DSE Physics | Electrostatics and Capacitors Tutor Prep Guide | 45-minute plan, field-line reasoning, four common errors

2026-11-05

Tutor lesson-prep guides · Secondary Physics

What this lesson fixes: students substitute straight into F = kQ1Q2 / r squared with microcoulombs and centimetres, get a number that looks plausible, and still lose the whole mark because the direction is wrong.

Objectives — By the end the student can (1) sketch the field lines for two opposite charges, two like charges and a uniform field, and state the direction of the net force at a third point within 60 seconds; (2) convert microcoulombs to coulombs and centimetres to metres before substituting into Coulomb’s law, and quote newtons as the unit of force; (3) use C = Q / V, E = V / d and half C V squared on a parallel-plate capacitor, and state the series and parallel rules for equivalent capacitance.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Unit conversion	Write 4 microcoulombs in coulombs and 6 centimetres in metres	Answering 4 C or 6 m means the prefixes need revision first
Vector addition	Two perpendicular forces of 3 N and 4 N: state the resultant	Answering 7 N means the parallelogram method needs revision
Inverse square	Double the separation r: by what factor does the force change	Answering half means the 1 over r squared relation is missing

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
5 min	Opening diagnostic	Put one question with microcoulombs and centimetres on the board; units must be converted before any working	Answer orally, no arithmetic needed
8 min	Three field-line rules	Show opposite, like and uniform-field diagrams; circle only direction, density and no crossing	Copy all three and write one rule each
12 min	Coulomb’s law drill	Model the same question in five steps: sketch, direction, units, substitute, state the unit	Follow along, reporting each step aloud
10 min	One independent question	New question: field strength at the midpoint between two equal charges	Tutor observes only, no prompts
6 min	Three capacitor formulas	Use C = Q / V, E = V / d and half C V squared on one parallel-plate set-up	Say which formula uses which known quantity
4 min	Exit check	Three quick questions; anything wrong is retaught on the same reasoning line	Answer orally, redo immediately if stuck

03 Teaching script (read it out)

Scene 1: Act one: draw the field before doing any algebra

The costliest habit in electrostatics is treating every question as pure algebra. The correct order is: sketch the two charges, mark the signs, draw arrows leaving the positive charge, let the arrow density stand for field strength, and only then ask what the question wants at the third point. Once the sketch exists, direction questions almost answer themselves. Without it, a student can compute 60 N correctly and still score zero because the direction line is wrong. In this act the tutor asks for one thing only: the first line of every electrostatics answer is a diagram, not an equation.

Ask the student: what is the first line of every electrostatics answer?

Scene 2: Act two: the fixed five-step Coulomb routine

Take charges of 4 microcoulombs and 6 microcoulombs separated by 6 centimetres. Step one, sketch and mark the direction. Step two, state whether the force is attractive or repulsive. Step three, convert units: 4 times 10 to the minus 6 coulombs and 0.06 metres. Step four, substitute into F = k Q1 Q2 over r squared with k equal to 9 times 10 to the 9. Step five, write the unit N and check the order of magnitude. The answer is 60 N. A student who skips step three lands on something like 6 times 10 to the 15 and should not be told the answer is wrong; ask instead which step the order of magnitude exposes.

Ask: if the unit conversion is skipped, by what factor does the answer inflate?

Scene 3: Act three: a capacitor is the same field seen from another side

A parallel-plate capacitor is just a uniform field trapped between two plates. With C equal to 2 microfarads and V equal to 12 volts, three questions chain together: charge Q = C V = 24 microcoulombs; with plate separation 2 millimetres, field strength E = V over d = 6000 volts per metre; stored energy half C V squared = 1.44 times 10 to the minus 4 joules. One set of numbers feeds three formulas, which is how students stop seeing them as three unrelated facts. Close with the trap: capacitors in series give an equivalent smaller than the smallest (2 microfarads with 3 microfarads gives 1.2), while in parallel they simply add (5 microfarads), the reverse of resistors.

Ask: the equivalent of 2 and 3 microfarads in series must be smaller than which number?

