A-Level Physics Required Definitions You Must Know for Full Marks

Overview

A-Level physics definitions are one of the few parts of revision where precision matters almost word for word. You may understand the concept well, but still miss marks if your answer is too broad, too casual, or missing a required detail such as direction, rate of change, or unit context.

The aim of this article is not to provide an endlessly long glossary to cram in one sitting. Instead, it is a living definitions guide. That means you should revisit it regularly and track your progress over time. Some definitions need exact recall. Others need a clear explanation in your own words plus the ability to apply them in a calculation, graph question, or practical context.

A useful way to think about A-Level physics definitions is to split them into three precision levels:

• Level 1: exact wording definitions — these are short statements where one missing phrase can cost the mark.
• Level 2: technical distinctions — pairs or groups of terms that students often confuse, such as scalar and vector, EMF and potential difference, or diffraction and refraction.
• Level 3: definition plus context — ideas that are easy to state but harder to use correctly, such as electric field strength, decay constant, or Young modulus.

This structure makes the page useful across AQA, Edexcel, and OCR courses, while still keeping it practical for UK exam preparation. For a broader view of topic order and study planning, see A-Level Physics Topics List with Best Revision Order and High-Value Skills.

One more point matters. Definitions do not live on their own. They connect directly to equations, units, practical work, and graph interpretation. If you revise them alongside those linked skills, they stick better and become easier to use in exam questions.

What to track

The best way to improve with required definitions A-Level physics students are expected to know is to track them by topic rather than trying to memorise everything at once. Below is a practical list of high-value definition areas and what to monitor in each one.

1. Mechanics definitions

These are often introduced early but tested throughout the course. Track whether you can define each term cleanly and distinguish it from nearby ideas.

• Scalar quantity — has magnitude only.
• Vector quantity — has magnitude and direction.
• Speed — distance travelled per unit time.
• Velocity — rate of change of displacement.
• Acceleration — rate of change of velocity.
• Force — interaction that can cause acceleration; in many exam contexts linked to rate of change of momentum.
• Momentum — product of mass and velocity.
• Work done — energy transferred by a force moving an object through a distance.
• Power — rate of energy transfer or rate of doing work.

What to track here: are you missing words like displacement instead of distance, or rate of change instead of just change? These small differences matter. If your mechanics wording is still GCSE-like, revisit the higher precision language. Our guides on Forces and Motion Revision and SI Units, Prefixes, and Conversions are useful support here.

2. Materials definitions

Materials questions often reward exact definitions because the terms are technical and closely linked.

• Stress — force per unit cross-sectional area.
• Strain — extension divided by original length.
• Young modulus — stress divided by strain, within the limit of proportionality.
• Elastic deformation — deformation from which the material returns to its original shape when the force is removed.
• Plastic deformation — permanent deformation after the force is removed.

What to track: whether you include the condition for Young modulus and whether you can explain elastic and plastic behaviour without vague wording.

3. Waves and optics definitions

Waves contain many familiar terms, but exam questions often depend on precise distinctions.

• Progressive wave — transfer of energy from one place to another without overall transfer of matter.
• Transverse wave — oscillations are perpendicular to the direction of energy transfer.
• Longitudinal wave — oscillations are parallel to the direction of energy transfer.
• Phase difference — difference in position within a cycle or difference in fraction of a cycle between two points.
• Coherence — waves with a constant phase difference and the same frequency.
• Diffraction — spreading of waves as they pass through a gap or around an obstacle.
• Refraction — change in direction because wave speed changes when entering a different medium.
• Superposition — when waves meet, the resultant displacement is the vector sum of individual displacements.

What to track: do you confuse diffraction with refraction, or phase difference with path difference? Can you define coherence well enough to explain why a stable interference pattern forms? For related revision, see A-Level Waves Revision and How to Draw and Interpret Physics Graphs.

4. Electricity definitions

This topic is full of terms that sound similar in ordinary language but mean different things in physics.

• Current — rate of flow of charge.
• Potential difference — energy transferred per unit charge between two points.
• Electromotive force (EMF) — energy supplied per unit charge by a source.
• Resistance — ratio of potential difference across a component to the current through it.
• Resistivity — property of a material defined by the relation between resistance, length, and cross-sectional area.
• Power — rate of energy transfer.
• Internal resistance — resistance inside a source that causes the terminal potential difference to be less than the EMF when current flows.

What to track: whether you can clearly separate EMF from potential difference and resistance from resistivity. Students often understand the circuit method but state the definition loosely. For deeper treatment, revisit A-Level Electricity Revision.

5. Fields definitions

Field language often feels abstract, so definitions need regular review.

• Gravitational field strength — force per unit mass.
• Electric field strength — force per unit charge.
• Potential — work done per unit charge or energy per unit charge in bringing a small positive charge from infinity to a point, depending on context and board phrasing.
• Uniform field — a field in which field strength is constant in magnitude and direction.

What to track: whether you can distinguish field strength from potential, and whether you can state units confidently.

6. Nuclear and quantum definitions

These terms are short, technical, and easy to misstate under time pressure.

• Isotopes — nuclei with the same number of protons but different numbers of neutrons.
• Decay constant — probability per unit time that a nucleus decays.
• Half-life — time taken for the number of undecayed nuclei, or the activity, to fall to half its original value.
• Activity — rate at which a sample undergoes radioactive decay.
• Photon — discrete packet of electromagnetic radiation energy.
• Ionisation — removal or addition of electrons from or to an atom or molecule to form an ion.

