electrostatics and electricity. electric charge static electricity: electric charge at rest due to...
Post on 01-Apr-2015
227 Views
Preview:
TRANSCRIPT
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
top related