Electric Field: NEB Class 11 Physics Guide

Curriculum boundary

From source charge to force on another charge

The CDC secondary curriculum includes electric field due to point charges and charge distributions; check the CDC Physics Grade 11 page. The concepts align with OpenStax electric field and its University Physics field treatment.

Review Electric Charges and Vectors Study Guide first.

Field concept

Separate source, field point and test charge

Source charges create a field. The field exists at a chosen point whether or not a test charge is present. A sufficiently small positive test charge defines direction without significantly disturbing the source distribution. Once E is known, force on any charge q is F=qE; a negative charge experiences force opposite to E.

Point source

Use inverse-square magnitude and radial direction

For point charge Q, E=k|Q|/r². The magnitude is independent of the test charge. Around positive Q vectors point radially outward; around negative Q they point inward.

Doubling distance reduces E to one quarter. A graph of E against 1/r² is linear through the origin for an ideal point charge, with gradient k|Q|.

Multiple sources

Add fields at one point, not charges by convenience

1. Mark field point P separately from source charges.
2. For each source, draw direction at P: away from +, toward −.
3. Calculate each magnitude using its distance to P.
4. Resolve into common axes.
5. Add x and y components.
6. Recover resultant magnitude and direction.

Visual model

Read field lines without making them physical

• Lines start on positive charge or infinity and end on negative charge or infinity.
• The tangent gives field direction.
• Closer line spacing represents stronger field in a consistent diagram.
• Line number is drawn proportional to source magnitude by convention.
• Field lines never cross because E has one direction at a point.
• Lines are a visualisation; the field exists between drawn lines.

Uniform parallel lines model a nearly uniform field, such as the central region between large oppositely charged plates when edge effects are neglected. Curved, varying-density lines represent non-uniform fields.

Electrostatic equilibrium

Free charges rearrange until the internal field vanishes

In an ideal conductor at electrostatic equilibrium, the field inside conducting material is zero; otherwise free charges would continue moving. Excess charge resides on the surface, and the field immediately outside is perpendicular to the surface. Sharp curvature can produce larger surface charge density and stronger nearby field.

Electrostatic shielding follows from conductor charge rearrangement. Real protection depends on a continuous conducting enclosure and safe grounding/context; never use classroom electrostatics as permission to approach live power lines or lightning.

Mapping and simulation

Predict vectors before revealing a field map

Place source charges on a coordinate grid and choose five field points. At each point draw qualitative component arrows, rank magnitudes by charge and distance, then calculate. Only afterward compare with PhET Charges and Fields.

A simulation uses ideal point charges and a display scale. It cannot replace discussion of measurement uncertainty, finite charge size or disturbance by a physical probe. Record which prediction changed and why.

Exam readiness

Common mistakes and practice tasks

• Do not include the test charge in E=k|Q|/r².
• Define E direction with a positive test charge.
• Reverse force, not field, for a negative placed charge.
• Add fields as vectors at the same point.
• Use each source-to-point distance.
• Do not let field lines cross.
• Distinguish zero field from absence of sources.
• State symmetry and point-charge assumptions.

1. Find E 0.20 m from +5 μC.
2. Find force on −2 nC at that point.
3. Solve midpoint fields for like and unlike pairs.
4. Find a zero-field point for unequal like charges on a line.
5. Sketch a dipole and two unequal like charges.
6. Explain conductor equilibrium and shielding.

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References and next steps

Sources and related study guides

• CDC Nepal: Physics Grade 11
• CDC Nepal: Secondary Curriculum
• OpenStax: Electric Field
• OpenStax University Physics: Electric Field
• OpenStax: Field Lines
• PhET: Charges and Fields

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

Continuous distributions

From point charges to distributed sources

A real charged rod or plate can be divided into many small elements dq. Each element contributes dE with its own direction and distance, and the total is the vector integral. Symmetry may cancel components before integration.

For a uniformly charged ring, fields from opposite elements cancel sideways components on its axis while axial components add. At the centre the net field is zero even though every element contributes. Moving along the axis produces a nonzero field that first increases and eventually falls with distance.

This extension explains why adding total charge at the geometric centre is not generally valid. Treating a distribution as a point charge requires distance much larger than its size or a symmetry result valid outside the distribution.

Uniform-field model

Connect parallel plates, force and motion

Between large oppositely charged parallel plates, far from edges, the field can be approximated as uniform: vectors have nearly constant magnitude and point from the positive plate to the negative plate. Edge regions curve and weaken the uniform model.

A suspended charged droplet can involve electric force, weight and possibly drag or buoyancy. Draw all forces before equating magnitudes. “Stationary” means zero resultant, not absence of field or gravity.