SI base units and derived units

Every quantity in physics is built from just seven SI base units. Edexcel expects fluency with these six:

• kilogram (kg) — mass · metre (m) — length · second (s) — time
• ampere (A) — electric current · kelvin (K) — temperature · mole (mol) — amount of substance

Everything else is derived — you build it from the base units using a defining equation:

Homogeneity check: a correct equation has the same base units on both sides. If they do not match, the equation is definitely wrong. (Matching units does not prove it right — a missing factor of ½ is dimensionless.)

Prefixes, standard form and orders of magnitude

Physics spans 10⁻¹⁵ m (a nucleus) to 10²⁶ m (the observable Universe). Prefixes keep the numbers readable:

• T (10¹²) · G (10⁹) · M (10⁶) · k (10³)
• c (10⁻²) · m (10⁻³) · µ (10⁻⁶) · n (10⁻⁹) · p (10⁻¹²) · f (10⁻¹⁵)

Order-of-magnitude estimate: examiners award marks for a sensible power of ten with clear assumptions stated. Being within a factor of ten is the target — not three significant figures.

Which is a base unit?

Base unit, derived unit, or prefix?

Tap an item, then tap the box it belongs in.

Precision, accuracy and the two kinds of error

Two words examiners will not let you swap:

• Precision — how closely repeated readings agree with each other (small spread).
• Accuracy — how close a reading is to the true value.

Readings can be precise but inaccurate — tightly grouped around the wrong answer. That is the fingerprint of a systematic error.

• Systematic error — shifts every reading the same way (zero error on a micrometer, a mis-calibrated ammeter). Repeating does not help; you must correct or re-calibrate.
• Random error — scatters readings either side of the true value (reaction time, judging a scale). Repeat and mean to reduce it.

Resolution is the smallest change an instrument can show. A metre rule reading to 1 mm has a resolution of 1 mm — but that alone does not make it accurate.

Precise, accurate, or neither?

Calculating and combining uncertainties

To combine uncertainties in a calculation, Edexcel uses three rules:

• Adding or subtracting quantities → add the absolute uncertainties.
• Multiplying or dividing → add the percentage uncertainties.
• Raising to a power n → multiply the percentage uncertainty by n.

The power rule bites: a cube of side l has V = l³, so a 1% uncertainty in l becomes a 3% uncertainty in volume. Measuring the side accurately matters three times over.

Your turn — percentage uncertainty

Your turn — combining uncertainties

Your turn — the power rule

Graphs, gradients and straight-line form

Physicists bend equations into the shape y = mx + c so that a graph gives a gradient with physical meaning.

• Pendulum: T = 2π√(l/g) → square it: T² = (4π²/g) l. Plot T² against l; gradient = 4π²/g.
• Internal resistance: V = ε − Ir. Plot V against I; gradient = −r, intercept = ε.
• Capacitor discharge: exponentials become straight by taking logs — ln Q = ln Q₀ − t/RC, gradient = −1/RC.

Uncertainty from a graph: draw the steepest and shallowest lines that still pass through all the error bars. Uncertainty in the gradient ≈ (max gradient − min gradient) ÷ 2.

Your turn — unit conversion

Your turn — using SI units

Reading the units

Match the quantity to its base units

Tap an item on the left, then its partner on the right.

Significant figures

Models, validation and peer review

A physicist builds a model, uses it to make a testable prediction, and then tries to break it with experiment. If the data disagree, the model is modified or replaced.

• Validation — an idea is only accepted once other scientists can reproduce the result.
• Peer review — before publication, independent experts check the method, the analysis and the claims.
• Anomalies — a rogue point is investigated and repeated, not quietly deleted.

Example: Newtonian gravity predicted planetary orbits superbly — but not Mercury's perihelion precession. General relativity kept everything Newton got right and fixed the discrepancy. Models are replaced by better models, not by opinions.

The big ideas to know

That is the whole of Edexcel Topic 1 — the skills every later topic is marked against. Press Finish to see your score.