A-Level Physics Equations and Constants You Should Know

Overview

This guide is designed as a practical A-Level physics formula sheet in article form. Instead of giving you an unstructured list, it groups equations and constants into the topic areas students most often revise separately: mechanics, materials, electricity, waves, quantum and nuclear physics, and fields. That matters because most exam mistakes are not caused by forgetting a formula completely. They usually happen because a student mixes two similar equations, forgets the units on a constant, or applies a familiar equation in the wrong context.

Think of this page as a revision hub rather than a replacement for your course specification. Different exam boards phrase topics slightly differently, and some include optional material or give equations in a data sheet. Even so, the core relationships below are useful across most A-Level courses and introductory physics study paths.

As you revise, focus on four things for every equation:

• Name: what physical idea the equation represents.
• Symbols: what each quantity means and what unit it usually uses.
• Conditions: when the equation applies and when it does not.
• Links: how it connects to other equations in the same topic.

If you build those four habits, your a level revision physics work becomes more stable and less dependent on memorizing isolated formulas.

Topic map

Use this map to find the equations and constants you are most likely to revisit by topic.

1. Mechanics

This is often where students first start building a real equation toolkit. The key groups are motion, forces, momentum, and energy.

• SUVAT equations: for constant acceleration problems.
  v = u + at
  s = ut + 1/2 at²
  v² = u² + 2as
  s = (u + v)t / 2
• Newton's second law: F = ma
• Weight: W = mg
• Momentum: p = mv
• Impulse: FΔt = Δp
• Kinetic energy: E_k = 1/2 mv²
• Gravitational potential energy near Earth: ΔE_p = mgΔh
• Work done: W = Fs cos θ
• Power: P = E/t and P = Fv
• Efficiency: efficiency = useful output / total input

Constants to know: gravitational field strength, g, is commonly taken as about 9.81 m s^-2 unless your question states otherwise or your course uses a rounded value.

Revision note: in mechanics, the hardest part is often not the algebra but the choice of model. Ask: is acceleration constant, is a force balanced, and is energy conserved? If you need a refresher on motion formulas, see Kinematics Equations Explained: When to Use Each SUVAT Formula. For force questions, pair this page with Free Body Diagrams Explained: Rules, Examples, and Common Mistakes and Newton’s Laws of Motion Problems With Step-by-Step Solutions.

2. Materials and deformation

These equations often look simple, but they rely on careful definitions and unit handling.

• Density: ρ = m/V
• Stress: stress = force / area
• Strain: strain = extension / original length
• Young modulus: E = stress / strain
• Elastic potential energy: E = 1/2 F x and E = 1/2 kx²
• Hooke's law: F = kx

Revision note: students often confuse stress and pressure because both use force over area and share the same unit, the pascal. Keep the physical context clear. Stress belongs to materials under load. Pressure often belongs to fluids or gas systems.

3. Electricity

This is one of the most equation-heavy parts of A-Level Physics, and it rewards organized revision.

• Charge: Q = It
• Potential difference: V = W/Q
• Resistance definition: R = V/I
• Power: P = IV, P = I²R, P = V²/R
• Energy transferred electrically: E = VQ and E = Pt
• Resistivity: R = ρL/A

Constants to know: elementary charge, e = 1.60 × 10^-19 C, is one of the most useful constants in particle and electricity questions.

Revision note: do not memorize power formulas as unrelated facts. Start from P = IV, then combine with V = IR to generate the others. That gives you more flexibility in exam questions. For worked circuit practice, use Ohm’s Law Problems With Answers and Full Working and Electric Circuits Explained: Series vs Parallel With Worked Examples.

4. Waves and optics

Waves questions usually feel easier once you separate the general relationships from the topic-specific details.

• Wave speed: v = fλ
• Phase difference: fraction of cycle × 2π rad, or fraction of cycle × 360°
• Refractive index: n = c/v
• Path difference condition for constructive and destructive interference: depends on whole-number or half-number multiples of wavelength, depending on the setup

Constants to know: speed of light in vacuum, c = 3.00 × 10⁸ m s^-1.

Revision note: with waves, language matters. Be able to define wavelength, frequency, phase difference, coherence, and superposition in words, not only symbols. That often turns a memorized equation into a usable exam answer. For a focused revision companion, see Waves Physics Revision Guide: Speed, Frequency, Wavelength, and More.

5. Circular motion and simple harmonic motion

These topics introduce equations that students often remember incompletely.

• Centripetal acceleration: a = v²/r
• Centripetal force: F = mv²/r
• Angular speed: ω = 2πf = 2π/T
• Simple harmonic motion condition: a ∝ -x
• SHM acceleration form: a = -ω²x
• Maximum speed in SHM: v_max = ωA

Revision note: the minus sign in SHM is not decorative. It shows that acceleration is directed towards the equilibrium position. If you learn what the sign means physically, the equation becomes much easier to remember.

6. Thermal physics and gases

These equations connect particle ideas to measurable quantities.

• Specific heat capacity: ΔQ = mcΔθ
• Ideal gas equation: pV = nRT
• Molecular form: pV = NkT

Constants to know: gas constant R = 8.31 J mol^-1 K^-1; Boltzmann constant k = 1.38 × 10^-23 J K^-1.

Revision note: always check whether a gas question uses amount of substance, n, or number of molecules, N. Mixing the two forms is a common error.

7. Fields and gravitation

Field equations are manageable if you sort them into force, potential, and energy.

