1. Electrical Energy, Electric Fields & DC Circuits
Chapters
Electrical Energy, Electric Fields
& DC Circuits
Herriman High Honors Physics

2. Other Forms of Stored Energy: Chemical Energy
Stored in the Chemical Bonds that make up a substance
Often released by combustion (burning)
Released as
kinetic energy
Heat
Light
Sound
*** Demonstration ***
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3. 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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4. Herriman High Honors Physics
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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5. 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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6. (9 x 109 N•m2/C2)(+10 x 10-6 C)(-15 x 10-6 C)/(1.5 m)2
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?
Solution
F = 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
Practice A
P. 566 #1 & 3
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7. Herriman High Honors Physics
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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8. Herriman High Honors Physics
Sample Problem
Calculate the strength of an electric field at a point 30 cm from a point charge Q = +3 µC
Solution
E = kQ/r2 =
(9 x 109 N•m2/C2)(+3 x 10-6 C)/(0.3 m)2
= N/C
Practice D
P. 575 # 1 & 3
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9. Chapter 17: Electrical Energy & Current
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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10. 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 C
1 V = 1 Joule/1 Coloumb
Work = Volts * Charge = Vq
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11. An Old Equation – with a twist
Remember that the equation for the strength of an electric field is given by
E = F/Q
now we have
V = W/Q where W = F x d so
V/d = E or V = Ed
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12. Herriman High Honors Physics
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
Practice A
P # 1 & 3
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13. 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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14. Herriman High Honors Physics
Sample Problem
What charge is stored when a 0.5 F capacitor is attached to a 9 volt source?
Solution
q = CV = (0.5 F)(9 V)
= 4.5 Coloumbs
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15. Herriman High Honors Physics
Capacitance
To calculate the capacitance of a plate capacitator
C = Kε0A/d
where K = the dielectric constant
ε0 = the permitivity constant 8.85 x C2/N•m2
A = the area of the plates in m2
d = the distance between the plates in meters
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16. Herriman High Honors Physics
Sample Problem
What is the capacitance of a capacitor consisting of 2 plates, each having an area of 0.5 m2, separated by 2 mm of mica?
Solution
C = Kε0A/d
= (7)(8.85 x C2/N•m2)(0.5 m2)/(.002 m)
= 1.55 x 10-9 F = 1.55 nF
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17. Herriman High Honors Physics
Energy and Capacitors
Energy stored in capacitors is electric potential energy.
𝑃 𝐸 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 = 1 2 𝑄∆𝑉
Where Q is the charge on one plate and ΔV is the voltage or potential difference
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18. Herriman High Honors Physics
Sample Problem
A capacitor connected to a 12 V battery holds 36 µC of charge on each plate. What is the capacitance of the capacitor and how much electrical potential energy is stored in the capacitor?
Solution
𝐶= 𝑄 𝑉 = 3.6 𝑥 10 −5 𝐶 12 𝑉 =3 𝑥 1 0 −6 𝐹
𝑃𝐸= 1 2 𝑄𝑉= 1 2 𝐶 𝑉 2 =(0.5)(3 𝑥 1 0 −6 𝐹)(12 𝑉 ) 2 =2.2 𝑥 1 0 −4 𝐽
Practice B
P. 607
#2 & 4
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19. Electric Current & Resistance
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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20. Herriman High Honors Physics
Electric Current
Electron Flow is from – terminal to + terminal.
Conventional Current is from + terminal to – terminal.
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21. Herriman High Honors Physics
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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22. Herriman High Honors Physics
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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23. Herriman High Honors Physics
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?
Solution
R = V/I =
(1.5 V)/(0.3 A) = 5 Ω
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24. Herriman High Honors Physics
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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25. Herriman High Honors Physics
Resistor Color Code
Bright
Black
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%
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26. Red = 2, Green = 5, Blue = 6 and Silver = 10%
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 = ± 10% Or
R = 25 MΩ ± 10%
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27. Herriman High Honors Physics
Resistivity
Spools or lengths of wire each have their own Resistance.
Resistivity of these items can be calculated using the equation:
R = ρL/A
Where ρ is a constant, L is length, and A is cross sectional area of the wire.
Practice D
P. 615 #1,3,& 5
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28. Herriman High Honors Physics
Electric Power
Power = Work/time
In an Electical System P = QV/t
So P = VI = I2R = V2/R
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29. Herriman High Honors Physics
Sample Problem
Calculate the resistance of a 40 Watt headlight which is designed to run on 12 Volts.
Solution
R = V2/P
R = (12 V)2/40 Watts = 36 Ω
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30. Herriman High Honors Physics
Sample Problem
Calculate the resistance of a spool of
copper wire which is 20 m long and
has a cross sectional area of
3.4 x 10-6 m2?
Solution
R = ρL/A=
(1.68 x 10-8Ω•m)(20 m)/(3.4 x 10-6 m2) =
1.14 x Ω
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31. Herriman High Honors Physics
Chapter 18: 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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32. Herriman High Honors Physics
Sample Problem
Calculate the voltage and current when 3 batteries (1.5 V, 0.25 A are connected in
A) Series
B) Parallel
Solution
a) Et= E1 + E2 + E3 =1.5 V V 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 A A = 0.75 A
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33. Herriman High Honors Physics
DC Circuits
Resistance in Series
Rt=R1+R2+R3. . .
Resistance in Parallel
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34. Herriman High Honors Physics
Sample Problem
Calculate the resistance when a 5 Ω, 6 Ω, and 3 Ω resistor are connected in
A) Series
B) Parallel
Solution
a) Rt=R1+R2+R3 = 5 Ω+ 6 Ω+ 3 Ω = 14 Ω b)
Rt= 1.43 Ω
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35. Herriman High Honors Physics
In Class Practice
Practice A, P ,3, & 5
Practice B, p & 4
Do Practice C, p & 2 on board
Do Practice D, p. 662 on board
Herriman High Honors Physics