reads to ±0.1 °C, more precise than liquid type
Force
Force meter (spring balance)
N
Read at eye level to avoid parallax
Current
Ammeter
A (mA, µA)
Connected in series
Potential difference
Voltmeter
V (mV)
Connected in parallel
Angle
Protractor
degrees
±0.5°; align the baseline, read at eye level
A ruler and a micrometer both measure length — but you’d never measure the diameter of a wire with a ruler. Always ask “what’s the smallest thing I need to resolve?” and pick the instrument whose precision matches. That single decision is often worth a mark on its own.
Reading a Ruler
A ruler measures small distances to the nearest millimetre. The trick is to line the object up against the scale carefully and read at eye level. Note that the object below runs from 0 to the 9 cm mark, giving a length of 9.0 cm.
Line the object up with zero and read the far end against the scale — a ruler resolves to the nearest millimetre.
Analogue vs Digital Instruments
Many instruments — ammeters, voltmeters, force meters — come in two styles. Knowing the trade-offs lets you pick well and read carefully.
Analogue
Digital
Display
Needle moving over a scale
Numerical readout
Reading
You judge the needle’s position
Gives a specific value directly
Precision
Lower; limited by the scale
Higher; can show mV, µA, etc.
Main pitfalls
Parallax and zero errors
Readout may flicker between values
On an analogue scale, your line of sight matters. Reading from an angle makes the needle appear to line up with the wrong mark — that’s parallax error. Always read from directly in front, perpendicular to the scale.
Reading an analogue scale from the side shifts the apparent needle position — always view perpendicular to the scale.
Current and Potential Difference
Two electrical instruments trip students up constantly, and it comes down to how they’re connected. An ammeter measures current (charge per unit time), so it goes in series — the current has to flow through it. A voltmeter measures the potential difference between two points, so it goes in parallel across the component.
The ammeter sits in the main loop (series); the voltmeter branches off across the lamp (parallel).
ammeter
→ measures current →
in SERIES
•••
voltmeter
→ measures p.d. →
in PARALLEL
Quick recap: pick the instrument whose precision fits the task; read to the scale’s smallest division; avoid zero and parallax errors; ammeter goes in series, voltmeter in parallel.
WE 1
A student needs to measure the diameter of a thin copper wire and the time for a pendulum to complete 10 swings. State the most appropriate instrument for each, and give one reason for your choice.
Diameter of a thin wire
use a micrometer (not a ruler)
the diameter is well under a millimetre, so a ruler isn’t precise enough
→ a micrometer resolves much smaller lengthsTime for 10 swings
use a stopwatch, and time 10 swings rather than 1
dividing by 10 reduces the effect of human reaction time
→ a stopwatch, timing many swings for better accuracy
WE 2
A student sets up a circuit to find the current through a lamp and the potential difference across it. (a) State how the ammeter and voltmeter should each be connected. (b) The analogue ammeter reads slightly above zero with no current flowing. Name this error and state how to correct for it.
Part (a) — connections
ammeter in series with the lamp (so the current flows through it)
voltmeter in parallel across the lamp (to read the p.d. across it)
→ series for current, parallel for p.d.Part (b) — the error
this is a zero error
note the reading with no current, then subtract it from every measurement→ zero error, corrected by subtracting the offset
💡 Top tips
Justify your instrument choice by its precision: match the smallest division to the size of what you’re measuring
Ammeter = series, voltmeter = parallel. A memory hook: the Ammeter joins the mAin line; the Voltmeter branches oVer the component
Beat parallax by reading analogue scales perpendicular, at eye level
Time many cycles (e.g. 10 swings) and divide, to shrink the effect of reaction time
⚠ Common mistakes
Swapping the connections — putting the ammeter in parallel or the voltmeter in series
Ignoring a zero error instead of subtracting it from every reading
Reading an analogue scale from an angle (parallax) rather than straight on
Choosing a ruler for something that needs a micrometer — mismatched precision loses marks
That wraps up Tool 1: Experimental Techniques. You can now think through the safety, ethics and environmental side of a practical, and choose and read the right instrument for every variable — exactly the practical judgement your Internal Assessment is marked on.
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