Eduvista · Class 12 Physics
Electric Charges and Fields
A focused lesson and practice set. This page covers the concepts listed below; it is not a claim of complete chapter or syllabus coverage.
Learn the essentials
Charge and force
Like charges repel and unlike charges attract. Coulomb's law gives force magnitude F = k|q₁q₂|/r² for point charges separated by distance r.
Electric field
Electric field at a point is force per unit positive test charge: E = F/q. Its direction is the force on a positive test charge.
Superposition
Fields from multiple charges add as vectors. Direction matters: equal magnitudes can cancel or reinforce.
Worked example
Two point charges remain fixed, but their separation doubles. How does force change?
- Start with Coulomb's law: F ∝ 1/r² when charges stay fixed.
- Replace r by 2r: F′/F = r²/(2r)² = 1/4.
- The magnitude becomes one quarter. The attractive or repulsive nature stays the same.
Memory trick
For inverse-square laws, scale the distance first: double distance → quarter force; halve distance → four times force.
Common mistakes
- Adding electric fields as scalars without directions.
- Using the sign of charge inside a force-magnitude calculation.
- Confusing electric field with force; E is force per unit charge.
Practice recall
Tap a card to reveal the answer. These are original Eduvista study cards.
Original practice quiz
Choose one answer per question. You can change it and see why it is right or wrong.
Quick revision sheet
- Charge is conserved and quantised.
- Coulomb's law: F ∝ |q₁q₂|/r².
- E = F/q for a positive test charge.
- Use vectors for field superposition; draw a direction diagram.
Original Eduvista explanations and practice aligned to the named NCERT topic. Check your current textbook and school syllabus for the full prescribed scope. NCERT textbooks ↗
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