Download Lecture 22

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?
• r = r0 (1 + aDT) or Dr = aDT
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 Tower
Potential Energy from Gravity
PE = mgh
Forces 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 Tower
Potential Energy from Gravity
PE = mgh
Forces 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 Tower
Potential Energy from Gravity
PE = mgh
Forces water down [pressure]
Look at it this way…
Short one = part that is same width as one on left, plus part infinitely wide…
Math
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Shorter pipe = more flow
Shorter resistor = less resistance
Fatter pipe = more flow
Fatter resistor = less resistance
Resistor with Larger area = less resistance
R = rL/A: r = 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
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“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
Takes effort to raise ball up: Battery
Increase PE of “ball” (current charges)
Ball rolls down hill
PE  KE
Rolls around track
DE = 0
Rolls into Elevator
KE  PE
Circuit
Current made up of “+” charges
Call them “holes”
I
R1
+
-
“+” charges s exit + terminal
Flow through circuit
Return to “-” terminal
Need return path for current flow
Circuit
Current made up of “+” charges
Call them “holes”
I
R1
“+” charges s exit + terminal
Flow through circuit
Return to “-” terminal
Need return path for current flow
What happens here?
+
-
-
+
Circuit
Current made up of “+” charges
Call them “holes”
I
R1
“+” charges s exit + terminal
Flow through circuit
Return to “-” terminal
Call “-” zero volts as reference here
+
-
0V
Indicates “ground” reference
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
A
E
D
0V
B
Distance
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
A
E
D
0V
B
Distance
Voltage Constant in a wire!
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
B
E
D
0V
C
Distance
Voltage in resistor?
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
B
E
D
0V
Distance
Voltage in resistor?
Not constant: Why linear?
Resistance increases with length…. R=rL/A
C
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
C
E
D
0V
Voltage in ?
D
Distance
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
C
E
D
0V
Voltage in ?
Wire: ~ constant
D
Distance
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
C
E
D
0V
Voltage in ?
Wire: ~ constant
D
Distance
E
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
C
E
D
0V
Voltage in ?
Wire: ~ constant
D
E
Distance
F
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
F
E
D
0V
Voltage in Battery?
Voltage Source?
A
Distance
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
F
E
D
0V
Distance
Voltage in increases from “-” to “+”
Nor clear internal workings
No matter, just worry about terminal areas
A
Voltage in a given area
1.5V
B
I
R1
A
+
V
C
0
F
A
E
D
0V
B
C
Distance
Complete Circuit
Voltage ends where it began… (Loop)
F
A
Voltage Loop Math
1.5 V Battery
B
I
A
+
C
F
E
D
0V
VA – VA = 0
VAA = VA – VA
VAB = VA – VB
VAA = VAB + VBC + VCD + VDE + VEF + VFA = 0
VAA = 0 + 1.5V + 0V + 0 V + 0V + (-1.5V) = 0
Useful trick
Find any loop in a circuit
Voltage around entire loop must be zero
Powerful….
Back to Energy
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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
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Big 2!
V = IR
P = IV
Mix & Match
P=I2R, P=V2/r, etc…
Energy Conservation
• Energy in = Energy Out
• Power in = Power Out
B
I
A
Power into Circuit: From Battery
Power Out of Circuit: Resistor
R’s Convert Electricity to Heat, light, etc,,,
Toaster?
+
-
F
E
0V
Charge Conservation
• Charge in = Charge Out
• Current in = Current Out
B
I
A
Current into Circuit: From Battery
Current flowing through : Resistor, Wires
IBAT = IWIRE = IR
No other way to go!
+
-
F
E
0V
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…