H2 Physics Energy & Fields Notes | A-Level 9478

Q: What does A-Level Physics: 4) Energy & Fields Guide cover?
A: From energy stores and power equations to field lines and equipotential surfaces, this post demystifies Section I Topic 4 of the 2026 H2 Physics syllabus.

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.

Concrete example: how to use this page

If a question says "friction" or "air resistance", do not assume mechanical energy is conserved. Write the initial energy store, final energy store, and work done by non-conservative forces before substituting numbers.

Use the H2 Physics notes hub to hop between this topic, the preceding mechanics refreshers, and later electrostatics/capacitance chapters without losing the syllabus thread.

Route map: choose the energy method first

This map keeps the main decisions separate. The common mistake is to see a force and immediately use $F = ma$, even when an energy equation is faster.

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 $s$:

$$W = F s \cos\theta.$$

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

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 $v^{2} = u^{2} + 2 a s$ and $W = F s$ with $F = m a$

$$W = m a s = \tfrac{1}{2} m (v^{2} - u^{2}) = \Delta E_k.$$

Taking the object from rest $u = 0$ gives the familiar

$$E_k = \tfrac{1}{2} m v^{2}.$$

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

$$g = \frac{F}{m}.$$

Units: $\pu{N.kg-1}$.

Lines point towards masses.

4.2 Electric field

Define field strength

$$E = \frac{F}{q}.$$

Units: $\pu{N.C-1}$.

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.

Field-work sign checkpoint

Before using energy language in a field question, decide who is doing the work and whether the object moves with or against the field force.

Misconception check: field arrows show the force direction for a test mass in gravity or a positive test charge in electricity. Energy change follows the work done by that force, not just the shape of the path.

Worked check: a positive charge of $\pu{2.0e-6 C}$ moves through a potential difference of $\pu{150 V}$. The change in electric potential energy is

$$\Delta E = q\Delta V = (2.0 \times 10^{-6})(150) = \pu{3.0e-4 J}.$$

If it moves along an equipotential instead, $\Delta V = 0$, so $\Delta E = 0$ even if the path is curved.

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,

$$P = F v$$

because $v = \dfrac{s}{t}$.

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. Use syllabus pacing as a guide: Paper 2/3 average ~1.6 min/mark; Paper 4 ~3 min/mark.
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 a helper formula or the trapezium rule in Sheets to estimate the area numerically.
• Quote final energies to the same s.f. as the least precise raw input.

Need structured practice on Energy and Fields? Our H2 Physics tuition programme covers this topic with weekly problem sets and Paper 4 practical drills.

Comprehensive revision pack

9478 Section I, Topic 4 Syllabus outcomes

Candidates should be able to:

• (a) show an understanding that physical systems can store energy, and that energy can be transferred from one store to another.
• (b) give examples of different energy stores and energy transfers, and apply the principle of conservation of energy to solve problems.
• (c) show an understanding that work is a mechanical transfer of energy, and define and use work done by a force as the product of the force and displacement in the direction of the force.
• (d) derive, from the definition of work done by a force and the equations for uniformly accelerated motion in a straight line, the equation $E_k = \tfrac{1}{2} m v^2$

Concept map (in words)

Energy questions have three checkpoints: identify the store, choose the transfer (work, heating, radiation), and quantify with equations. Fields provide the force backdrop that links potential energy to work. Use energy-first reasoning for speed (e.g., convert GPE to KE), then verify with dynamics if required.

Key definitions & formulae

Derivations & reasoning to master

1. Work-energy theorem: derive $\Delta E_k = \int \vec{F} \cdot d\vec{s}$

Worked example 1 - multi-store energy track

A $\pu{0.80 kg}$ block is released from rest and slides down a $\pu{3.0 m}$ rough slope (angle $25^{\circ}$). The frictional force on the slope is constant at $2.0 \pu{N}$. At the bottom, it runs onto a smooth surface and compresses a spring ($k = 420 \pu{N.m-1}$

Solution (energy):

$$m g (s \sin 25^{\circ}) - F_{\text{fric}} s = \tfrac{1}{2} k x^{2}.$$

Taking $g = \pu{9.81 m.s-2}$,

$$x = \sqrt{\dfrac{2\left(0.80\times 9.81\times 3.0\times \sin 25^{\circ} - 2.0\times 3.0\right)}{420}} \approx \pu{0.137 m}.$$

Worked example 2 - electric + gravitational potential

An electron is released from rest midway between two parallel metal plates $\pu{1.5 cm}$ apart with potential difference $\pu{120 V}$. Determine its speed when it reaches the positive plate, neglecting gravity. (Assume the field is uniform between the plates.)

Strategy: use energy conversion $\tfrac{1}{2} m v^{2} = e\Delta V$. From the midpoint to a plate, the potential change is $\Delta V = \pu{60 V}$

$$v = \sqrt{\dfrac{2 e \Delta V}{m_e}} \approx \sqrt{\dfrac{2\left(1.60\times 10^{-19}\right)\left(60\right)}{9.11\times 10^{-31}}} \approx \pu{4.6e6 m.s-1}.$$

This is equivalent to the constant-field method $E = V/d$ with $a = eE/m_e$.

Practical & data tasks

• Map gravitational potential using Pasco field plotting board; compare with the theoretical $1 / r$ graph.
• Record force vs extension for a rubber band (non-Hookean) and integrate numerically using spreadsheet trapezium rule.
• Conduct a small-scale efficiency audit: measure electrical energy in (power meter) vs mechanical output (lifting a mass).

Common misconceptions & exam traps

• Forgetting minus signs in potential expressions (gravitational and electric).
• Mixing up field strength $\left(\pu{N.kg-1}\right)$ with potential $\left(\pu{J.kg-1}\right)$

Quick self-check quiz

1. What is the work done by a $\pu{30 N}$ force acting $60 ^\circ$ to the direction of motion over $\pu{4.0 m}$? - $60 \pu{J}$.
2. State the relationship between electric field strength and potential gradient. - $E = -\dfrac{d\Phi}{dx}$

Revision workflow

1. Create a comparison table of gravitational vs electric fields (strength, potential, energy conversions).
2. Re-do two past-paper questions that mix energy with dynamics (e.g., block-spring, roller coaster).
3. Practise sketching field lines and equipotentials for point masses/charges and parallel plates from memory.
4. Summarise common efficiency contexts (motors, generators, power plants) with sample calculations.

Practice Quiz

Test yourself on the key concepts from this guide.

9 Further reading

• SEAB H2 Physics 9478 syllabus (2026)
• Save My Exams - First Law of Thermodynamics
• Wikipedia - Work (physics)
• YouTube - Deriving $E_k = \tfrac{1}{2} m v^{2}$

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 $F v \rightarrow P$ conversions tonight.

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