Basic Laws EEE105 Electric Circuits Anawach Sangswang Dept. of Electrical Engineering KMUTT Ohm’s Law Resistor Materials with a characteristic behavior of resisting the flow of electric charge Resistance: An ability to resist the flow of electric current, measured in ohm (Ω) where ρ is the resistivity of the material in ohm-meters l is the length in meters A is the cross-sectional area in m 2 2 l R A ρ = Ohm’s Law Georg Simon Ohm (1787-1854: German) Ohm’s law: the voltage across a resistor (R) is directly proportional to the current (i) flowing through the resistor Mathematical expression: or Note: The direction of current (i) and the polarity of voltage (v) must conform with the passive sign convention 3 iR v = v i ∝ v R i = Ohm’s Law Two extreme possible values of R: 0 (zero) and ∞ ∞ ∞ (infinite) are related with two basic circuit concepts: short circuit and open circuit. 4 0 v iR = = lim 0 R v i R →∞ = = Short circuit Open circuit
7
Embed
L2 basic laws - KMUTTstaff.kmutt.ac.th/~Anawach.San/eee105/L2.pdf · Kirchhoff’s Laws: KCL KCL also applies to a closed boundary 13 The total current entering the closed surface
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Basic Laws
EEE105 Electric Circuits
Anawach Sangswang
Dept. of Electrical Engineering
KMUTT
Ohm’s Law Resistor
Materials with a characteristic
behavior of resisting the flow
of electric charge
Resistance: An ability to resist
the flow of electric current,
measured in ohm (Ω)
where ρ is the resistivity of the material in ohm-meters
l is the length in meters
A is the cross-sectional area in m2
2
lR
Aρ=
Ohm’s Law
Georg Simon Ohm (1787-1854: German)
Ohm’s law: the voltage across a resistor (R) is
directly proportional to the current (i) flowing
through the resistor
Mathematical expression:
or
Note: The direction of current (i) and the polarity
of voltage (v) must conform with the passive
sign convention
3
iRv =
v i∝v
Ri
=
Ohm’s Law
Two extreme possible values of R: 0 (zero)
and ∞∞∞∞ (infinite) are related with two basic
circuit concepts: short circuit and open circuit.
4
0v iR= = lim 0R
vi
R→∞= =
Short circuit Open circuit
Fixed resistors
5
Variable resistors Ohm’s Law
Linear and nonlinear resistors
Conductance is the ability of an element to conduct electric current; it is the reciprocal of resistance R and is measured in mhos or siemens.
6
v
i
RG ==
1
Ohm’s Law
The power dissipated by a resistor:
Note
The power dissipated in a resistor is a nonlinear
function of either current or voltage
The power dissipated in a resistor is always
positive
The resistor always absorbs power and is a passive
element (incapable of generating energy)
7
R
vRivip
22 ===
Example
Calculate the current i, the
conductance G and the
power p
The current
The conductance
8
3
306
5 10
vi mA
R= = =
×
3
1 10.2
5 10G mS
R= = =
×
330(6 10 ) 180p vi mW−= = × =
2 3 2 3(6 10 ) 5 10
180
p i R
mW
−= = × ⋅ ×
=
2 2 330 0.2 10
180
p v G
mW
−= = ⋅ ×
=
Power
or
or
Branch, Nodes A branch represents a single element such as
a voltage source or a resistor
A node is the point of connection between
two or more branches
9
Original circuit
Equivalent circuit
Loop, Series, Parallel A loop is a closed path in a circuit
An independent loop contains at least 1 branch
which is not a part of any other independent
loop or path sets of independent equations
A network with b branches, n nodes, and l
independent loops satisfies
Series: 2 or more elements share a single node
and carry the same current
Parallel: 2 or more elements are connected to
the same two nodes and have the same
voltage across them10
1−+= nlb
Example Number of branches, nodes, series and
parallel connection
11
Kirchhoff’s Laws: KCL
Kirchhoff’s current law (KCL) states that
“the algebraic sum of currents entering a
node is zero”
“The sum of the currents entering a node is equal to
the sum of the currents leaving the node”12
01
=∑=
N
nni
Applying KCL: 1 2 3 4 5( ) ( ) 0i i i i i+ − + + + − =
Rearranging the equation
1 3 4 2 5i i i i i+ + = +
Kirchhoff’s Laws: KCL
KCL also applies to a closed boundary
13
The total current
entering the closed
surface is equal to
the total current
leaving the surface
Kirchhoff’s Laws: KCL
14
I + 4-(-3)-2 = 0
⇒I = -5A
This indicates that the
actual current for I is
flowing in the
opposite direction.We can consider the whole
enclosed area as one “node”.
Example
Example: Determine the current I
Kirchhoff’s Laws: KVL
Kirchhoff’s voltage law (KVL) states that
“the algebraic sum of all voltages around a
closed path (or loop) is zero”
15
01
=∑=
M
mnv
1 2 3 4 5 0v v v v v− + + − + =
2 3 5 1 4v v v v v+ + = +or
Sum of voltage drops = Sum of voltage rises
Kirchhoff’s Laws: KVL
Series-connected voltage sources
Note:
2 different voltages cannot be connected in parallel
2 different currents cannot be connected in seriess16
1 2 3 0abV V V V− + + − = 1 2 3abV V V V= + −
Kirchhoff’s Laws: KVL
Example: Determine vo and i
Apply KVL around the loop
17
12 4 2 4 0o oi v v− + + − − =
The Ohm’s law at the 6-ohm
resistor gives 6ov i= −
12 4 2( 6 ) 4 ( 6 ) 0i i i− + + − − − − = 8i A= −
48ov V=
Example
Determine the current i
18
1 2 3 0a bV V V V V− + + + + =
1 1 2 2 3 3, , V IR V IR V IR= = =
The Ohm’s law at the each
resistor gives
KVL:
1 2 3 0a bV IR V IR IR− + + + + =
1 2 3
a bV VI
R R R
−=
+ +
Series Resistors
Two or more elements are in series if they are
cascaded or connected sequentially
and consequently carry the same current
19
1 2 1 2v v v iR iR= + = +1 2 eq
v vi
R R R= =
+ 1 2eqR R R= +
1 21 1 2 2
1 2 1 2
, R R
v iR v v iR vR R R R
= = = =+ + Voltage divider
Series Resistors, Voltage Division
The equivalent resistance of any number of
resistors connected in a series is the sum of
the individual resistances
The voltage divider can be expressed as
20
∑=
=+⋅⋅⋅++=N
nnNeq RRRRR
121
vRRR
Rv
N
nn +⋅⋅⋅++=
21
Parallel Resistors, Current Division Parallel connection: elements