A7LAM CSCA ACADEMYCSCA Academy / Physics / Electromagnetism
CSCA Physics · Lesson 7 of 16Electric fields
Champ électrique · المجال الكهربائي
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Charges push and pull each other through an electric field. Coulomb's law gives the force; field strength and potential describe the space around charges.
Key ideas
- Like charges repel; unlike charges attract.
- Coulomb's law: F = kq₁q₂/r², k = 9 × 10⁹ N·m²/C².
- Field strength E = F/q (N/C); field lines go from + to −.
- Potential difference U_AB = W_AB/q; in a uniform field U = Ed.
Formulas to know
- Point charge: E = kQ/r²
- Work by the field: W = qU
- Capacitor: C = Q/U
Worked example
Charges of 2 μC and 3 μC are 0.3 m apart. Force between them?
- Formula: F = kq₁q₂/r²
- Replace: F = 9×10⁹ × 2×10⁻⁶ × 3×10⁻⁶ / 0.09
- Compute: F = 0.054 / 0.09
Answer: F = 0.6 N (they repel)
Practice questions
1. The distance between two charges is halved. The force becomes:
half · 2 times larger · 4 times larger ✓ · a quarter
F ∝ 1/r².
2. A 2 C charge moves through a potential difference of 6 V. Work done by the field?
3 J · 12 J ✓ · 8 J · 0.33 J
W = qU = 2 × 6 = 12 J.
3. Uniform field E = 200 N/C. Potential difference across 0.05 m?
10 V ✓ · 4,000 V · 0.25 V · 200 V
U = Ed = 200 × 0.05 = 10 V.