Do Now (11/25/13): Pass in your HW What do you know about electric current? What is resistance?

AP PHYSICS Ch. 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) 10V 0V e

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. Ohm’s Law Electron Flow (-) Conventional Current 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. Length 2. Area 3. Material (ρ = resistivity)  Table on page 501 for T = 20°C 4. 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 Ωm  R = Ω/m 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. ρ T = resistivity at a given temperature ρ o = resistivity at a reference temperature α = temperature coefficient of resistivity T o = reference temperature

Example Problem Find the resistance of a 20m-long, 2.05mm radius copper wire at 52°C. (α copper = °C -1 ) R = Ω

Electric Power A resistor gives off energy in the form of heat in a circuit  electrical potential energy  thermal energy. How much energy is lost each second in the nichrome wire for the Tarzan Project?

Power Usage Energy costs $$ $ for every kilowatt- hour (kWh) for the first 300kWh in Examples: How many joules of energy is 1 kilowatt hour?  1kWh = 3.6x10 6 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 labs A 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 carrier v=drift speed of carriers A = area of conductor

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: 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 38 Due: ( )

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.

Do Now (12/2/13): Pass in your homework In 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: R1R1 R2R2 R3R3 R1R1 R2R2 R3R3

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. 4Ω4Ω7Ω7Ω10Ω 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. 7Ω7Ω 9V0V 4Ω4Ω 10Ω

Combination Circuits: PROBLEM 11 IN TEXTBOOK (CH. 18) If you have finished number, begin working on #9 Once 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. 1Ω1Ω 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. 1Ω1Ω 9V emf 50Ω 12V V terminal = 9.06volts

Example Problem #2 Find the current in the 5Ω resistor in the complex circuit shown in the figure. R E 4Ω4Ω 9V 5Ω5Ω 6V 5A I2I2 I1I1 I 1 = 0.56A

Example Problem#3 Find the voltage drop across the 3Ω resistor in the complex circuit shown in the figure. 3Ω3Ω 4Ω4Ω 6V 9Ω9Ω 12V I1I1 I2I2 Voltage Drop = 1.83V I3I3 7Ω7Ω

Combination of Capacitors Series Combination: the charge on each capacitor is the same. Parallel Combination: C1C1 C2C2 C3C3 C1C1 C2C2 C3C3

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. 4uF4uF 12V 7uF7uF 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. 2uF2uF 9V 4uF4uF 8uF8uF

Multiple Choice Answers (Unit 9) 1. D 2. A 3. A 4. A 5. E 6. All choices are incorrect 7. E 8. B 9.C 10. C & E 11. B 12. A