NEB Class 11 • Physics • Focused Revision

Electric Charges Study Guide: NEB 11 Physics

Turn electrostatic facts into a study system: track electrons, conserve charge, predict force direction, calculate magnitude and defend the point-charge model.

Six-session revision sequence
Charging-process and Coulomb-law drills
Vector checks, practical critique and timed test

Charge sign determines attraction or repulsion.

Revision boundary

Connect charge language, process and force

Use the CDC secondary curriculum and CDC Physics Grade 11 page for current scope. Read the paired Electric Charges concept guide and compare it with OpenStax charge conservation and Coulomb’s law.

A complete solution moves through five layers: microscopic electron story, system conservation, sign and direction, magnitude and model limit.

Knowledge map

Build four connected representations

Particles

Electron surplus or deficiency and q=ne.

Process

Friction, contact, induction, polarisation and grounding.

Force

Pair direction, inverse-square magnitude and superposition.

Evidence

Qualitative demonstrations, units, assumptions and safety.

Make one page with arrows between the layers. For example: a negative rod near a neutral conductor causes separation; grounding transfers electrons; removing ground freezes a net charge; Coulomb interaction then predicts force.

Six sessions

A spaced electrostatics plan

Session	Focus	Exit evidence
1	Signs, particles, q=ne	Ten electron-count items
2	Conservation and charging	Four process diagrams
3	Conductors, insulators, grounding	Five explanation cards
4	Coulomb magnitude and scaling	Eight calculations
5	Vector superposition	Line and right-angle problems
6	Timed mixed test	80% without repeated unit or direction error

Use 50-minute blocks: ten minutes closed-book retrieval, ten minutes correction, twenty-five minutes mixed problems and five minutes error logging. Retest after one, three and seven days.

Process rehearsal

Draw charge motion in time order

Label each object initially neutral, positive or negative.
Show electron movement with arrows; do not move protons between ordinary solids.
Mark any grounding connection and its removal.
Separate polarisation from net charging.
Check total system charge before and after.
Write the final sign under each isolated object.

Induction with a negative rod

A negative rod near a neutral conductor repels mobile electrons to the far side. Grounding lets some electrons leave to Earth. Disconnect the ground while the rod remains; then remove the rod. The conductor is left net positive and its charge redistributes.

Contrast with contact: touching the negative rod can transfer electrons directly and often leaves the conductor with the same sign as the rod.

Calculation routine

Convert units before squaring distance

Sketch charges and separation.
Predict attraction or repulsion.
Convert μC, nC and centimetres to SI.
Calculate pair magnitudes independently.
Draw all force vectors on the target.
Add components and interpret.
Check inverse-square scaling and symmetry.

Pair force

+5.0 μC and −2.0 μC separated by 0.25 m give F≈(8.99×10⁹)(10×10⁻¹²)/0.0625≈1.44 N, attractive.

Three charges on a line

Place +1 μC at x=0, +2 μC at x=0.30 m and −1 μC at x=0.60 m. On the middle positive charge, the left positive repels right with about 0.200 N; the right negative attracts right with about 0.200 N. Net is about 0.400 N right, not zero.

Scaling without arithmetic

If one charge triples and separation becomes half, force multiplies by 3/(1/2)²=12. State the factor before substituting numbers.

Error log

Code the first incorrect decision

Error	Code	Repair
Moved protons during rubbing	particle	Redraw electron transfer
Called polarised paper charged	net	Sum both signs
Used centimetres in r²	unit	Convert before squaring
Subtracted forces by sign only	vector	Draw arrows on one target
Ignored body size	model	Compare size with separation

Redo each corrected item after 24 hours. If a code repeats, write a contrast problem: induction versus contact, neutral versus uncharged, or collinear versus perpendicular superposition.

Final checkpoint

Thirty-five minute readiness test

Explain conservation and quantisation with one numerical example.
Draw charging by contact and induction.
Explain polarisation attraction.
Calculate two pair forces and one scaling factor.
Solve a three-charge line problem.
Add two perpendicular electric forces.
Evaluate a static demonstration and two limitations.
State point-charge and vacuum assumptions.