04 Question types and worked steps

Type	Example	Steps	Answer
Force between two point charges	Charges of 4 microcoulombs and 6 microcoulombs separated by 6 centimetres	Sketch and direction, convert units, substitute k Q1 Q2 over r squared, state N	60 N along the line joining them, repulsive
Field at a midpoint	Two equal like charges separated by 2a: field at the midpoint	Draw both field vectors, compare directions, subtract	Zero, and the reason (equal magnitude, opposite direction) is required
Parallel-plate capacitor	C = 2 microfarads, V = 12 V, plate separation 2 mm: find Q, E and stored energy	Q = C V, then E = V over d, then half C V squared	24 microcoulombs, 6000 volts per metre, 1.44 times 10 to the minus 4 joules
Series and parallel capacitance	2 microfarads and 3 microfarads, in series and in parallel	Reciprocals add in series; values add in parallel	1.2 microfarads in series, 5 microfarads in parallel

Sanity check (teach this): Orders of magnitude must agree. Charges go into the formula in coulombs, not microcoulombs: two charges of this size 6 centimetres apart give tens of newtons, not thousands and certainly not 10 to the 15. The energy from half C V squared comes out at 10 to the minus 4 joules, consistent with 24 microcoulombs of charge. An equivalent series capacitance must be smaller than 2 microfarads, so an answer of 5 means the student swapped the rules. Direction is the cheapest check of all: between two positive charges the fields cancel, between a positive and a negative they add, so one glance back at the sketch confirms or kills the answer.

05 Independent practice (3 questions)

Question	Answer	What to watch
Two positive charges of 3 microcoulombs separated by 9 centimetres: find the force	Convert units first, then substitute, then state the direction	0.09 metres used, answer 10 N
C = 5 microfarads, V = 20 V: find Q and the stored energy	Two separate lines with units on both	100 microcoulombs and 1.0 times 10 to the minus 3 joules
4 microfarads and 12 microfarads in series across 16 V: equivalent capacitance and total charge	Equivalent first, then charge, which is the same on both in series	3 microfarads and 48 microcoulombs

06 Common mistakes and what to say

Mistake	Student writes	Say this
Substituting microcoulombs directly	F = 9 times 10 to the 9 times 4 times 6 over 36, answered as 6 times 10 to the 9 N	Your formula is fine, your units are not. 4 microcoulombs is 4 times 10 to the minus 6 coulombs, so write that line before you touch the calculator.
Field direction reversed	Two positive charges, midpoint described as the strongest point	At the midpoint of two positive charges one field points left and the other right. Are you adding them or subtracting them?
Series capacitance treated like resistors	2 microfarads and 3 microfarads in series answered as 5	Resistors add in series; capacitors do the opposite. Add the reciprocals and see whether the answer is smaller than 2.
Energy computed as C V instead of half C V squared	2 microfarads at 144 answered as 2.88 times 10 to the minus 4 joules	That is the energy if the full voltage were applied the whole time. The voltage climbs from zero to 12, so you take half.

07 Homework (paper, section and question numbers)

Series and parallel consolidation on the same subject page — Section 07, questions 1 to 3
Secondary Physics prep index: preview the next chapter — Pick one electromagnetism chapter from the index and read the objectives section

08 Exit check (last 5 minutes)

Question	Answer	Decision
What must happen before Coulomb’s law is applied	Unit conversion: microcoulombs to coulombs and centimetres to metres	Only passes if units are mentioned
Field at the midpoint of two equal like charges	Zero, because the two fields are equal in magnitude and opposite in direction	The reason is required, not just the word zero
Equivalent capacitance of 2 and 3 microfarads in series	1.2 microfarads, smaller than the smallest component	Both the number and the smaller-than rule

09 Tutor FAQ

The student says the formulas are already memorised

Memorising is about a third of this chapter. Marks are lost on units and direction, so the routine is: diagram on line one, unit conversion on line two. Fixing the habit matters more than another formula sheet.

Should the value of k be memorised

DSE papers normally supply k as 9 times 10 to the 9 newton metre squared per coulomb squared, but students should recognise that order of magnitude so they can sanity-check their own answers.

Is the school paper as hard as DSE

School papers often merge capacitors into a field question, which DSE rarely does. The 45 minutes here is classroom pacing, not an exam time limit.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.