Ch 2: Basic Components and Electric Circuits Flashcards
Kirchhoff’s Voltage Law
The sum of the voltages across each element of a loop/mesh is zero.
Σ Vn = 0
Kirchhoff’s Current Law
The sum of all currents entering and exiting a given node is 0.
Σ in = 0
Ohm’s Law
V = IR
The voltage across an element is the product of the current through the element and the resistance of the element.
Current:
Definition
Mathematical Relation to Charge
Unit
Current is the charge moved through an element, per unit time.
i = dq/dt
Unit: Ampere(A) = 1 coulomb/second
Sum of Power in a circuit
The sum of absorbed power for each element is zero.
The sum of the absorbed power equals the sum of the supplied power.
All power that goes into a circuit must be absorbed by the circuit elements.
Σpabsorbed = Σpsupplied
Definition of Network
Interconnection of two or more simple circuit elements
Definition of
Circuit
A network with at least one closed path
Circuit Diagrams:
Element Polarity:
Passive Elements
vs
Voltage Source
For Passive Elements:
The terminal receiving positive current is labeled with a ( + )
The other terminal is negative ( - )
For a Voltage Source,
The terminal supplying positive current is +, the other is negative, -
Independent Current Source
An ideal element in which the current through the element is constant,
independent of the voltage across it.
This is accomplished with active elements beyond the scope of analyzing the circuit in question.
SI Unit of Resistance
Ohm - Ω
1 Ω = 1 V/A
Power, Voltage and Current relationship
Power is the product of current and voltage
p = iv
or
P = IV
Passive Sign Convention
Determines how power absorbed/supplied by element within the circuit is represented.
When an element ABSORBS power, the terminal that current flows into is positive. Power is treated as “positive” (+p)
When an element GENERATES power, the terminal that current flows OUT OF is positive.
Power is treated as “negative”.
(-p)
Dependent Sources
Voltage or Current Sources whose source quantity is controlled by a voltage or current somewhere else in the circuit.
These sources are represented with diamonds instead of circles.
Active Elements
vs
Passive Elements
Active Element
An element capable of furnishing an average power greater than zero to some external device, over an infinite time interval.
Usually voltage or current sources.
Passive Element
An element that cannot supply an average power greater than zero over an infinite time interval.
Conductance:
Definition
Relation to voltage
Units
Conductance (G) is the inverse of resistance. It is the amount of current that passes through an element, per unit of voltage.
The ratio of current to voltage.
G = i/v = 1/R
Unit: Siemens (S) in SI
an older unit is the “mho”, represented by the inverted omega symbol, or Ω-1
Independent Voltage Source
An ideal element that has a constant terminal voltage, independent of the current through it.
Represented by a circular element (independent) with Plus and minus signs at the appropriate terminals.
SI Unit
of Work or Energy
The Joule (J)
1 J = 1 kg*m2 / s2
SI Unit
of Power
Watt (W)
Work/Energy over time
1 W = 1 J/s
Linear Dependent Source
A dependent current or voltage source,
whose output current or voltage is proportional only to the first power of a specified current or voltage variable in the circuit.
Typically related by a value “k”
SI:
Base Units and Symbols (7)
Base Quantity
Name
Symbol
length
meter
m
mass
kilogram
kg
time
second
s
electric current
ampere
A
Temperature
Kelvin
K
Amount of Substance
mole
mol
Luminous Intensity
candela
cd
Relationship
between
Current and Charge
Current is the rate of charge moved through an element
i = dq/dt
or
q(t) = ∫i dt + q(t0)
SI Unit of
Voltage
Volt ( V )
1 V = 1 Joule/Coulomb
SI Unit of
Charge
Coulomb (C)
The total charge that passes through a cross section of wire in 1 second at 1 ampere of current
Static Representation: Q
Instantaneous Amount: q(t) or q
Voltage Division
Place two resistors in series. the voltage across each resistor can be calculated simply, proportionally to voltage across both. The voltage is “divided” between the two resistors
V1 = V * R1/(R1 + R2)
V2 = V * R2/(R1 + R2)
Current Division
Similar in concept to voltage division.
Resistors are placed in parallel, effectively dividing the total current into the current through each resistor.
i1 = i * R1/(R1 + R2)
i2 = i * R2/(R1 + R2)
What is Voltage?
Voltage is a difference in charge between two points.
Equal to the energy required to move a unit of charge between two places.
Charge of
a single Electron
-1.602 x 10-19 Coulombs
Power
Definition
Equation
Units
Power is the rate of energy provided or consumed per unit time.
Equation: p = vi
Units: Watts (W)
Three Characteristics
of an
Ideal Circuit
- Electrical Effects propagate instantly through the whole system (assuming a lumped system)
- Net charge of every component in the system is ZERO. (Charge in = Charge out)
- No magnetic Couplings
Current, Voltage, Power
Properties of an Ideal Circuit
- Current measured on various parts of a connecting line are the same
- Voltage across 2 terminals is the same, regardless of which side of the terminal the measurement is made
- Sum of element powers in a circuit equals zero.
- Generated Power = Absorbed Power
Ideal Basic Circuit Element:
Three Attributes
- Only has two terminals which are the points it connects to other elements
- It can be fully described in terms of relationship between current and voltage
- It cannot be subdivided into other elements