Electron movement is explicit.
System boundaries are defined for conservation.
SI conversions precede substitution.
Direction is separate from magnitude.
Vectors are drawn on the same target.
The answer includes a model limit.

Use PhET Coulomb’s Law after predicting. For online or physical NEB tuition, call 9846662070.

Frequently asked questions

Questions about studying electric charges

Should I start with equations?

Start with electron transfer, system charge and force direction. The equation then supplies magnitude without hiding the physical story.

How do I remember induction order?

Approach, ground, remove ground while the charged object remains, then remove the object. Draw the sequence rather than memorising words alone.

How do I prevent unit errors?

Write a conversion line for every prefix and distance before multiplying or squaring.

Why do I draw forces before adding?

Charge signs determine each vector direction, and only vectors acting on the same target may be added for its resultant.

What score shows readiness?

Aim for at least 80% on a fresh mixed set with no repeated particle, unit, direction or model error.

Where can I get electric-charge tuition?

Call 9846662070 for current online or physical NEB tuition options, schedules and fees.

References and next steps

Sources and related study guides

Continue with Electric Field and use the Vectors Practice Set for component drills. Curriculum scope and sources were checked on 2 August 2026; follow current CDC, NEB and college instructions if requirements change.

Transfer workbook

Five contrast drills

Neutral versus uncharged: explain why a neutral atom still contains charge. Polarisation versus induction: show local separation without grounding, then net charging with grounding. Magnitude versus direction: calculate Coulomb magnitude with absolute values, then select attraction or repulsion. Pair versus resultant: draw every pair force on one target before summing. Ideal versus real object: compare point-charge assumptions with finite conducting spheres.

Vector transfer

Three equal positive charges occupy three corners of a square of side a. At the empty corner, two adjacent-source fields each have magnitude kQ/a² along perpendicular directions; the diagonal-source field has magnitude kQ/(2a²) along the diagonal. Resolve all three on the same axes before adding.

After solving, rotate the square 45°. The physical resultant rotates with the configuration; its magnitude stays the same. This test reveals whether coordinates or electrostatic geometry controlled the reasoning.

Worked revision set

Six questions that change the first decision

1. Conservation audit

Two initially neutral objects are rubbed and object A gains −6.4 nC. If the pair is isolated, object B gains +6.4 nC. Writing only “B becomes positive” misses the equal magnitude required by conservation.

2. Quantisation check

A stated isolated charge 2.00×10⁻¹⁹ C is not an integer multiple of e≈1.602×10⁻¹⁹ C. Treat this as rounded measurement data or question the ideal isolated-particle interpretation; do not invent a fractional electron in ordinary charging.

3. Induction order

If a negative rod is removed before the ground, electrons can return while the conductor remains connected to Earth, leaving little intended net charge. The order is part of the mechanism.

4. Scaling

Changing q₁→2q₁, q₂→3q₂ and r→2r multiplies force by 6/4=1.5.

5. Direction trap

A negative target between a positive source on the left and negative source on the right is attracted left and repelled left, so both forces add left.

6. Model boundary

Two large charged plates separated by a small gap should not automatically be treated as point charges at their centres. Geometry and distribution determine the suitable field model.

Write a one-line reason before each calculation. Retest after three days with signs and positions swapped; if the method changes only because the diagram looks unfamiliar, return to pair directions and component arrows.

Weekly transfer: create a three-column page labelled observation, electron story and force model. Fill it for rubbing, contact, induction, polarisation and discharge. Then write a new example from Kathmandu daily life—clothes, comb, plastic chair or dry dust—without assuming the material signs until evidence or a triboelectric comparison is supplied. Mark conservation separately from the final sign. Repeat after a humid day and explain leakage without saying charge was destroyed.

Finish with a 20-minute oral test: explain why neutral paper is attracted, why like charges can produce zero resultant at one point, why force changes by four when distance halves, and why induction can charge without contact. Give one counterexample to each vague slogan. Keep the recording and repeat it after a week; improvement should appear as clearer system boundaries, electron paths, vector directions and model limits, not merely faster recall.

Review the attempt after twenty-four hours and explain each correction without opening the original answer.

Persist.

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