Ch. 20 - ElectricityCh. 20 -

ElectricitySection 20.1Section 20.1Electric Charge Electric Charge

and Static and Static ElectricityElectricity

p. 600p. 600

Electric ChargeElectric ChargeCauses subatomic particles to

attract/repel2 types

• + and –Everything affected by charge

Neutral atom = protons & e-’s

Gain e-’s = - ion Lose e-’s = + ionExcess/shortage of e-’s

produce net chargeCoulomb (C) - SI unit of

electric charge 6.24 x 1018 e-’s = 1 C

Electric ForcesElectric ForcesOpposite charges attractElectric force depends on charge &

distance2x distance = ¼ electric forceStronger than gravityHold atoms together

Electric FieldsElectric FieldsThe effect electric charge has on

other chargesStrength depends on amt of

charge producing field & distance from charge

Exerts forces on charged object placed in field.

More net charge object has, greater force on it

Static Electricity and ChargingStatic Electricity and Charging

Static Electricity net accumulation of electric charges

on an objectCharge can be transferred by

Friction Contact Induction

Static Electricity and ChargingStatic Electricity and Charging

During charge transfer, total charge same before & after transfer Law of conservation of charge

Charging by frictionCharging by frictione-’s move from hair to balloon

Atoms in rubber >attraction for e-’s• Balloon net – charge• Hair net + charge

- - -+ + ++ + +

Charging by contactCharging by contactTouch electrically charged object – you

become chargedSphere still has net charge, but reduced

Van de Graaff generator

Charging by Induction Transfer of charge w/o contact btwn

materials Walk across carpet

Pick up extra e-’s; net – charge Repels e-’s in doorknob Leaves net + charge on doorknob closest

to hand Doorknob overall charge neutral, but

charges moved within it

Static DischargeOccurs when pathway forms for

charges to moveAir becomes charged when hand near

doorknobAir provides path for e-’s

Static Discharge – lightning

Charge built up from friction btwn moving air masses in clouds

- charge in bottom of cloud induces + charge in ground

Charge in cloud increasesAttraction increasesAir charged – pathway for

e-’s Lightning 5:06

Chapter 20.2Chapter 20.2

Electric Current and Electric Current and Ohm’s LawOhm’s Law

p. 604p. 604

Electric CurrentElectric CurrentContinuous flow of

electric chargeSI unit is ampere, or

amp (A)2 types:

• Direct current (DC)• Alternating current

(AC)

Direct CurrentDirect Current

Charge flows in 1 directionMost battery opperated devices

• Flashlights• Remotes

Alternating CurrentAlternating CurrentCurrent that regularly reverses its

directionElectric current in homes, businesses,

& schools often use AC

Electric current in a flashlightElectric current in a flashlighte-’s flow from - to + terminalCurrent in opposite direction!Scientists define currentcurrent as direction + direction +

charges flowcharges flow

ConductorsConductorsmaterial that allows e-’s to flow easily

• Metals like copper and silverMade of ionsions in a lattice

• ions - atoms that gained or lost e-’sIons not free to moveEach ion has e-’s loosely held

• Free e-’s conduct charge

InsulatorsInsulatorsmaterial that doesn’t allow e-’s to move

through it easily e- are tightly held ex: plastic, wood, rubber, glass

ResistanceResistancee-’s in electric curent collideopposesopposes flow of e-’s

electrical energy converted to thermal energy & light

measured in ohms () Thickness, length, temp

affect resistance

Tungsten - high resistance

Resistance depends on..Resistance depends on.. wire thickness

• more resistance in thinner wires– milkshake in thin v.s.

thick straw wire length

• more resistance in longer wires Temp

• high resistance at high temps• e-’s collide more often

SuperconductorsSuperconductorsCan resistance ever be 0?Superconductors produce near zero

resistance when cooled to low temps.

VoltageVoltagePotential Difference (voltage)(voltage)

diff in electrical potential btwn 2 places in electric field• Similar to PE diff of water at top and

bottom of fountain large separation of charge creates high

voltage• Large fountain – high PE

“push” causing e- to move from - to + measured volts (V)

Voltage sourcesVoltage sources

Fountains pump water to top, voltage source increases PE of electric charges batteries solar cells generators

Ohm’s LawOhm’s Law

•increasing voltage increases current•Increasing resistance decreases current

A lightbulb with a resistance of 160 is plugged into a 120-V outlet. What is the current flowing through the bulb?

GIVEN:

R = 160 V = 120 V

I = ?

I

V

R

ElectroscopeElectroscope

Electroscope instrument that

detects presence of electrical charges

leaves separate when they gain + or - charge