Electric Charges: NEB Class 11 Physics Guide

Learning boundary

What the electric-charge topic connects

The CDC secondary curriculum places electric charge and field in Grade 11 Physics; check the CDC Physics Grade 11 page for current Nepal materials. This guide follows the institutional treatments of charge and conservation and Coulomb’s law.

Prerequisites are scientific notation, vectors, force and inverse-square relationships. Review Vectors Study Guide and Physical Quantities before multi-charge calculations.

Core properties

Conservation, quantisation and sign

Positive and negative are labels for two kinds of electric charge. An electron carries −e and a proton +e. A neutral object has zero net charge, not an absence of charged particles. In ordinary charging processes electrons transfer; protons remain bound inside nuclei.

Charge conservation means the total charge of an isolated system remains constant. Rubbing can leave one object negative and the other equally positive because electrons move between them. Quantisation is expressed q=ne, where n is an integer for net charge on an isolated object under ordinary conditions.

Charging processes

Track electrons and grounding explicitly

Induction sequence matters: bring the charged rod near, ground the conductor, remove the ground while the rod remains, then remove the rod. Reversing the last two steps can destroy the intended final charge.

Material response

Conductors and insulators differ in charge mobility

Conductors allow some charges to move readily through the material, whereas charges are more locally bound in insulators. This is a model distinction, not a statement that insulators contain no charge. Humidity and surface contamination can let static charge leak away, so demonstrations vary with conditions.

Grounding connects an object to Earth’s large charge reservoir. A grounded conductor can gain or lose electrons depending on the nearby charge and circuit path. Do not say that Earth “removes all charge”; describe the direction of electron flow for the situation.

Pair force

Apply Coulomb’s law with direction

For point charges separated by r in vacuum, F=k|q₁q₂|/r² with k≈8.99×10⁹ N·m²/C². The forces lie along the joining line and form an equal-and-opposite Newton’s-third-law pair. Charge signs choose attraction or repulsion; the magnitude formula uses absolute values.

For extended bodies, the point-charge model works when size is negligible compared with separation or when symmetry permits an equivalent description. Otherwise divide the distribution into elements and sum or integrate.

Superposition

Add forces as vectors, one source at a time

1. Draw the target charge and every source.
2. Decide attraction or repulsion for each pair.
3. Calculate each magnitude independently.
4. Draw force arrows on the same target charge.
5. Resolve non-collinear forces into x and y components.
6. Add components and recover magnitude and direction.

Observation and safety

Use static demonstrations as qualitative evidence

Rub a clean plastic rod with an approved cloth and bring it near small paper pieces or a lightweight suspended pith ball. Record attraction, repulsion after contact and discharge with humidity changes. These observations show force and charge transfer qualitatively; they do not directly measure elementary charge or prove a precise inverse-square law.

Keep electrostatic demonstrations away from mains circuits, flammable vapours and sensitive electronics. Never imitate lightning experiments outdoors. Use PhET Coulomb’s Law in a predict–observe–explain cycle after doing the scaling calculation.

Exam method

Common mistakes, practice tasks and readiness

• Neutral is not chargeless: distinguish net and constituent charge.
• Conservation is checked over a defined system.
• Electron transfer direction is stated.
• Microcoulombs are converted to coulombs.
• Separation is squared in SI units.
• Attraction or repulsion is added after magnitude.
• Multiple forces are added as vectors.
• Point-charge assumptions are visible.

1. Find electron number for −8.01×10⁻¹⁹ C.
2. Explain induction with a negative rod.
3. Calculate force for +4 μC and +6 μC at 0.30 m.
4. Predict the change when distance triples.
5. Solve a three-charge line problem.
6. Explain neutral-object attraction by polarisation.

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References and next steps

Sources and related study guides

• CDC Nepal: Physics Grade 11
• CDC Nepal: Secondary Curriculum
• OpenStax: Charge and Conservation
• OpenStax: Conductors and Insulators
• OpenStax: Coulomb’s Law
• PhET: Coulomb’s Law

Continue with the Electric Charges Study Guide and then Electric Field. Curriculum scope and sources were checked on 2 August 2026; follow current CDC, NEB and college instructions if requirements change.

Integrated transfer

Separate interaction, system and material effects

A charged plastic ruler attracts neutral paper, a contacted metal sphere shares charge, and an induced conductor gains opposite sign after grounding. These observations use different mechanisms. For each, name the system boundary, mobile charges, final net charge and force source.

In a dry classroom, static effects may persist; humid surfaces allow leakage. The change does not violate conservation—charge moves through paths outside the simplified object pair. A good explanation expands the system rather than claiming charge disappeared.

Model extension

How the medium and scale affect Coulomb force

In a uniform dielectric material, electrostatic interactions are reduced relative to vacuum by the material permittivity in the simple macroscopic model. Write F=|q₁q₂|/(4πεr²) when the medium is specified, with ε=ε₀ε_r. Do not insert a relative permittivity unless the question defines the material and model.

At atomic scales, classical point-charge electrostatics remains a useful force law but full material behaviour can involve quantum structure and microscopic polarisation. At classroom scales, surface leakage, humidity and finite shapes often matter before such corrections.