A-Level Physics — 4) Energy & Fields (IP-Friendly Guide)

TL;DR
Energy keeps moving but never disappears — track the stores, measure the work, picture the fields and you will turn Paper 1 MCQs into freebies. This guide converts the SEAB bullet points into exam-grade check-lists, mini-drills and WA timing hacks.

1 Energy stores & transfers

The syllabus now uses the stores model: kinetic, gravitational, elastic, chemical, nuclear, internal and thermal.

An *energy transfer is any process that decreases one store while increasing another, with the total remaining constant (principle of conservation of energy).

You gain marks by

1. naming the store,
2. naming the mechanism,
3. stating “total energy is conserved”.

Mini-drill: Identify the two main stores and the transfer mechanism when a phone slides off a desk, hits the carpet and stops.

2 Work done by a force

Work is the mechanical transfer of energy. For a constant force \(F\) acting through displacement \(s\):

\[ W = Fs \cos \theta. \]

For Weighted Assessment 1 (WA1), sometimes questions set the force to be in the same direction as displacment so \(\theta = 0\) and hence \(W = Fs\).

Exam cue: quote both the numerical answer and the store changed — SEAB frequently awards a follow-up mark for stating “work done increases kinetic energy”.

3 Kinetic energy

Starting from \(v^2 = u^2 + 2as\) and \(W = Fs\) with \(F = ma\):

\[ W = (ma) \space s = \tfrac12 m(v^2 - u^2) = \Delta E_k. \]

Taking the object from rest \((u = 0)\) gives the familiar

\[ E_k = \tfrac12 m v^2. \]

Check-list: always attach \(\pu{J}\) and quote to three s.f. unless the question states otherwise.

4 Concept of a field

A field is a region where a body experiences a force without direct contact. Visualise it with arrows (field lines) or “slicing planes” (equipotentials).

4.1 Gravitational field

Define field strength

\[ g = \frac{F}{m} \space (\pu{N.kg-1}). \]

Lines point towards masses.

4.2 Electric field

Define field strength

\[ E = \frac{F}{q} \space (\pu{N.C-1}), \]

where \(q\) is positive by convention.

4.3 Equipotential surfaces

Field lines cross equipotentials at right angles. No work is done moving along an equipotential.

WA hack: draw one equipotential ring then add arrows — examiners see the concept instantly.

5 Potential energy

Elastic energy equals the area under the force-extension graph — triangular if Hookean, trapezoidal if not.

Mini-drill: Sketch a non-Hookean graph and shade the work done when stretching from 0 cm to 5 cm.

6 Power & efficiency

Power is the rate of energy transfer

\[ P = \frac{E}{t} = \frac{W}{t}. \]

For a constant force,

\[ P = Fv \]

because \(v = s/t\).

Efficiency

\[ \text{Efficiency} = \frac{\text{useful output}}{\text{total input}} \times 100\%. \]

Real devices suffer heat, sound and friction losses — a typical electric motor in WA problems lands in the 70-90 % band.

7 Three WA timing rules

1. 1 mark ≈ 1.5 min — same as SEAB design.
2. Label units first; numbers follow.
3. When in doubt, state conservation of energy — it rescues method marks even if arithmetic falters.

8 Bridge to Paper 4 practical

• Overlay field-line diagrams with equipotential maps in Logger Pro.
• Use =TRAPZ() in Sheets to integrate a force-extension curve.
• Quote final energies to the same s.f. as the least precise raw input.

9 Further reading

• SEAB H2 Physics 9478 syllabus (2026)
• Save My Exams — First Law of Thermodynamics
• Wikipedia — Work (physics)
• YouTube — Deriving \(E_k = \tfrac12 mv^2\)
• Physics Classroom — Electric Field & Movement of Charge
• Isaac Physics — Gravitational Field
• TechTarget — Electric Field Strength
• YouTube — Equipotential Lines & Surfaces
• Save My Exams — Elastic Potential Energy
• Physics Classroom — Power
• Keysight — Mechanical Power Cheatsheet
• Khan Academy — Elastic Potential Energy
• Investopedia — Efficiency

10 Call-to-action

Parents: book a 45-min Energy & Fields clinic one week before WA 1.
Students: screenshot the power equations and try three Fv -> P conversions tonight.

Last updated 14 Jul 2025. Next review when SEAB issues the 2027 draft syllabus.