Do Now (11/25/13): Pass in your HW What do you know about electric current? What is resistance?
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Transcript of Do Now (11/25/13): Pass in your HW What do you know about electric current? What is resistance?
Do Now (11/25/13):
Pass in your HWWhat do you know about electric current?What is resistance?
AP PHYSICSCh. 17 & 18
Electric Current and Simple Circuits
Electric Cells and Batteries
A source of constant voltage or potential difference, or electromotive force (emf)
The voltage that exists between the terminals of a battery depends on what the electrodes are made of and their relative ability to be dissolved or give up electrons.
If a potential or voltage is applied across a conductor, or other pathway, (-) charge will flow from low to high potential.
The flow of charge per unit time Unit – 1 coulombs per second = 1 ampere
Electrical Current (I)
t
QI
10V 0Ve
Current in a Simple Circuit
In a simple circuit current is proportional to the voltage of the battery and inversely proportional to the resistance of the circuit.
R
VI
IRV Ohm’s Law
ElectronFlow (-)
ConventionalCurrent Flow(+)
*SEE PhET BATTERY-RESISTOR*
Tarzan Level 5
What is the resistance for every inch of the nichrome wire used for cutting the string?
R = Ω /inch
Resistance of Material
Resistance of a conductor (wire) depends upon:1. Length2. Area3. Material (ρ = resistivity)
Table on page 501 for T = 20°C4. Temperature
Resistivity is an intrinsic property of a material (Resistivity is a measure of a material’s ability to conduct electricity)
Example: Find the resistance per unit length of a copper wire commonly used in homes. The diameter is 2.05mm and ρ = 1.68x10-8 Ω•m R = 0.005 Ω/m
A
LR
Units: Ohms (Ω)
Temperature Dependence of R
At higher temperature, the atoms of a metal are moving faster and more rapidly, so they interfere more with the flow of electricity.
ooT TT 1ρT = resistivity at a given
temperature
ρo = resistivity at a reference
temperature
α = temperature coefficient of resistivity
To = reference temperature
Example Problem
Find the resistance of a 20m-long, 2.05mm radius copper wire at 52°C. (αcopper = 0.0068 °C-1)
R = Ω
Electric Power
A resistor gives off energy in the form of heat in a circuit electrical potential energy
thermal energy.
t
qVP
time
nsformedenergy tra
RIIVP 2How much energy is lost each second in the nichrome wire for the Tarzan Project?
Power Usage
Energy costs $$$0.10792 for every
kilowatt-hour (kWh) for the first 300kWh in 2008.
Examples:How many joules of energy
is 1 kilowatt hour? 1kWh = 3.6x106 J
How much would it cost to leave a 100W light bulb on for 24 hours? 1 year? Cost = $0.26/day & $94.54/year
100W
Practice:
Work with your group to complete the multiple choice questions in Chapter 17 (on a separate sheet)
Will be collected
Do Now (11/26/13):
Pass in your capacitor labsA lightbulb is rated at 120 V and 75 W. The
bulb is powered by a 120 V direct-current supply.
1.Find the current in the bulb. 2.Find the resistance.
Drift Speed
Current in a conductor:
N= number of mobile charge carriers per unit volume
q=charge on each carrierv=drift speed of carriersA = area of conductor
AnqvI d
Practice
Work with your group to complete the conceptual questions in Chapter 17.
Use different colored writing utensils for each person
Ch.18 Homework Assignment
Read: 18.1-18.3 and 18.5 – 18.7 (no math in last section)
Textbook (pg. 514)Questions: 1, 3-5, 7, 10 – 14 and
16 Problems: 1, 4, 5, 13, 26, 27, 31,
33 and 38Due: (2.21.08)
High Voltage Power Transmission
High voltages are used to transfer power over DC power lines to reduce power losses in the wires.
The following graph shows the amount of power transmitted over a 2 mile copper wire (r=2.5mm) compared to the power dissipated in the wire for different voltages.
RIIVP 2
Do Now (12/2/13):
Pass in your homeworkIn your own words, what is the difference
between parallel and series circuits?What is the role a battery?What is the role of a resister?What is current?
Combination of Resistors
Series Combination: Parallel Combination:
321 RRRReq 321
1111
RRRReq
R1 R2 R3
R1
R2
R3
Example Problem
Three resistors, 4Ω, 7 Ω , and 10 Ω , are connected together in series, and then connected to a 9V battery. Find the equivalent resistance of the group, and find the voltage across and current through the 7 Ω resistor.
4Ω 7Ω 10Ω
9V
Example Problem
Three resistors, 4Ω, 7 Ω , and 10 Ω , are connected together in series, and then connected to a 9V battery. Find the equivalent resistance of the group, and find the voltage across and current through the 7 Ω resistor.
7Ω9V 0V
4Ω
10Ω
Combination Circuits:
PROBLEM 11 IN TEXTBOOK (CH. 18)If you have finished number, begin working
on #9Once you have completed #9, solve for the
currents and voltages across each resistor
Real Batteries
Real batteries are not a source of constant voltage.
They can be modeled by a constant source of voltage (emf or electromotive force) and a small resistor in series.
The terminal voltage is the actual voltage supplied by the battery.
1Ω 10V
emf
Kirchoff’s Rules (for circuits)
Everything you know about solving series and parallel circuits ceases to work as soon as there are multiple sources of voltage.
THE RULES:1. Current Rule: at any junction point, the sum of all
currents entering the junction must equal the sum of all currents leaving the junction.
Note: this rule is based on conservation of charge.
2. Voltage Rule: the sum of the changes in potential (voltage) around any closed path of a circuit must be zero
1. Note: this rule is based on the conservation of potential (energy).
The Strategy
First assign numbers and directions to currents in all branches. (It doesn’t matter whether or not you guess the direction correctly).
Apply the current rule to any one junction in the circuit.
Apply the voltage rule as many times as you need to have enough equations to solve all of the unknowns for the entire circuit.
The # of independent equations you need to solve a problem is the same as the # of unknown currents you have in the problem.
Example Problem #1
Find the terminal voltage of the 9V battery shown in the diagram.
1Ω 9V
emf
50Ω12V
Vterminal = 9.06volts
Example Problem #2
Find the current in the 5Ω resistor in the complex circuit shown in the figure.
R E
4Ω9V
5Ω6V
5A
I2
I1 I1 = 0.56A
Example Problem#3
Find the voltage drop across the 3Ω resistor in the complex circuit shown in the figure.
3Ω
4Ω6V
9Ω 12VI1
I2
Voltage Drop = 1.83V
I3
7Ω
Combination of Capacitors
Series Combination: the charge on each
capacitor is the same.
Parallel Combination:
321 CCCCeq 321
1111
CCCCeq
C1 C2 C3
C1
C2
C3
Example Problem #1
Three capacitors, 4uF, 7 uF , and 10uF , are connected together in series, and then connected to a 12V battery. Find the equivalent capacitance of the group, and find the charge stored by the 10uF capacitor.
4uF
12V
7uF 10uF
Example Problem #2
Find the equivalent capacitance of the combination of capacitors shown in the figure, and find the charge on and potential difference across the 4uF capacitor.
2uF
9V
4uF
8uF
Multiple Choice Answers (Unit 9)
1. D2. A3. A4. A5. E6. All choices are
incorrect7. E8. B9.C
10. C & E11. B12. A