# Electrical Current

```Chapter 1: Introduction
1
Outlines
1. Recognize interrelationships of electrical
engineering with other fields of science and
engineering.
2. List the major subfields of electrical
engineering.
3. List several important reasons for studying
electrical engineering.
2
4. Define current, voltage, and power,
including their units.
5. Calculate power and energy, as well as
determine whether energy is supplied or
absorbed by a circuit element.
6. State and apply basic circuit laws.
7. Solve for currents, voltages, and powers in
simple circuits.
3
Electrical systems have two
main objectives:
z To gather, store, process, transport, and
present information
z To distribute and convert energy between
various forms
4
Electrical Engineering Subdivisions
z Communication
systems
z Computer systems
z Control systems
z Electromagnetics
z Electronics
z Photonics
z Power systems
z Signal processing
z (Circuit design)
z (Biomedical engineering)
5
Why Study Electrical Engineering?
zBecause it is everywhere,…
6
7
Electrical Current
Electrical current is the time rate of flow
of electrical charge through a conductor
or circuit element. The units are
amperes (A), which are equivalent to
coulombs per second (C/s).
8
Electrical Current
dq (t )
i (t ) =
dt
t
q (t ) = ∫ i (t )dt + q (t0 )
t0
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10
11
Direct Current
Alternating Current
When a current is constant with time,
we say that we have direct current,
abbreviated as dc. On the other hand,
a current that varies with time, reversing
direction periodically, is called
alternating current, abbreviated as ac.
12
.
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14
15
Voltages
The voltage associated with a circuit element
is the energy transferred per unit of charge
that flows through the element. The units of
voltage are volts (V), which are equivalent to
joules per coulomb (J/C).
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19
POWER AND ENERGY
p(t ) = v (t )i (t )
t2
w = ∫ p(t )dt
t1
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21
22
KIRCHHOFF’S CURRENT LAW
zThe net current entering a node is zero.
zAlternatively, the sum of the currents
entering a node equals the sum of the
currents leaving a node.
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24
25
zpp. 19
zFig. 1.18
26
Series Circuits
27
KIRCHHOFF’S VOLTAGE LAW
The algebraic sum of the voltages
equals zero for any closed path (loop) in
an electrical circuit.
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29
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32
Find voltages
Find series/parallel connection
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34
35
36
37
38
Resistors and Ohm’s Law
a
b
v = iR
vab = iab R
39
Conductance
1
G=
R
i = Gv
40
Resistance Related to Physical
Parameters
R=
ρL
A
41
Voltage, current and power
42
43
44
Using KVL, KCL, and Ohm’s Law
to Solve a Circuit
45
15 V
iy =
=3A
5Ω
ix + 0.5ix = i y
ix = 2 A
46
v x = 10ix = 20 V
Vs = v x + 15
Vs = 35 V
47
Problem Set
z6, 14, 19, 23, 28, 33, 48, 57
48
```