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1 Energy stores & transfers
2 Work done by a force
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.
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If you only have...
Focus on...
1 second
Track where energy starts and where it ends
10 seconds
Energy stores, work done, and power
100 seconds
The stores table and first transfer examples
10 minutes
Efficiency, fields, equipotentials, and exam traps
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.
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).
Energy store
Main transfer mechanism(s)
Everyday or exam-style example
Typical conversion (“from → to”)
Kinetic (movement)
Mechanical work (friction, collision)
Car brakes to a stop on a road
Kinetic → Thermal (tyre & road heat)
Gravitational potential
Mechanical work (free-fall, lifting)
Roller-coaster car descending first drop
GPE → Kinetic (plus small Thermal via air resistance)
Elastic potential
Mechanical work (stretch/compress)
Drawn bow string launches an arrow
Elastic → Kinetic (arrow) + Sound
Chemical
Electrical work (cell), Heating, Mechanical work
AA battery powers a torch bulb
Chemical → Electrical → Thermal + Light
Nuclear
Radiation
U-235 fission in a reactor core
Nuclear → Thermal (steam) → Electrical
Thermal / Internal
Heating, Radiation, Mechanical work
Hot coffee cooling on a desk
Thermal in Object → Thermal + Radiation into the Surroundings
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=Fscosθ.
For Weighted Assessment 1 (WA1), sometimes questions set the force to be in the same direction as displacement so θ=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 v2=u2+2as and W=Fs with F=ma:
W=mas=21m(v2−u2)=ΔEk.
Taking the object from rest u=0 gives the familiar
Ek=21mv2.
Check-list: always attach 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=mF.
Units: N⋅kg−1.
Lines point towards masses.
4.2 Electric field
Define field strength
E=qF.
Units: 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
Store
Expression
Gravitational (near Earth)
Eg=mgh
Electric (point charges)
Ee=krQq
Elastic (spring obeying Hooke)
Ee=21kx2
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=tE=tW.
For a constant force,
P=Fv
because v=ts.
Efficiency
Efficiency=total inputuseful output×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
Use syllabus pacing as a guide: Paper 2/3 average ~1.6 min/mark; Paper 4 ~3 min/mark.
Label units first; numbers follow.
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 Ek=21mv2.
(e) recall and use the equation Ek=21mv2 to solve problems.
(f) show an understanding of the concept of a field as a region of space in which bodies may experience a force associated with the field.
(g) define gravitational field strength at a point as the gravitational force per unit mass on a mass placed at that point, and define electric field strength at a point as the electric force per unit charge on a positive charge placed at that point.
(h) represent gravitational fields and electric fields by means of field lines (e.g. for uniform and radial field patterns), and show an understanding of the relationship between equipotential surfaces and field lines.
(i) show an understanding that the force on a mass in a gravitational field (or the force on a charge in an electric field) acts along the field lines, and the work done by the field in moving the mass (or charge) is equal to the negative of the change in potential energy.
(j) distinguish between gravitational potential energy, electric potential energy and elastic potential energy.
(k) recall that the elastic potential energy stored in a deformed material is given by the area under its force-extension graph and use this to solve problems.
(l) define power as the rate of energy transfer.
(m) show an understanding that mechanical power is the product of a force and velocity in the direction of the force.
(n) show an appreciation for the implications of energy losses in practical devices and solve problems using the concept of efficiency of an energy transfer as the ratio of useful energy output to total energy input.
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
Concept
Relation / value
Work done by constant force
W=Fscosθ
Work done by variable force
W=∫F⋅ds
Gravitational potential
Φg=−rGM (point mass); g=−drdΦ
Electric potential
Φe=krQ; E=−drdΦ
Elastic potential energy
E=21kx2
Power
P=dtdE=Fv
Efficiency
Efficiency=total inputuseful output
Equipotential property
W=0 along an equipotential surface
Derivations & reasoning to master
Work-energy theorem: derive ΔEk=∫F⋅ds and show it equals the net work.
GPE near Earth vs far field: connect mgΔh (near) with GMm(r11−r21)
Power for lifting: show P=mgv for a constant-speed hoist; discuss inefficiency due to motor heat.
Area under F-x: prove elastic energy equals ∫F,dx; generalise to non-linear springs using numerical integration.
Worked example 1 - multi-store energy track
A 0.80kg block is released from rest and slides down a 3.0m rough slope (angle 25∘). The frictional force on the slope is constant at 2.0N. At the bottom, it runs onto a smooth surface and compresses a spring (k=420N⋅m−1). Find the maximum compression of the spring.
Solution (energy):
mg(ssin25∘)−Ffrics=21kx2.
Taking g=9.81m⋅s−2,
x=4202(0.80×9.81×3.0×sin25∘−2.0×3.0)≈0.137m.
Worked example 2 - electric + gravitational potential
An electron is released from rest midway between two parallel metal plates 1.5cm apart with potential difference 120V. Determine its speed when it reaches the positive plate, neglecting gravity. (Assume the field is uniform between the plates.)
Strategy: use energy conversion 21mv2=eΔV. From the midpoint to a plate, the potential change is ΔV=60V.
