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Chapter 32
Electrostatics
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Electric Charge and Electric Field
Static Electricity – Unmoving charge Two types
Positive – lack of electrons Negative – excess electrons
Like charges - Repel Opposite Charges - Attract
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Electric Charges
Charge can be induced by rubbing an object – View demonstrations
Charge is detected using an electroscope.
Charge can travel via a conductor. Poor conductors are insulators.
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Force Exerted by Charges
Coulomb’s Law F = kQ1Q2/r2
k = 9 x 109 N•m2/C2
Positive solution – repulsion Negative solution - attraction
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Sample Problem
Two charges, Q1 = +10 µC, and Q2 = -15 µC, are separated by 1.5 meters.
What is the electrostatic force acting between them?
SolutionF = kQ1Q2/r2 =
(9 x 109 N•m2/C2)(+10 x 10-6 C)(-15 x 10-6 C)/(1.5 m)2
= -0.6 N
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Conductors and Insulators
A good conductor transfers charge easily.
A good insulator inhibits the transfer of charge.
A good conductor is a poor insulator and a good insulator is a poor conductor.
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Creation of Static Charges
Two ways to create a static charge are: Charging by contact (friction) – when
electrons are transferred from one object to another by touching.
Induction – when a charge is transferred by bringing one object near another without actually touching
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Chapter 33
Electric Fields and Potential
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Electric Field
Field – Affect that acts at a distance, without contact Examples
Electric Field Gravitational Field
Electric Field Strength – E = F/q = kQ/r2
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Sample Problem
Calculate the strength of an electric field at a point 30 cm
from a point charge Q = +3 µC
SolutionE = kQ/r2 =
(9 x 109 N•m2/C2)(+3 x 10-6 C)/(0.3 m)2
= 300000 N/C
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Electrical Energy Electrical Energy is generated from other
forms of energy and transmitted over power lines and/or stored in batteries
Vocabulary Voltage (V)
Force in an electrical system; Volt = Work/Charge = W/q = Joule/Coloumb
Current (I) Rate in an electrical system = Charge/time = q/t
=Coloumb/sec = 1 Ampere
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Energy in Electrical System
Volts =Work/charge = V =W/q Work is measured in joules (the same
as energy) Charge is measured in Coloumbs (C) The charge on an electron is 1.6 x 10-
19 C 1 V = 1 Joule/1 Coloumb
Work = Volts * Charge = Vq
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Sample Problem
How much work is needed to move a 10 μC charge to a point where the potential is 70 V?
W = Vq = (70 V)(10 x 10-6 C) = 7 x 10-4 J
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Electrical Energy Storage Electrical Energy can be stored in two
ways: Batteries
Long term storage, even flow of charge Storage ability measured in Volts
Capacitors Short term storage, releases charge all at once (boost
in charge) Storage capacity measured in Farads (F) 1 Farad = 1 Coloumb/Volt Mathematically Charge = Capacitance * Voltage = q
= CV
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Chapter 34
Electric Current
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Electric Current
Circuit – A continuous path connected between the terminals of a power source.
Current – Flow of Charge I = ΔQ/Δt Current is measured in
Coloumbs/Sec which is called an Ampere.
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Electric Current
Electron Flow is from – terminal to + terminal.
Conventional Current is from + terminal to – terminal.
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Sample Problem
A steady current of 2.5 Amps passes through a wire for 4 minutes. How much charge passed through any point in
the circuit?Solution
Q = IΔt (2.5 C/s)(240 s) = 600 C
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Ohm’s Law
Resistance – how much the conductor slows down the flow of electrons through it.
Resistance is measured in Ohms (Ω)
Ohm’s law -In any Circuit:V = IR or R = V/I
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Sample Problem
A small flashlight bulb draws a current of 300 mA from a 1.5 V battery. What is the resistance
of the bulb?SolutionR = V/I =
(1.5 V)/(0.3 A) = 5 Ω
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Resistor Color Code Resistors are banded in order to
describe the amount of resistance they provide. Each resistor is banded with 4 stripes.
Band Represents
1 First Digit
2 Second Digit
3 Multiplier
4 Tolerance
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Bright Black 0
Boys Brown 1
Remember Red 2
Our Orange 3
Young Yellow 4
Girls Green 5
Become Blue 6
Very Violet 7
Good Grey 8
Wives White 9
Gold 5%
Silver 10%
None 20%
Resistor Color Code
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Sample Problem
Calculate the resistance of a resistor which is banded with the
following colors: Red, Green, Blue, Silver.Solution
Red = 2, Green = 5, Blue = 6 and Silver = 10% R = 25000000 ± 10%
OrR = 25 MΩ ± 10%
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Electrical Power
Electrical Power is measured in Watts.
Power = current x voltageP = IV or P = I2R or P = V2/R
Since Energy is Power x Time electrical energy is often measured in Kilowatt•hours or power x time.
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Chapter 35
DC Circuits
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DC Circuits Batteries Connected in Series
Increase Voltage Et= E1 + E2 + E3. . .
Produce the Same Current It= I1 = I2 = I3. . .
Batteries Connected in Parallel Produce the Same Voltage
Et= E1 = E2 = E3. . . Increase Current
It= I1 + I2 + I3. . .
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Sample Problem
Calculate the voltage and current when 3 batteries (1.5 V, 0.25 A are connected in
A) SeriesB) Parallel
Solutiona) Et= E1 + E2 + E3 =1.5 V + 1.5 V + 1.5 V = 4.5 V
It= I1 + I2 + I3= 0.25 A
b) Et= E1 = E2 = E3=1.5 V
It= I1 + I2 + I3=0.25 A + 0.25 A + 0.25 A = 0.75 A
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DC Circuits
Resistance in SeriesRt=R1+R2+R3. . .
Resistance in Parallel
...1111
321 RRRRt
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Sample Problem
Calculate the resistance when a 5 Ω, 6 Ω, and 3 Ω resistor are connected in
A) SeriesB) Parallel
Solution
a) Rt=R1+R2+R3 = 5 Ω+ 6 Ω+ 3 Ω = 14 Ωb)
Rt= 1.43 Ω
30
21
30
10
30
5
30
6
3
1
6
1
5
11111
321 RRRRt