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Lecture - 2 Basic circuit laws. Outline. Electrical resistance. Ohm’s law. Conductance. Power in resistor. Open circuit and short circuit. Branches, nodes, loops. Kirchhoff's current law Kirchhoff's voltage law. Electrical Resistance. - PowerPoint PPT Presentation
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Lecture - 2
Basic circuit laws
Outline• Electrical resistance.
• Ohm’s law.
• Conductance.
• Power in resistor.
• Open circuit and short circuit.
• Branches, nodes, loops.
• Kirchhoff's current law
• Kirchhoff's voltage law.
Electrical Resistance• Resistance is the capacity of materials to impede the flow of
current or, more specifically, the flow of electric charge. • The circuit element used to model this behaviour is the
resistor.• Most materials exhibit measurable resistance to current. • The amount of resistance depends on the material.• Metals such as copper and aluminium have small values of
resistance, making them good choices for wiring used to conduct electric current.
• The resistance of the wire is so small compared to the resistance of other elements in the circuit that we can neglect the wiring resistance to simplify the diagram.
Ohm's Law• For purposes of circuit analysis, we must reference the current
in the resistor to the terminal voltage. We can do so in two ways: – in the direction of the voltage drop across the resistor,– in the direction of the voltage rise across the resistor.
Conductance• The reciprocal of the resistance is referred to as conductance,
is symbolized by the letter G, and is measured in Siemens (S).Thus:
• An 8 Ω resistor has a conductance value of 0.125 S.
SR
G ,1
Power in a resistor • Regardlessof voltage polarity and current direction, the power
at the terminals of a resistor is positive. Therefore, a resistor absorbs power from the circuit.
• Power equations:
Rvp
Rip2
2
GvpGip
2
2
vip
Example 1• In each circuit in the figure:a) Calculate the values of v and i.b) Determine the power dissipated in each resistor.
Example 1a)• The voltage va in (a) is a drop in the direction of the current in the resistor,
va= ( 1 ) ( 8 ) = 8 V.
• The current ib in the resistor with a conductance of 0.2 S in (b) is in the direction of the voltage drop across the resistor. ib = (50)(0.2) = 10 A.
• The voltage vc in (c) is a rise in the direction of the current in the resistor,vc = -(1)(20) = -20 V.
• The current id in the 25 Ω resistor in (d) in the direction of the voltage rise across the resistor,id =-50/25= - 2 A.
Example 1b) The power dissipated in each of the four resistors is:
Wp
Wp
Wp
Wp
S
10025
)50(
2020
)20(
500)2.0()50(
88)8(
2
25
2
20
22.0
2
8
Short Circuit as Zero Resistance
• An element or wire with R=0 is called a short circuit.
• Just drawn as a wire (line).
Open circuit
• An element or (wire) with is called open circuit.
• Just omitted.
R
Branches• Branch: a single two-terminal element in a circuit.
• Segments of wire are not counted as elements.
• Examples: voltage source, resistor, current source.
Nods• Node: the point of connection between two or more branches.
• May include a portion of the circuit (more than a single point).
• Essential Node: the point of connection between three or more branches.
Loops• Loop: any closed path in the circuit.
Overview of Kirchhoff’s Laws
• The foundation of circuit analysis is:– the defining equations for circuit elements (e.g. Ohm’s law).– Kirchhoff’s current law(KCL).– Kirchhoff’s voltage law(KVL).
• The defining equations tell us how the voltage and current within a circuit element are related.
• Kirchhoff’s laws tell us how the voltage and currents in different branches are related.
Kirchhoff’s Current Law (KCL)• KCL: The algebraic sum of all the currents at any node in a
circuit equals zero.• The sum of currents entering a node is equal to the sum of the
currents leaving a node.
Example 2• Sum the currents at each node in the circuit shown in the
figure. --------------------------------
In writing the equations, we use a positive sign for a current leaving a node. The four equations are
node a i1 + i4 - i2 – i5 = 0,node b i2 + i3 - i1 - ib - ia = 0,node c ib - i3 - i4 - ic = 0,node d i5 + ia + ic = 0.
