Transcript Lecture2 - Texas A&M University

Current, Voltage and
Resistance
ENTC 210: Circuit Analysis I
Rohit Singhal
Lecturer
Texas A&M University
Administrative Tasks
Fix Exam Schedule
 Lab details

 TA –
Mr. Pankaj Bhagawat
 Sections Merge
Atoms and their structure
electron
neutron
proton
Atomic Structure
Mass of an Electron = 9.11 x 10-28 gm.
 Mass of a Proton = 1.672 x 10-24 gm.
 Proton is ~1836 times heavier than the
electron

Atomic Structure
Unit of Charge = Coulombs
 Charge on electron = charge on a proton =
1.6 x 10-19 C
 1 Coulomb = Charge on 6.242 x 1018
electrons

Coulomb’s Law

Like charges repel, opposites attract
= k Q1 Q2 / r2
 k = 9 x 109 (units?)
F
Coulomb’s Law

Like charges repel, opposites attract
= k Q1 Q2 / r2
 K = 9 x 109 N m2/C2
F
Conduction
In metals, the electrons are “more free”
than the insulators.
 Whenever there is a charge present at one
end, the electrons flow to (or away) from
that charge.

Current
Rate of flow of charge
 1 Amp = 1 Coulomb / 1 Second.

Question





If a laptop needs constantly needs 2 Amps
current from a battery, how many electrons are
drained from the battery in one hour?
1 Amp = 6.242 x 1018 electrons/second
2 Amp = 12.484 x 1018 electrons/second
In one hour - > 3600 x 12.484 x 1018 electrons
Answer is 4.49 x 1022 electrons
Question
What’s the weight of all those electrons?
 4.49 x 1022 x 9.11 x 10-28 gm
 4.09 x 10-5 gm

Equations
I = Q/t
Q=Ixt
 t = Q/I

Potential
Every particle of mass m raised to a height
h above the earth’s surface has a potential
energy m.g.h
 This potential energy can be raised by
raising the particle a little higher
 When the particle is set free, it travels to
the point of least potential.

Electric Potential
Similarly, a charge wants to travel to a
lower “electric” potential.
 A negative charge on the other hand,
wants to travel to a higher potential.
 Each point in a circuit has a potential.

Voltage
Voltage is always measured between two
points.
 It is defined as the difference of potential
between the two points.
 Measured in volts

Volts

1 volt is defined as the potential difference,
which results in an energy exchange of 1
Joule due to the movement of 1 Coulomb
across it.
DC Voltage Supply
Conductivity

Copper is the most
popular conductor.
Metal
Conductivity (%)
Silver
105
Copper
100
Gold
70.5
Aluminum
61
Tungsten
31.2
Nickel
22.1
Iron
14
Constantan 3.52
Nichrome
1.73
Calorite
1.44
Resistance

Resistance is proportional to length
length
direction of current flow
Resistance

Resistance is inversely proportional to the
cross sectional area
direction of current flow
Resistance


R = ρ L/A
ρ is the resistivity of
the material (units?)
Material
ρ (10-8 Ohm-Metres)
Silver
Copper
1.645
1.723
Gold
Aluminum
Tungsten
2.443
2.825
5.485
Nickel
Iron
Tantalum
Nichrome
Tin Oxide
Carbon
7.811
12.299
15.54
99.72
250
3500
American Wire Gage (AWG) sizes
AWG #
Diameter (in)
Ω /1000ft.
0000
0.46
0.0490
000
0.409
0.0618
0
0.325
0.0983
1
0.289
0.1240
2
0.257
0.1563
4
0.204
0.2485
10
0.102
0.9989
14
0.0640
2.525
28
0.0126
64.90
Color Coding
5 Bands of code (3 are mandatory)
 Bands 1 - 3  the value of the resistor
 Band 4  the range (tolerance)
 Band 5  the reliability

Color Code (Band 1-3)
Color
Value
Black
0
Brown
1
Red
2
Orange
3
Yellow
4
Green
5
Blue
6
Violet
7
Gray
8
White
9
Example
2
6
x
103
= 26 K Ohms
Band 3 (special cases)

Gold = 0.1
 Red

Blue Gold = 2.6 Ohm
Silver = 0.01
 Red
Blue Silver = 0.26 Ohm
More Bands
Band 4
Tolerance
Gold
5%
Silver
10%
None
20%
Band 5
Reliability
(after 1000 Hrs of use)
Brown
1%
Red
0.1%
Orange
0.01%
Yellow
0.001%
Example
= 26 K Ohms ± 5%, 1 in 100,000 fails after 1000 hrs of use