Schedule

• Today: Finish Current & Resistance – Chapter 22– Activity 4 due nominally

• Thursday:– Activity 5

• Next Tuesday:– Review & Ch. 22

• Next Thursday:– Exam I: Ch 20-22

• Problems based on HMWK, activities 1,2,3,4,5

Activity Grades

• A+ (100) = Professional, Correct• A (95) = Correct, small errors• B (85) = At least one major error• C (75) = Several Major errors• F (50) = Attended, Turned in something• 0 = Did not do activity

• Use Staples. Kill Trees

HMWK 1

• Misprint last week – Should be due Thursday:

• Can extend to Tueday (HMWK 2 Due next Tues)

• 20 Q13; P 1, 15, 24, 32, 49, 51, 57, 62, 65

• 21 Q17; P 8, 27, 38, 46, 48, 54, 56, 62, 73

HMWK 2

• Due Beginning of Class Next Tues

• CH22: – Q 11; P 16, 33, 37, 42, 45, 46, 48, 50, 53

Chapter 22

• Ideas– Voltage, Current Resistance– Charge Conservation– Resistivity– Same as water…

Chapter 22

• Voltage, Current, Resistance

• Current: Flow of Electricity– Units = A = C/s– Electrons flowing though wires (opposite…)– Think water flowing through a pipe– Charge Conserved: Current in = Current Out

Chapter 22

• Voltage, Current, Resistance

• Voltage: Electromotive “Force”– Units = V (Seen these, no?)– Potential that causes electrons to move– Think pressure forcing water to flow

Chapter 22

• Voltage, Current, Resistance

• Resistance: Opposition to flow– Units: Ohms ()– Think size of pipe that can restrict water flow– Or think water wheel

• Turns electricity into heat, uses electricity (toaster)

Relationship

• “Flow” proportional to “pressure”

• Current proportional to voltage

• Larger resistance inhibits current

• Current inversely proportional to resistance

• Combined: V=IR

Back to Energy

• Power = Watts (W)

• Power = J/s [Energy per second]

• Volts = J/C

• Energy = V x C

• Power = Energy / time = V x C/s

• Power = IV

Electricity Equations

• Big 2!

• V = IR

• P = IV

• Mix & Match

• P=I2R, P=V2/r, etc…

Startup Problems

• Hints: V=IR, P=IV

• What is the resistance of a 120W standard light bulb? What does the 120W mean?

• Hint: What is standard voltage?

• Wall voltage is 120V

• Voltage constant, current can vary…

Startup Problems

• Hints: V=IR, P=IV

• What is the resistance of a 120W standard light bulb? What does the 120W mean?

• 120W means the light bulb converts 120 watts of electrical power to light & heat [also a form of light?] when 120V is applied to it.

Startup Problems

• Hints: V=IR, P=IV• What is the resistance of a 120W standard light

bulb? What does the 120W mean?

• Math• P=IV 120W = I (120V)• I = 1 A [standard circuit 10-15 A]• V=IR• R = V/I = 120 V / 1A = 120 .

Resistivity

• Materials have a definable opposition to flow: resistivity

• Resistors = objects that do so– Like Light bulb

• Examine the water pipe analogy

Water tower

Water TowerPotential Energy from Gravity

PE = mghForces water down [pressure]

Two “hoses” one skinny, one fat, which one allows more water to flow through?More flow = less resistance [more conductance]

Water tower

Water TowerPotential Energy from Gravity

PE = mghForces water down [pressure]

Two “hoses” one short, one long, which one allows more water to flow through?More flow = less resistance [more conductance]

Water tower

Water TowerPotential Energy from Gravity

PE = mghForces water down [pressure]

Look at it this way…Short one = part that is same width as one on left, plus part infinitely wide…

Math

• Shorter pipe = more flow• Shorter resistor = less resistance• Fatter pipe = more flow• Fatter resistor = less resistance• Resistor with Larger area = less resistance• R = L/A: = resistivity

– Resistivity depends on type of material– Resistance also depends on geometry– Intrinsic property (independent of V, I, etc…)

Picture

• R = L/A: r = resistivity

• A: Cross Sectional Area

• How would resistance change if you doubled the width?

• How much would Area increase?

• A=w2: A’ = (2w)2 = 4w2

• New Area (A’) is 4x larger

• This means resistance is 4x smaller

Reading from Chapter 1. The charge carriers in metals are

A. electrons B. positrons C. protons D. a mix of protons and electrons

Slide 22-2

1. The charge carriers in metals are A. electrons

Slide 22-3

Answer

Protons are fixed to the atoms, only electrons can move

2. A battery is connected to a resistor. Increasing the resistance of the resistor will A. increase the current in the circuit. B. decrease the current in the circuit. C. not affect the current in the circuit.

Slide 22-4

2. A battery is connected to a resistor. Increasing the resistance of the resistor will

B. decrease the current in the circuit.

Slide 22-5

Answer

V = IRI = V/RV is same (same battery) so if R increases, I must decreaseSame push, more resistance so less “flow”

3. What is the overall charge of a current carrying wire? (Think the wire that connects the power to this laptop) A. Positive B. Negative C. Neutral D. No Idea

Slide 22-6

Answer

3. What is the overall charge of a current carrying wire? (Think the wire that connects the power to this laptop)

C. Neutral

Slide 22-6

Think about zero current:Equal # of electrons & protons

Current: Current in = current outNot adding charge, just pushing it throughSo total # electrons & protons still equal, just that the electrons are mocing

Break & Try some Book Problems