What to track: are you giving a process description or an actual definition? For example, half-life must refer to halving, not simply “how quickly something decays”.

7. Practical and data language

Definitions are not limited to theory topics. Practical wording can also earn or lose marks.

• Independent variable — the variable you deliberately change.
• Dependent variable — the variable you measure or observe.
• Control variable — a variable kept constant to make the test fair.
• Uncertainty — interval within which the true value is expected to lie.
• Systematic error — consistent shift in results in the same direction.
• Random error — unpredictable variation causing scatter in readings.
• Resolution — smallest change in a quantity that can be measured by an instrument.

What to track: whether you can use these definitions in a practical write-up, graph question, or method evaluation rather than only reciting them. This links closely with exam technique and required practical physics work.

Cadence and checkpoints

To get full marks definitions physics questions ask for, revision has to be repeated. A one-off memorisation session fades quickly. A better method is to build a recurring schedule with small checkpoints.

Weekly checkpoint

Choose one topic each week and test 8 to 12 definitions from memory. Cover the answers and write each one in a single sentence. Then mark each definition using a simple traffic-light system:

• Green — exact and confident.
• Amber — mostly right but missing a key phrase.
• Red — incorrect, vague, or confused with another term.

This takes around 10 to 15 minutes and gives a clear picture of progress.

Monthly checkpoint

Once a month, revisit all major topics and look for patterns. Are your red definitions clustered in one area such as electricity or waves? Are you repeatedly losing precision on rate-based definitions like current, acceleration, and activity? This is where the article becomes a tracker rather than just a list.

Create a simple table with these columns:

• Definition
• Topic
• Last tested
• Status
• Common mistake
• Next review date

The “common mistake” column is particularly useful. For example:

• Velocity — wrote “distance” instead of “displacement”.
• EMF — wrote “energy used” instead of “energy supplied”.
• Coherence — forgot “constant phase difference”.

Checkpoint before topic tests or mocks

In the week before an assessment, test definitions in mixed order instead of by topic. This is harder, but more realistic. Exam questions do not announce that three similar definitions are coming in a neat group. Mixed retrieval helps you separate related terms properly.

Final exam checkpoint

Near the exam, focus on the shortest, highest-risk definitions first. These are the ones where a single missing phrase can drop the mark. Then link them to equations and units. If you can state the definition, write the equation, and give the unit, your understanding is usually more secure.

For equations practice alongside definitions, keep the SI units guide and, where useful, the exam board comparison guide nearby.

How to interpret changes

If you are revisiting this page regularly, the main value is not only in testing definitions but in seeing what your mistakes tell you.

If your wording is usually close but not exact

This suggests your conceptual understanding is decent, but your recall is not sharp enough for exam conditions. The solution is short, frequent retrieval practice rather than rereading notes. Flashcards, blurting, and quick oral testing work better than passive review.

If you keep mixing pairs of terms

This often means you need comparison practice. Put the two terms side by side and force yourself to state the difference in one line. Good examples include:

• speed vs velocity
• distance vs displacement
• EMF vs potential difference
• resistance vs resistivity
• diffraction vs refraction
• field strength vs potential

When definitions are confused in pairs, isolated memorisation is usually not enough.

If you know the definition but cannot use it

This is a sign that the term is not connected strongly enough to questions, graphs, or practicals. For example, you may know the definition of current but struggle to use it in a drift of charge explanation, or know stress and strain separately but mix them on a graph. In that case, combine definition revision with worked examples and graph interpretation.

If some topics stay red for weeks

This usually points to one of three problems: the underlying concept is weak, the vocabulary is too abstract, or you are revising too many similar definitions at once. Slow down and learn fewer terms in a more connected way. Build mini-sets such as “waves language”, “electricity language”, or “nuclear language”.

If your definitions improve but marks do not

Then the issue may be application or exam technique rather than recall. Definitions can earn direct marks, but many more marks depend on using them in explanations. That is where past-paper practice matters. A definition should become a sentence you can deploy accurately inside a longer answer, not just a standalone card in a deck.

When to revisit

This guide works best when you return to it on purpose rather than only when panic starts. There are a few moments when revisiting definitions is especially useful.

• At the start of a new topic — learn the core language early so the lessons make more sense.
• At the end of each month — check which definitions stayed secure and which faded.
• Before required practical write-ups — review variables, uncertainty, resolution, and error language.
• Before mocks — switch from topic-by-topic learning to mixed self-testing.
• After every marked paper — add any missed terminology to your personal tracker.
• When you change revision focus — for example, moving from mechanics into electricity or waves.

A simple action plan is enough:

1. Pick one topic from this article.
2. Write five definitions from memory without notes.
3. Mark them for precision, not just general meaning.
4. Record your common mistakes.
5. Retest the same five after 48 hours and again after one week.

If you want this page to stay useful, treat it as a master checklist. Add your own definitions from lessons, mark schemes, and corrections from past papers. Over time, you will build a personal A-Level physics glossary that reflects the terms you actually lose marks on.

The calm truth is that full-mark definition answers are usually built through repetition, not brilliance. Precise language improves when you revisit it regularly, notice your recurring errors, and keep linking words to equations, units, graphs, and practical situations. That is why this is worth returning to throughout your A-Level physics revision, not just at the end.