• Newton's law of gravitation: F = Gm₁m₂/r²
• Gravitational field strength: g = F/m
• Gravitational potential: V = W/m
• Electric force: F = QE
• Electric field strength: E = F/Q and, for uniform fields, E = V/d
• Coulomb's law: F = 1/(4πɛ₀) × Q₁Q₂/r²
• Electric potential: V = W/Q

Constants to know: gravitational constant G, permittivity of free space ɛ₀, and elementary charge e. Exact values are often supplied, but you should still recognize them and know where they fit.

Revision note: many students confuse electric field strength E with energy E. When revising, say the quantity out loud: “field strength” versus “energy.” It sounds basic, but it prevents misreading under exam pressure.

8. Quantum and nuclear physics

These topics are compact, but the equations are high value in revision because they connect constants directly to physical meaning.

• Photon energy: E = hf
• Wave-particle relation: c = fλ for electromagnetic waves in vacuum
• Photoelectric equation: hf = φ + E_k,max
• Mass-energy equivalence: E = mc²
• Radioactive decay: dN/dt = -λN
• Decay activity: A = λN
• Half-life relation: linked to the decay constant λ by a logarithmic relationship often provided or derived in course work

Constants to know: Planck constant h = 6.63 × 10^-34 J s; speed of light c; elementary charge e.

Revision note: this topic is a good example of why constants matter. If you know what h represents physically, E = hf becomes easier to remember and easier to apply.

Related subtopics

A good equations hub should point you to the places where formulas become usable. If your revision feels stuck, it usually means you do not need more equations. You need better connections between equations, diagrams, and worked examples.

Here are the subtopics worth pairing with this page:

• Equation selection: knowing several formulas is not enough. You need to identify the givens, unknowns, and topic clues in a question.
• Units and prefixes: milli, micro, kilo, mega, and giga cause many avoidable mark losses.
• Rearranging formulas: weak algebra can make a familiar physics equation feel harder than it is.
• Graph interpretation: many constants and physical quantities appear as gradients or areas under graphs.
• Definitions: examiners often test ideas in words before they test them numerically.
• Data sheet use: if your course provides equations, revision should focus on recognition, interpretation, and application rather than raw memorization alone.

If you want a broader companion page, Physics Formulas Cheat Sheet: The Essential Equations Students Keep Forgetting is useful for cross-topic review. If you are moving up from earlier study, GCSE Physics Equations List: What You Need to Memorize and What to Understand helps connect prior knowledge to A-Level expectations.

One practical way to revise related subtopics is to build a three-column page for each chapter:

1. Equation: write the relationship clearly with symbols.
2. Meaning: explain in one sentence what the equation describes.
3. Exam use: add one short note such as “constant acceleration only” or “uniform electric field only.”

This makes your revision notes much more useful than copying a formula sheet without commentary.

How to use this hub

The best way to use an a level physics equations resource is not to read it once from top to bottom. Return to it in stages.

Stage 1: Build recognition

At the start of a topic, use this hub to identify the main equations and constants. Do not try to memorize everything at once. Instead, ask: which equations are core, which are derived, and which appear only in certain question styles?

Stage 2: Add meaning

For every equation, define each symbol and write the standard SI unit. This is where many students quietly fix their understanding. For example, if you know that resistivity has units and belongs to a material rather than a whole component, the equation R = ρL/A becomes much easier to use correctly.

Stage 3: Practice retrieval

Close your notes and write down the equations for one topic from memory. Then check what you missed. Retrieval practice is usually more effective than rereading because it shows you exactly where your memory is still weak.

Stage 4: Solve mixed problems

Move from single-step questions to mixed-topic questions. That is where real exam preparation starts. In a mechanics question, for example, you may need a force equation, an energy equation, and a graph interpretation in the same answer.

Stage 5: Build an error log

Keep a short list of the formulas you misuse, not just the ones you forget. Typical entries include:

• using a SUVAT equation when acceleration is not constant
• confusing voltage with energy transferred
• forgetting to convert centimetres to metres before using SI units
• mixing up field strength and potential
• using the wrong area or length in resistivity questions

This turns your mistakes into a personalized revision guide.

Stage 6: Revise constants with context

Do not learn constants as detached numbers. Link each one to a topic and a physical meaning:

• g belongs to weight, motion, and gravitational potential energy near Earth.
• c belongs to waves, relativity contexts, and refractive index work.
• h belongs to photons and quantum physics.
• e belongs to charge and particle processes.
• R and k belong to gas equations.

That is more reliable than trying to memorize a list of symbols in isolation.

When to revisit

Come back to this hub whenever your revision needs structure rather than more pages of notes. In practice, that usually means five moments.

• At the start of a new topic: use the topic map to see where the new equations fit into the bigger course.
• Before a topic test or mock exam: review the equations and constants for just the chapters being assessed.
• After doing past-paper questions: add missed formulas and misunderstandings to your own summary notes.
• When new subtopics appear in class: expand your equation map instead of leaving formulas scattered across notebooks.
• In final revision: use this page as a checklist for spaced recall, not passive reading.

For a practical next step, choose one topic today and make a one-page summary with these headings: equation, symbols, units, conditions, and one worked example. Then test yourself on it tomorrow without notes. If you repeat that process across the course, this page becomes more than an a level physics formula sheet. It becomes the framework behind your exam prep.

As your course expands, this hub is also worth revisiting when you notice new overlaps between topics. Electricity links to materials through resistivity. Waves links to quantum ideas through frequency and photon energy. Mechanics links to fields through energy and force models. The more clearly you see those links, the less physics feels like a list of separate chapters and the more it starts to feel like one connected subject.