CEE103 - Lecture No 02.pdf applied electricity

lecture

1. CEE103 –Applied Electricity &
Electronics
Course Instructor: Engr. Riaz H. Junejo
Lecture No. 02
Topic: System of Units, Charge and Current
Semester: Fall 2020
Department of Electrical & Computer Engineering
CUI Wah Campus.

2. 1.1 Introduction
▪An electric circuit is an interconnection of electrical elements.
▪Objective of circuit analysis is not to study various applications of the circuits, but
to solve and troubleshoot the circuits.
▪How does it respond to a given input?
▪How do the interconnected elements in the circuit interact?

3. 1.2 System of Units
▪We deal with measurable quantities.
▪Measurements must be in a standard way.
▪Such an international measurement standard is the International System of Units.
▪In this system (called SI system), there are six principal units (see the table) from
which the units of all other physical quantities can be derived.
The SixBasic SI Units
Quantity Basic Unit Symbol
Length meter m
Mass kilogram kg
Time second s
Electric Current ampere A
Thermodynamic
temperature
kelvin K
LuminousIntensity candela cd

4. System of Units (Continued)
▪The advantage of the SI units is that it uses prefixes based on the power of 10 to
relate larger and smaller units to the basic unit (see the table).
The SI Prefixes
MultiplierPrefix Symbol MultiplierPrefix Symbol
10
18
Exa E 10
−1
Deci d
10
15
Peta P 10
−2
Centi c
10
12
Tera T 10
−3
Milli m
10
9
Giga G 10
−6
Micro ??????.
10
6
Mega M 10
−9
Nano n
10
3
Kilo k 10
−12
Pico p
10
2
Hecto h 10
−15
Femto f
10
1
Deka da 10
−18
atto a

5. 1.3 Charge and Current
▪Charge is an electrical property of the atomic particles of which matter consists.
▪It is measured in coulombs (C).
▪A particle can be:
▪Positively charged
▪Negatively charged, or
▪Neutral.
▪Charge �on an electron is: �=−1.602×10
−19
C
▪While charge on proton is +1.602×10
−19
C
▪Neutron is electrical neutral quantity.
▪In one coulomb of charge there are Τ1�=6.24×10
18
electrons.
▪The only charges that can exist in nature are the integral multiples of �.
▪Law of conservation of charges.
▪Charges can be transferred from one place to another.

6. Charge and Current (Continued)
▪When a conducting wire is connected to a battery, the charges are compelled to
move. That motion of charges create a current.
▪Electric current is the time rate of change of charges.
▪It is conventional to take the current flow as the movement
of positive charges, that is, opposite to flow of negatives.
▪Current is measured in amperes (A)
▪Mathematically, ??????=
????????????
????????????
. . . (1) ⇒�=׬
??????
0
??????
??????��. . . (2)
▪Hence, 1 ampere = 1 coulomb/second.

7. Charge and Current . . .
▪Eq. (1) shows that the current need not be a constant.
▪Anyway, if the current does not change with time, we call it a direct current (dc).
▪A dc current is denoted by the symbol ??????.
▪An alternating current (ac) is a current that varies with time.
▪An ac current is represented by the symbol ??????��??????(�).

8. Example
▪How much charge is represented by 4600 electrons?
▪Solution: Since each electron has −1.602×10
−19
??????
▪Hence, charge 4600 electrons will have a charge of: 4600���������×
(−1.602×10
−19
??????/��������) =−7.369×10
−16
??????

9. Example:
▪The total charge entering a terminal is given by ��=10−10�
−2??????
�??????, find the
current at �=1.0�
▪Solution: Since ??????=
????????????
????????????
▪So, here it=
??????
????????????
10−10�
−2??????
�??????
▪⇒??????�=20�
−2??????
�??????
▪And current at �=1.0�????????????����:??????1.0=20�
−2
�??????
▪Or ??????1.0=2.707�??????

10. Example:
▪The current flowing through an element is given by:
??????=ቊ
4??????,0<&#3627408481;<1
4&#3627408481;
2
,&#3627408481;>1
Calculate the charge entering the element from &#3627408481;=0&#3627408481;&#3627408476;&#3627408481;=2&#3627408480;
▪Solution: Since ??????&#3627408481;=
????????????
????????????
⇒&#3627408478;&#3627408481;=׬??????&#3627408481;&#3627408465;&#3627408481;
▪So, here in this example, &#3627408478;=׬
0
2
??????(&#3627408481;)&#3627408465;&#3627408481;
▪⇒&#3627408478;=׬
0
1
4&#3627408465;&#3627408481;+׬
1
2
4&#3627408481;
2
&#3627408465;&#3627408481;
▪⇒&#3627408478;=4&#3627408481;
0
1
+4
??????
3
3
1
2
▪⇒&#3627408478;=4+
4
3
8−1=13.33??????

11. Thank You!