STATIC ELECTRICITY
(Ch. 22)
PH 104 w/ dr. g
Lec 23
Electrostatics…
”Electrically charged” object:
• Exerts a force:
• Can release energy (sparks)
(video1) (video2)
• Can attract or repel
(demos)
• Stronger than gravity
• Decreases with distance: another inverse square law (like gravity)
Really big sparks:
Electrostatics…
”Electrically charged” object:
• Exerts a force:
• Can release energy (sparks)
(video1) (video2)
• Can attract or repel
(demos)
• Stronger than gravity
• Decreases with distance: another inverse square law (like gravity)
•Charge: two kinds = ‘positive’ / ‘negative’
•Opposite charges attract, like charges repel
yStable matter: electrically neutral.
yAtom : equal number of (-) electrons and (+) protons: ZERO net charge
yCharged object:
ySurplus of electrons = negatively charged
yShortage of electrons = positively charged
Electrostatics…
Electrostatics…
Charging Process: how objects get charged
Charging Process: how objects get charged
MOBILITY of electrons in object: depends on composition
• Conductors: electrons are more mobile
• Examples: most metals
• Charge gets evenly distributed along the surface.
• Insulators: electrons are less mobile
• Examples: thermal insulators (amber, plastic, hair, etc.)
• Excess charge on surface from polarization:
• Each atom/molecule can get polarized :
• Nearby charge induces a (+) end and ((-) end
• Charge on surfaces unbalanced : (+) side and ((-) side
• By friction:
• Rubbing: comb + hair, shoes + carpet, clothes +
upholstery
• By contact:
• Transfer of charge into neutral object (or between
opposite charges)
• “Grounding” = neutral object is “charge reservoir” :
neutralizes charge.
• Remedy for static cling !
Electrostatics…
Electrostatics…
Charging Process: how objects get charged
Charging Process: how objects get charged
• By induction:
• By induction:
• Nearby charge polarizes neutral object: (+) end and (-) end
• Nearby charge polarizes neutral object: (+) end and (-) end
• Physically separate the opposite poles: oppositely charged objects!
• OR: “Grounding”: excess charge at one end neutralized by charge reservoir
• Physically separate the opposite poles: oppositely charged objects!
• OR: “Grounding”: excess charge at one end neutralized by charge reservoir
Back at the gas station: grounding could neutralize ALL charge: no more spark!
Electrostatics…
Electrical force – exerted by charges
Coulomb’s Law:
• Repulsive or attractive force (1 & 2):
•
magnitude = k q1 q2 / r2
• r = separation distance
• Inverse-square law!
• q= amount of charge, in Coulombs (C)
• k = 9 x 109 Æif force in N and d in meters.
• Compare with gravitational force:
•
•
•
Both action-at-a-distance, both follow inverse square law
k >> G
Gravitational is attractive only
(But: electrons do not fall into nucleus; protons in nucleus don’t fly apart.
WHY??? ….)
Each case: Charge is transferred. Charge is CONSERVED.
LIGHTNING: result of charging by induction
Cloud: Polarized: + charge on top, - charge below
+ charge induced on the ground below.
Discharge = transfer of charge : cloud bottom to ground