Herriman High Honors Physics Chapters 16 - 18 Electrical Energy, Electric Fields & DC Circuits.
-
Upload
virginia-melina-hall -
Category
Documents
-
view
214 -
download
1
Transcript of Herriman High Honors Physics Chapters 16 - 18 Electrical Energy, Electric Fields & DC Circuits.
Herriman High Honors Physics
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 ***
Herriman High Honors Physics
Electric Charge and Electric Field
Static Electricity – Unmoving charge Two types
Positive – lack of electrons Negative – excess electrons
Like charges - Repel Opposite Charges - Attract
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.
Herriman High Honors Physics
Force Exerted by Charges Coulomb’s Law
F = kQ1Q2/r2
k = 9 x 109 N•m2/C2
Positive solution – repulsion Negative solution - attraction
Herriman High Honors Physics
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
Practice AP. 566 #1 &
3
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
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
SolutionE = kQ/r2 =
(9 x 109 N•m2/C2)(+3 x 10-6 C)/(0.3 m)2
= 300000 N/C
Practice DP. 575 # 1 &
3
Herriman High Honors Physics
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
Herriman High Honors Physics
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
Herriman High Honors Physics
An Old Equation – with a twist
Remember that the equation for the strength of an electric field is given by
E = F/Qnow we have
V = W/Q where W = F x dso
V/d = E or V = Ed
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 AP. 599 # 1 & 3
Herriman High Honors Physics
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
Herriman High Honors Physics
Sample Problem What charge is stored when a 0.5 F
capacitor is attached to a 9 volt source?
Solutionq = CV = (0.5 F)(9 V)
= 4.5 Coloumbs
Herriman High Honors Physics
Capacitance To calculate the capacitance of a
plate capacitatorC = Kε0A/d
where K = the dielectric constant
ε0 = the permitivity constant 8.85 x 10-12 C2/N•m2
A = the area of the plates in m2
d = the distance between the plates in meters
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?
SolutionC = Kε0A/d
= (7)(8.85 x 10-12 C2/N•m2)(0.5 m2)/(.002 m)
= 1.55 x 10-9 F = 1.55 nF
Herriman High Honors Physics
Energy and Capacitors Energy stored in capacitors is
electric potential energy.
Where Q is the charge on one plate and ΔV is the voltage or potential difference
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
Herriman High Honors Physics
Practice BP. 607 #2 & 4
Herriman High Honors Physics
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.
Herriman High Honors Physics
Electric Current Electron Flow is from – terminal to
+ terminal. Conventional Current is from +
terminal to – terminal.
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
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
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?SolutionR = V/I =
(1.5 V)/(0.3 A) = 5 Ω
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 Represents1 First Digit2 Second
Digit3 Multiplier4 Tolerance
Herriman High Honors Physics
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
Herriman High Honors Physics
Sample Problem
Calculate the resistance of a resistor which is banded with
the following colors: Red, Green, Blue, Silver.
SolutionRed = 2, Green = 5, Blue = 6 and Silver =
10% R = 25000000 ± 10%
OrR = 25 MΩ ± 10%
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
Herriman High Honors Physics
Electric Power Power = Work/time In an Electical System P = QV/t So P = VI = I2R = V2/R
Herriman High Honors Physics
Sample Problem Calculate the resistance of a 40
Watt headlight which is designed to run on 12 Volts.
SolutionR = V2/P
R = (12 V)2/40 Watts = 36 Ω
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?
SolutionR = ρL/A=
(1.68 x 10-8Ω•m)(20 m)/(3.4 x 10-6 m2) = 1.14 x 10-12 Ω
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 VoltageEt= E1 = E2 = E3. . .
Increase CurrentIt= I1 + I2 + I3. . .
Herriman High Honors Physics
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
VIt= I1 + I2 + I3= 0.25 A
b) Et= E1 = E2 = E3=1.5 VIt= I1 + I2 + I3=0.25 A + 0.25 A + 0.25 A = 0.75 A
Herriman High Honors Physics
DC Circuits Resistance in Series
Rt=R1+R2+R3. . . Resistance in Parallel
...1111
321 RRRRt
Herriman High Honors Physics
Sample ProblemCalculate 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