Electricity & magnetism · Topic 14 of 17

Electric Charge, Static Electricity & Fields

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Physics · static electricity

Charge is positive or negative; like charges repel, unlike attract, and the force grows with the charges and falls with distance squared.

01 · The ruleLike charges repel, unlike attract

02 · Coulomb's lawF = k·q1·q2 / r²

03 · Distance2× the distance → ¼ the force

Gain electrons → negative charge; lose electrons → positive charge must know

Charged objects attract neutral ones by polarizing them

Electric field E = F/q, in newtons per coulomb (N/C)

Friction: rubbing transfers electrons from one material to another

Contact: touching shares charge, both end with the same sign

Induction: no touching; the object ends with the opposite charge

Coulomb's law: F = kq₁q₂/r² (k ≈ 9.0 × 10⁹ N·m²/C²) must know

Charge unit: coulomb (C); electron −1.60 × 10⁻¹⁹ C, proton +1.60 × 10⁻¹⁹ C

Point charge field: E = kQ/r²

Like charges repel; unlike charges attract must know

Double one charge → 2× force; double the distance → ¼ the force must know

Charge is conserved: rubbing moves charge, never creates it

Field lines: away from +, toward −; closer = stronger; never cross

3.0 μC and 4.0 μC, 0.30 m apart → F ≈ 1.2 N (repel)

36 N at distance r → 9 N at distance 2r

0.020 N on 4.0 μC → E = 5.0 × 10³ N/C

Conductors let charge move (metals); insulators don't (glass, rubber) must know

Commonly confused

	Friction	Induction
Touching?	Yes, the materials are rubbed together	No, the charged object is only brought near
What moves	Electrons pass from one material to the other	Charges shift within the conductor (or to ground)
Result	Two objects with equal, opposite charges	Object charged opposite to the nearby charge
Example	Balloon rubbed on hair	Grounded metal sphere near a charged rod

Trap: positive = electrons lost, not protons gained

Show 3 more facts

Conductor: excess charge on the surface; field inside = 0

1 C ≈ the charge of 6.25 × 10¹⁸ protons

F = qE; a negative charge is pushed against the field

Pearl

Likes repel, opposites attract, and twice as far apart means one-fourth the force.

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