Agenda Today –Finish Chapter 25 Monday –Simple Circuitry (ch. 26) Tues Lab & Quiz on Ch. 24-25 Finish 26 next week then…. –Freedom?
Post on 18-Jan-2016
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Agenda
• Today– Finish Chapter 25
• Monday– Simple Circuitry (ch. 26)
• Tues Lab & Quiz on Ch. 24-25
• Finish 26 next week then….– Freedom?
Temp Dependence of Resistivity
• What happens when you turn on a light?
• When do light bulbs burn out?• What did you learn about the
resistance of light bulbs in lab?
• How does resistivity change in metals with temperature?
• = 0 (1 + T) or = T
Resistance
• Resistivity is a property inherent to material (and Temp)
• Example: All copper has same resistivity
• Examine a Resistor
Seen water analogy?Resistor like water hose
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…)
Relationships
• Voltage– Water Pressure– Forces current to flow– Electron flow vs. Current flow?
• Current– Amount of flowing water– Charge traveling through per second
• Resistance– Impededes Current Flow
Relationship
• “Flow” proportional to “pressure”
• Current proportional to voltage
• Larger resistance inhibits current
• Current inversely proportional to resistance
• Combined: V=IR
“EMF”
• Electromotive Force?
• Silly archaic words for voltage?– Voltage more like Energy than force…
• Usually used in non-ideal batteries
• Examine somewhat more with non-ideal voltage sources in circuits
Voltage Loop
• Think of voltage like energy
Ball rolls down hillPE KE
Rolls around trackE = 0
Rolls into ElevatorKE PE
Takes effort to raise ball up: BatteryIncrease PE of “ball” (current charges)
CircuitCurrent made up of “+” charges
Call them “holes”
R1I
+
-
“+” charges s exit + terminalFlow through circuitReturn to “-” terminalNeed return path for current flow
CircuitCurrent made up of “+” charges
Call them “holes”
R1I
+
-
“+” charges s exit + terminalFlow through circuitReturn to “-” terminalNeed return path for current flowWhat happens here?
- +
CircuitCurrent made up of “+” charges
Call them “holes”
R1I
+
-
“+” charges s exit + terminalFlow through circuitReturn to “-” terminalCall “-” zero volts as reference here
0 V
Indicates “ground” reference
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FA B
Distance
V
0
1.5V
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FA B
Distance
V
0
1.5V
Voltage Constant in a wire!
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FB C
Distance
V
0
1.5V
Voltage in resistor?
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FB C
Distance
V
0
1.5V
Voltage in resistor?Not constant: Why linear? Resistance increases with length…. R=L/A
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FC D
Distance
V
0
1.5V
Voltage in ?
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FC D
Distance
V
0
1.5V
Voltage in ?Wire: ~ constant
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FC E
Distance
V
0
1.5V
Voltage in ?Wire: ~ constant
D
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FC E
Distance
V
0
1.5V
Voltage in ?Wire: ~ constant
D F
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FF A
Distance
V
0
1.5V
Voltage in Battery?Voltage Source?
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FF A
Distance
V
0
1.5V
Voltage in increases from “-” to “+”Nor clear internal workingsNo matter, just worry about terminal areas
Voltage in a given area
R1I
+
-
0 V
A
B
C
DE
FA B
Distance
V
0
1.5V
Complete CircuitVoltage ends where it began… (Loop)
C F A
Voltage Loop Math
I
+
-
0 V
A
B
C
DE
F
VA – VA = 0VAA = VA – VA
VAB = VA – VB VAA = VAB + VBC + VCD + VDE + VEF + VFA = 0VAA = 0 + 1.5V + 0V + 0 V + 0V + (-1.5V) = 0Useful trickFind any loop in a circuitVoltage around entire loop must be zeroPowerful….
1.5 V Battery
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…
Energy Conservation
• Energy in = Energy Out
• Power in = Power Out
I
+
-
0 V
A
B
E
F
Power into Circuit: From BatteryPower Out of Circuit: ResistorR’s Convert Electricity to Heat, light, etc,,,Toaster?
Charge Conservation
• Charge in = Charge Out
• Current in = Current Out
I
+
-
0 V
A
B
E
F
Current into Circuit: From BatteryCurrent flowing through : Resistor, Wires IBAT = IWIRE = IR
No other way to go!
Agenda
• Today– Finish Chapter 25
• Monday– Simple Circuitry (ch. 26)
• Tues Lab & Quiz on Ch. 24-25
• Finish 26 next week then….– Freedom?
• Summer / Other Res. Interest…
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