Chapter 1
Basic Concepts
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
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.
2
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
3
1.1 System of Units (2)
The derived units commonly used in electric circuit theory
Decimal multiples and
submultiples of SI units 4
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.
5
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)
6
1.3 Current (2)
• The direction of current flow
Positive ions Negative ions
7
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?
8
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
9
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)
vab 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. 10