Kirchhoff’s Voltage Law (KVL)• KVL: The algebraic sum of all the voltages around any closed
path in a circuit equals zero.
• 011 sc Vvvv
Example 3• Sum the voltages around each designated path in the circuit shown in
the figure.
--------------------------In writing the equations, we use a positive sign for a voltage drop. The four equations are
path a -v1 + v2 + v4 - vb - v3 = 0,
path b -va + v3 + v5 = 0,
path c vb - v4 - vc -v6 -v5 = 0,
path d -va - v1 + v2 -vc + v7 - vd = 0.
Example 4a) Use Kirchhoff's laws and Ohm's law to find io in the circuit shown in the
figure.b) Test the solution for io by verifying that the total power generated equals
the total power dissipated.
----------------------------a) We begin by redrawing the circuit and assigning an unknown current to
the 50Ω resistor and unknown voltages across the 10 Ω and 50 Ω resistors.
Example 4by applying KCL to either node b or c:
i1- io - 6 = 0
by applying KVL to the closed path cabc:-120 + 10io + 50i1 = 0
by solving the two equations:io = -3A and i1 = 3A
b) the power dissipated in the 50Ω and the 10Ω resistors are:p50Ω = (3)2 (50) = 450 Wp10Ω = (-3)2 (10) = 90 W
the power delivered to the 120V and the 6A sources are:p120V = -120 io = -120 (-3) = 360Wp6A = - v1 (6) = (-50 i1) (6) = (-150) (6) = -900 W
Example 5a) Use Kirchhoff's laws and Ohm's law to find the voltage vo in the figure.b) Show that your solution is consistent with the constraint that the total power
developed in the circuit equals the total power dissipated.
---------------------------------------Once io is known, we can compute vo and we need two equations for the two currents. Because there are two closed paths and both have voltage sources, we can apply Kirchhoff's voltage law to each to give the following equations:
10 = 6 is and 3 is = 2 io + 3 io
Solving for the current yields:is = 1.67 A and io = 1A
Example 5Applying Ohm's law to the 3 Ω resistor gives the desired voltage:
v0 = 3 io = 3 V
b) The power delivered to the independent voltage source is:p = (10)(-1.67) = -16.7 W.
and the power delivered to the dependent voltage source is:p= (3 is)(- io) = (5)(-1) = - 5 W.
The power delivered to the 6 Ω resistor is: p = (1.67)2(6) = 16.7 W
The power delivered to the 2 Ω resistor isp = (1)2(2) = 2 W
The power delivered to the 3 Ω resistor isp = (1)2(3) = 3 W
Summary• A resistor constrains its voltage and current to be
proportional to each other. The value of the proportional constant relating voltage and current in a resistor is called its resistance and is measured in ohms.
• Ohm's law establishes the proportionality of voltage and current in a resistor. Specifically,
v = iR• If the current flow in the resistor is in the direction of the
voltage drop across it, orv = -iR
• If the current flow in the resistor is in the direction of the voltage rise across it.
Summary• By combining the equation for power, p = vi, with Ohm's law,
we can determine the power absorbed by a resistor:
• Circuits are described by nodes and closed paths. • A node is a point where two or more circuit elements join.• When just two elements connect to form a node, they are said
to be in series. • A closed path is a loop traced through connecting elements,
starting and ending at the same node and encountering intermediate nodes only once each.
RVRip
22
Summary• The voltages and currents of interconnected circuit elements obey
Kirchhoff's laws:– Kirchhoff's current law states that the algebraic sum of all
the currents at any node in a circuit equals zero.– Kirchhoff's voltage law states that the algebraic sum of all
the voltages around any closed path in a circuit equals zero.
• A circuit is solved when the voltage across and the current in every element have been determined.
• By combining an understanding of independent and dependent sources, Ohm's law, and Kirchhoff's laws, we can solve many simple circuits.