DC Circuits - Sections 19.1 - 19.2

bahmotherElectronics - Devices

Oct 7, 2013 (3 years and 10 months ago)

134 views

Announcements Kirchhoff’s Rules Combinations of Resistors Final Questions
DC Circuits
Sections 19.1 - 19.2
DC Circuits
Announcements
Kirchhoff’s Rules Combinations of Resistors Final Questions
Reading Assignment
Read sections 19.3 - 19.4
Homework Assignment 3
Homework for Chapter 18 is due in class today
Homework Assignment 4
Homework for Chapter 19 (due at the beginning of class on Wednesday,September 22)
Q:1,4,7,19,22
P:2,8,26,32,38,52
DC Circuits
Announcements
Kirchhoff’s Rules
Combinations of Resistors Final Questions
Kirchhoff’s junction rule
At any
junction,the sum of the currents must equal zero
￿
junction
I = 0
Currents directed into the junction are denoted as +I
Currents directed out of a junction are denoted as −I
In other words...
This rule simply states that for a steady flow of charge,there is neither a build-up nor a depletion of charge at any
junction
DC Circuits
Announcements
Kirchhoff’s Rules
Combinations of Resistors Final Questions
Kirchhoff’s junction rule
At any
junction,the sum of the currents must equal zero
￿
junction
I = 0
Currents directed into the junction are denoted as +I
Currents directed out of a junction are denoted as −I
In other words...
This rule simply states that for a steady flow of charge,there is neither a build-up nor a depletion of charge at any
junction
Kirchhoff’s loop rule
The sum of the potential differences across all circuit elements around any
closed circuit loop must be zero
￿
closed loop
ΔV = 0
What does this mean?
Kirchhoff’s loop rule is simply a statement of conservation of energy!
DC Circuits
Announcements
Kirchhoff’s Rules
Combinations of Resistors Final Questions
Electromotive force
The electromotive force (emf) ε of a battery is the maximum possible voltage the battery can provide
between its terminals
Though its name implies that it is a force,emf is actually a potential difference (measured in volts)
Using Kirchhoff’s rules
Label the current and the current direction in each branch (just guess a direction if you don’t know for sure)
Use Kirchhoff’s junction rule to write down a current equation for each junction that gives you a different
equation (every junction in the circuit except one)
Use Kirchhoff’s loop rule to write down loop equations for as many loops as it takes to obtain,in
combination with the equations from the junction rule,as many equations as there are unknowns
Charges move from a high-potential to a low-potential,so if a resistor is traversed in the direction
of the current,the potential difference is denoted as −IR
If the resistor is traversed in the direction opposite the current,the potential difference is denoted
as +IR
If a source of emf (with no internal resistance) is traversed in the direction of the emf (from
negative to positive),the potential difference is denoted as +ε
If a source of emf (with no internal resistance) is traversed in the direction opposite the emf (from
positive to negative),the potential difference is denoted as −ε
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Resistors
A resistor is an electronic component that impedes the flow of electric current,converting some of its
energy into heat
When charge flows through a resistor,it experiences a drop
in electric potential given by
ΔV = IR
The power delivered to the resistor by the current is
P = I ΔV = I
2
R =
(ΔV)
2
R
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Resistors
A resistor is an electronic component that impedes the flow of electric current,converting some of its
energy into heat
When charge flows through a resistor,it experiences a drop
in electric potential given by
ΔV = IR
The power delivered to the resistor by the current is
P = I ΔV = I
2
R =
(ΔV)
2
R
Internal resistance
Due to internal resistance in the battery,the actual potential difference between a battery’s terminals (the so-called
terminal voltage) is less than ε
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Series combination
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Series combination
The current through each resistor is the same (I
1
= I
2
= I ) (why?)
The total potential difference ΔV
tot
across resistors connected in series is the sum of the potential
differences across the individual resistors (ΔV
tot
= ΔV
1
+ ΔV
2
+...)
The equivalent resistance is the algebraic sum of the individual resistances
R
eq
= R
1
+ R
2
+ R
3
+...
Therefore,the equivalent resistance of a series combination of resistors is always greater than any
individual resistance
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Parallel combination
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Series combination
The current through each resistor is the same (I
1
= I
2
= I ) (why?)
The total potential difference ΔV
tot
across resistors connected in series is the sum of the potential
differences across the individual resistors (ΔV
tot
= ΔV
1
+ ΔV
2
+...)
The equivalent resistance is the algebraic sum of the individual resistances
R
eq
= R
1
+ R
2
+ R
3
+...
Therefore,the equivalent resistance of a series combination of resistors is always greater than any
individual resistance
Parallel combination
The potential difference across each resistor is the same (ΔV
1
= ΔV
2
= V) (why?)
The total current I is the sum of the currents across the individual resistors (I = I
1
+ I
2
+...)
The inverse of the equivalent resistance is the algebraic sum of the inverses of the individual resistances
1
R
eq
=
1
R
1
+
1
R
2
+
1
R
3
+...
Therefore,the equivalent resistance of a parallel combination of resistors is always less than the smallest
individual resistance in the group
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Scenario
Two lightbulbs are connected in series to a power source.Treat a lightbulb as an example of a resistor.
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Scenario
Two lightbulbs are connected in series to a power source.Treat a lightbulb as an example of a resistor.
Question#1
If the filament of the first lightbulb fails,what would happen to the second lightbulb?
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Scenario
Two lightbulbs are connected in series to a power source.Treat a lightbulb as an example of a resistor.
Question#1
If the filament of the first lightbulb fails,what would happen to the second lightbulb?
Answer
It would go out
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Scenario
Two lightbulbs are connected in series to a power source.Treat a lightbulb as an example of a resistor.
Question#1
If the filament of the first lightbulb fails,what would happen to the second lightbulb?
Answer
It would go out
Question#2
If the filament of the second lightbulb fails,what would happen to the first lightbulb?
DC Circuits
Announcements Kirchhoff’s Rules
Combinations of Resistors
Final Questions
Scenario
Two lightbulbs are connected in series to a power source.Treat a lightbulb as an example of a resistor.
Question#1
If the filament of the first lightbulb fails,what would happen to the second lightbulb?
Answer
It would go out
Question#2
If the filament of the second lightbulb fails,what would happen to the first lightbulb?
Answer
It would go out
DC Circuits
Announcements Kirchhoff’s Rules Combinations of Resistors
Final Questions
Reading Assignment
Read sections 19.3 - 19.4
Homework Assignment 3
Homework for Chapter 18 is due in class today
Homework Assignment 4
Homework for Chapter 19 (due at the beginning of class on Wednesday,September 22)
Q:1,4,7,19,22
P:2,8,26,32,38,52
DC Circuits