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Class 9–10 · Physics
Static electricity
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Static electricity
Lessons in this chapter
Lessons
1Charge and atoms
Protons are positive, electrons negative and neutrons neutral. Neutral objects balance positive and negative charge. Electron transfer charges ordinary objects; like charges repel and unlike attract. Total charge is conserved.
Key relationship
q = ne; e ≈ 1.60×10⁻¹⁹ C
Worked example
Gain 10¹² electrons ⇒ q = −1.60×10⁻⁷ C
Next lesson →2Conductors and charging
Conductors allow charge to redistribute; insulators restrict it. Charging can occur through friction, contact or induction. Friction transfers charge rather than creating it. Moisture often helps charge leak away.
Key relationship
total isolated charge before = total isolated charge after
Worked example
Comb gains electrons while hair loses them; combined total remains unchanged.
Next lesson →3Induction and earthing
A nearby charge redistributes a conductor's charges without contact. Insulators can polarise. For charging by induction, ground while the source is near, disconnect ground first, then remove the source. Earth acts as a large charge reservoir.
Key relationship
negative source near grounded conductor → electrons may leave
Worked example
Negative rod near sphere; ground, unground, remove rod ⇒ sphere left positive
Next lesson →4Electroscope
Charge spreads through a conducting knob and leaves, causing the leaves to repel. An electroscope detects charge, but a previously neutral instrument's divergence alone does not identify its sign. Earthing can discharge it.
Key relationship
like charges → leaf repulsion
Worked example
Charge knob by contact ⇒ leaves diverge; ground instrument ⇒ leaves collapse
Next lesson →5Coulomb's law
Electrostatic force between point charges follows an inverse-square law along their connecting line. In air, use approximately k = 9×10⁹ N m²/C². The medium affects the interaction.
Key relationship
F = k abs(q₁q₂)/r²
Worked example
1 μC and 2 μC, r = 0.3 m ⇒ F = 0.2 N
Next lesson →6Electric field
Electric field is force per positive test charge. It points away from positive sources and towards negative ones. Field-line tangents give direction and relative density indicates strength; lines cannot intersect.
Key relationship
E = F/q; point-charge magnitude E = k abs(Q)/r²
Worked example
E = 200 N/C, q = +0.01 C ⇒ F = 2 N along field
Next lesson →7Potential and energy
Potential is potential energy per charge. A potential difference gives energy change per charge. Potential is scalar whereas field is vector. A point charge's potential varies as 1/r, not 1/r².
Key relationship
ΔEp = qΔV; point charge V = kQ/r; 1 V = 1 J/C
Worked example
q = 2 C, ΔV = +3 V ⇒ ΔEp = +6 J
Next lesson →8Capacitors
Two conductors separated by insulation store equal and opposite charges. Capacitance is charge magnitude divided by voltage. Larger plate area and smaller spacing generally increase it. Charged capacitors store energy even after a device is disconnected.
Key relationship
C = Q/V; U = ½CV²; unit farad
Worked example
C = 10 μF, V = 12 V ⇒ Q = 120 μC
Next lesson →9Lightning and electrostatic applications
A large potential difference can ionise air and produce a discharge. Lightning protection provides a suitably designed conductive path and grounding. Electrostatic attraction also supports photocopying, spray painting and particle collection.
Key relationship
large potential difference → electrical breakdown of air
Worked example
Toner adheres to selected charged areas in a copier.
Next lesson →Check your understanding
Try answering first, then open the question to compare your reasoning.
1. Charge after losing electrons?
Positive: fewer electrons than protons
2. Triple separation: Coulomb-force factor?
1/3² = 1/9
3. Field direction around a positive source?
Outwards, following force on a positive test charge
4. 12 J change for 3 C: potential difference?
ΔV = 12/3 = 4 V
5. 20 μF capacitor at 5 V: charge?
Q = CV = 100 μC
Sources and scope
Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.