Electrostatics and Electricity

ELECTRIC CHARGE Static Electricity: electric charge at rest due to electron transfer (usually by friction)

+

–+–+

–

+ +–+

–

+

–+–+

–– negative charge: excess (gain) of electrons

positive charge: deficiency (loss) of electrons

neutral: electrons equal protons (no net charge)

ELECTRIC CHARGE law of conservation of charge:

total charge stays constant (for every + charge produced, there is a – charge produced)

+

+

+

– –

–

+

+

–

–

ELECTRIC CHARGE law of conservation of charge:

total charge stays constant (for every + charge produced, there is a – charge produced)

+

+

+

– –

+

+

–

–

–

ELECTRIC CHARGE law of

electrostatics: like charges repel, unlike charges attract

ELECTRIC CHARGE Charge transfer

conductor: readily transfers charge (free electrons)

insulator: doesn’t transfer charge (electrons in bonds)

ELECTRIC CHARGE Charging by

Conduction direct

contact same sign permanent charge

divides evenly between objects

ELECTRIC CHARGE Charging by

Induction no contact opposite

sign temporary

unless grounded

Electric Charge Charge by Friction

The heat generated by rubbing two objects together energizes electrons causing them to transfer.

ELECTRIC CHARGE

Conductor that has induced charge by neighboring positive wall. Free electrons move towards the wall.

Insulator that has induced charge by neighboring positive wall. Molecules are polarized.

ELECTRIC CHARGE

Why does the water bend towards the cup?

ELECTRIC FORCE electric force is a fundamental force of

nature: holds atoms together, holds molecules together, causes friction & most forces (except gravity)

Amount of charge, q or Q: measured in coulombs, C 1.00 C = 6.25×1018 electrons charge of one proton or electron, e =

±1.60×10–19 C

ELECTRIC FORCE Coulomb’s Law: force between charges

depends on amounts of charge and distance between them inverse square law like the force of gravity Fe = kq1q2/r

2

Fe: electric force q: charger: distance between charges k: 8.99×109

Nm2/C2 +Fe: repulsion, –Fe: attraction

ELECTRIC CHARGE Grounding: discharging by connecting

to a large charge sink (such as earth) Charge Distribution: only on the

surface; spreads evenly on spherical conductor; stays put on insulator; concentrates at points

Spark Discharge: when charge is large enough, air ionizes and conducts the charge away (lightning)

ELECTRIC FORCE Electric field: region around a

charge where it exerts electric force on other charges

field lines: show direction & amount of force (by how close the lines are) on a + test charge

ELECTRIC FORCE electric fields exert force on

charged objects electric field strength, E: force

exerted on a charge by an electric field

E = F/q unit: N/C (Newtons/Coulomb), or V/m

(Volts/meter)

ELECTRIC FORCE constant electric fields are used to

accelerate charged particles field is constant between parallel plates

force F = qE change in kinetic energy K-K0 = Fd

d: distance traveled in electric field, K = ½mv2

CIRCUIT BOARD INTRO

ELECTRIC CIRCUITS Basic Circuit: conductor loop for

transferring energy load: energy user (bulb, resistor, heater,

motor)

source: energy provider (battery, generator)

ELECTRIC CIRCUITS Current, I: rate of

“flow” of electric charge. unit: ampere, A I = Q/t 1 A = 1

C/s Charge, Q, is measured in

Coulombs. Think of current as

the number of electrons that pass by a point each second!

ELECTRIC CIRCUITS Voltage , V: work done per charge

between two points, unit: volt, V The voltage is the “push” on the

current! Examples: Batteries, Electrical

Outlets, Capacitors.

ELECTRIC CIRCUITS Resistance, R:

opposition to charge flow, unit: ohm, resistance limits the flow

of current resistance turns electric

energy into heat (& light)

resistor: fixed resistance, symbol:

ELECTRIC CIRCUITS Ohm’s law: current is proportional

to voltage and inversely proportional to resistance: V = IR V: voltage, V I: current, A R:

resistance, Example: How much current is there

if the voltage is 6V and the Resistance is 3 ?

ANALYZING CIRCUITS Resistances in Series:

IT = I 1 = I2 = I3

VT = V1+V2+V3

RT = R1+R2+R3

adding resistors in series increases RT, decreases IT

removing one resistor stops current in the whole circuit

ANALYZING CIRCUITS

EXAMPLE CIRCUIT 1 - assume 12 V battery

RT=____ VT=____ IT=____ PT=____

R1= 8 V1=____ I1=____ P1=____

R2= 8 V2=____ I2=____ P2=____

ANALYZING CIRCUITS

EXAMPLE CIRCUIT 2 - assume 4 V per cell

RT=____ VT=____ IT=____ PT=____

R1= 8 V1=____ I1=____ P1=____

R2= 16 V2=____ I2=____ P2=____

ANALYZING CIRCUITS Resistances in Parallel:

IT = I1+I2+I3 VT = V1 = V2 = V3

1/RT = 1/R1+1/R2+1/R3

adding resistors in parallel decreases RT, increases I

removing one resistor stops current only in that branch

ANALYZING CIRCUITS

EXAMPLE CIRCUIT 3 - assume 12 V

RT=____ VT=____ IT=____

R1= 8 V1=____ I1=____

R2= 8 V2=____ I2=____

ANALYZING CIRCUITS

EXAMPLE CIRCUIT 4 - assume 12 VRT=____ VT=____ IT=____

R1= 1 V1=____ I1=____

R2= 2 V2=____ I2=____

UNIT 7 FORMULAS Fe = kq1q2/r2

k = 8.99×109 Nm2/C2

e = ± 1.60×10–19 C

F = qE K-K0 = Fd

I = Q/t V = W/Q

R = L/A V = IR P = VI = I2R E = Pt RT = R1+R2+R3

1/RT = 1/R1+1/R2+1/R3

1.00 kWh = 3.60×106 J