Q: What does A-Level Physics: 14) Electric Fields Guide cover? A: From Coulomb's law to capacitor energy graphs, this post unpacks Section V Topic 14 of the 2026 H2 Physics syllabus for IP students and parents.
TL;DR Electric field questions are the hinge between mechanics and circuits. Nail the three k-values (force, field, potential), the V/d shortcut for plates and the 21-factor for capacitor energy, and you will harvest marks across Papers 1-3 and the practical.
Need the rest of the electromagnetism refresh (currents, circuits, EMF)? Jump to the H2 Physics notes hub to stay in sequence with Topics 15–18 and grab the shared practice decks.
1 Coulomb's law: the force glue
The syllabus demands you recall and use
F=4πε01r2Q1Q2.
1.1 Quick-fire cues
SI unit for charge is C.
Sign mistakes: repulsive if charges share sign, attractive otherwise.
1.2 Mini-drill
Two +2.0nC charges sit 5.0cm apart in air. Calculate F. Answer: 1.4⋅10−5N (repulsive).
2 Radial electric fields & potentials
Because E=qF, the field around a point charge is
E=4πε01r2Q.
Integrating this field gives the scalar potential
V=4πε01rQ.
2.1 Zero reference
Infinity is the zero-potential reference unless the question states otherwise.
2.2 Potential energy pair
Two point charges form a system with
UE=4πε01rQ1Q2.
3 Potential gradients & equipotentials
Field lines show direction of E.
Equipotential surfaces are always perpendicular to field lines.
Mathematically, E=−drdV.
Exam cue: check sign conventions when taking gradients.
4 Uniform electric fields
Between parallel plates:
E=dV.
4.1 Force & motion
A charge q experiences F=qE. With constant a=mF, kinematics mirrors vertical projectile motion.
4.2 Mini-drill
An electron enters a 600V plate pair 8.0mm apart, velocity horizontal. Find vertical displacement after 1.0cm of travel.
5 Capacitance fundamentals
Definition:C=VQ. Unit: farad (F=C⋅V−1).
5.1 Energy store
Area under V-Q graph (triangle) gives
U=21QV=21CV2=21CQ2.
5.2 Exam-class graph sketch
Label axes, mark triangle area, read 21 factor directly.
6 Three timing hacks for WA & prelims
Copy units first - minimizes careless mistakes.
Vector-check every force sum.
Leave capacitor energy algebra until numbers are parked.
7 IP-specific pitfalls
Pitfall
Fix
Treating ε0 as 0
Memorise 8.85×10−12F⋅m−1.
Confusing F=qE and E=dV
Write a “fields toolkit” flashcard.
Forgetting 21 in energy
Draw the Q-V triangle before calculating.
Comprehensive revision pack
9478 Section V, Topic 14 Syllabus outcomes at a glance
Outcome (a) - recall Coulomb's law and apply it to point charges.
Outcome (b) - describe electric field strength and potential, including equipotentials.
Outcome (c) - analyse uniform fields between parallel plates and charged particle motion.
Start with Coulomb's inverse-square law. Divide by charge to get field strength; integrate to find potential. Between plates you can treat the field as uniform E=dV. Once you know E, the force on a charge is qE and motion follows mechanics. Capacitors store charge/energy; their energy formulas mirror triangular areas on Q-V graphs.
Key relations
Quantity
Expression / note
Coulomb's law
F=4πε01r2Q1Q2
Electric field (point charge)
E=qF=4πε01r2Q
Electric potential
V=4πε01rQ
Potential gradient relation
E=−drdV
Uniform field
E=dV
Work/energy of charge
W=qV
Capacitance definition
C=VQ
Capacitor energy
U=21CV2=21QV=21CQ2
Equivalent capacitance
Series: Ceq1=∑Ci1
Derivations & reasoning to master
Field-potential link: integrate Coulomb's law to obtain potential; differentiate back to recover field.
Parallel plate field: derive E=dV by combining force and work definitions.
Energy stored in capacitor: integrate V with respect to Q to show factor 1/2.
Charged particle deflection: combine F=qE with SUVAT relations to predict trajectories in oscilloscopes or mass-spectrometer velocity selectors.
Worked example 1 - potential due to multiple charges
Two charges +4.0nC and −2.0nC are 12cm apart. Find the potential and field at a point midway between them.
Approach: potential adds algebraically, field adds vectorially; choose direction from positive to negative charge, compute magnitude using inverse-square.
Worked example 2 - capacitor discharge
A C=220×10−6F capacitor charged to V=12V discharges through R=4.7×103Ω. Calculate the initial energy stored and the initial discharge current. Discuss how energy is dissipated.
Solution: U=21CV2, I0=RV. Mention exponential decay of current and conversion to thermal energy.
Practical & data tasks
Map equipotentials using conductive paper and voltmeter; sketch electric field lines from data.
Use apparatus to show Millikan-style oil drop balancing (qualitative) linking gravitational and electric forces.
Build RC circuit and record V(t) to verify exponential decay; extract time constant.
Common misconceptions & exam traps
Treating potential as vector quantity; it is scalar.
Forgetting that equipotentials are perpendicular to field lines.
Mixing units (mm vs m) when using V/d.
Neglecting the 21 factor in capacitor energy or forgetting charge changes when voltage changes.
Quick self-check quiz
What is the electric field midway between two equal positive charges? - Zero by symmetry (fields cancel).
A proton accelerated through 500V gains how much kinetic energy? - 500eV≈8.0×10−17 J.
Doubling plate separation while keeping voltage constant does what to field strength? - Halves it.
State two ways to increase capacitance of a parallel-plate capacitor. - Increase plate area; decrease separation; insert dielectric with higher εr.
How does equivalent capacitance change when capacitors are in parallel? - Add directly Ceq=C1+C2+…
Revision workflow
Re-derive Coulomb-potential-field relationships weekly to cement mathematical links.
Solve combined problems involving charged particle motion in uniform fields and capacitor energy.
Prepare a flashcard sheet of dielectric constants and typical capacitor applications.
Work through past-paper RC discharge questions to stay fluent with exponential decay graphs.
