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Basic Concepts - Chapter 1

This document provides an overview of basic electrical concepts covered in Chapter 1, including: 1) Systems of units for electrical quantities like charge, current, voltage, and power; 2) Definitions of electric charge, current, and voltage; and 3) Descriptions of circuit elements like resistors, sources, and dependent sources. Key points covered are the units and symbols used to represent different electrical quantities, definitions of direct and alternating current, and classifications of active and passive circuit elements.

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Rezha
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83 views14 pages

Basic Concepts - Chapter 1

This document provides an overview of basic electrical concepts covered in Chapter 1, including: 1) Systems of units for electrical quantities like charge, current, voltage, and power; 2) Definitions of electric charge, current, and voltage; and 3) Descriptions of circuit elements like resistors, sources, and dependent sources. Key points covered are the units and symbols used to represent different electrical quantities, definitions of direct and alternating current, and classifications of active and passive circuit elements.

Uploaded by

Rezha
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Basic Concepts - Chapter 1

1.1 Systems of Units.


1.2 Electric Charge.
1.3 Current.
1.4 Voltage.
1.5 Power and Energy.
1.6 Circuit Elements.

1
1.1 System of Units (1)

Six basic units


Quantity Basic unit Symbol
Length meter m
Mass kilogram Kg
Time second s
Electric current ampere A
Thermodynamic kelvin K
temperature
Luminous intensity candela cd
2
1.1 System of Units (2)

The derived units commonly used in electric circuit theory

Decimal multiples and


submultiples of SI units
3
1.2 Electric Charges

• Charge is an electrical property of the atomic particles of


which matter consists, measured in coulombs (C).

• The charge e on one electron is negative and equal in


magnitude to 1.602 × 10-19 C which is called as electronic
charge. The charges that occur in nature are integral
multiples of the electronic charge.

4
1.3 Current (1)

• Electric current i = dq/dt. The unit of ampere can be


derived as 1 A = 1C/s.
• A direct current (dc) is a current that remains
constant with time.
• An alternating current (ac) is a current that varies
sinusoidally with time. (reverse direction)

5
1.3 Current (2)

• The direction of current flow

Positive ions Negative ions

6
1.3 Current (3)

Example 1

A conductor has a constant current of 5 A.

How many electrons pass a fixed point on the


conductor in one minute?

7
1.3 Current (4)
Solution

Total no. of charges pass in 1 min is given by


5 A = (5 C/s)(60 s/min) = 300 C/min

Total no. of electronics pass in 1 min is given

300 C/min
−19
= 1 . 87 x10 21
electrons/min
1.602 x10 C/electron

8
1.4 Voltage (1)
• Voltage (or potential difference) is the energy required to move
a unit charge through an element, measured in volts (V).

• Mathematically, (volt)

–
v ab = dw / dq
w is energy in joules (J) and q is charge in coulomb (C).

• Electric voltage, vab, is always across the circuit element or


between two points in a circuit.
– vab > 0 means the potential of a is higher than potential of b.
– vab < 0 means the potential of a is lower than potential of b.

9
1.5 Power and Energy (1)

• Power is the time rate of expending or absorbing energy,


measured in watts (W).
dw dw dq
• Mathematical expression: p= = ⋅ = vi
dt dq dt
i i

+ +

v v

– –
Passive sign convention
P = +vi p = –vi
absorbing power supplying power
10
1.5 Power and Energy (2)

• The law of conservation of energy

∑p=0
• Energy is the capacity to do work, measured
in joules (J).
t t
• Mathematical expression w = ∫ pdt = ∫ vidt
t0 t0

11
1.6 Circuit Elements (1)
Active Elements Passive Elements

• A dependent source is an active


element in which the source quantity
is controlled by another voltage or
current.

• They have four different types: VCVS,


CCVS, VCCS, CCCS. Keep in minds the
Independent Dependant signs of dependent sources.
sources sources 12
1.6 Circuit Elements (2)

Example 2

Obtain the voltage v in the branch shown in Figure 2.1.1P for i2 = 1A.

Figure 2.1.1P

13
1.6 Circuit Elements (3)

Solution

Voltage v is the sum of the current-independent


10-V source and the current-dependent voltage
source vx.

Note that the factor 15 multiplying the control


current carries the units Ω.

Therefore, v = 10 + vx = 10 + 15(1) = 25 V

14

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