bASIC eLECTRIC THEORY

1. BASIC ELECTRICAL THEORY
By: Engr. Salman Hameed

2. Charge

3. The Starting Point: Elements,
Atoms and Charge

5. Charge
• Charge
– Force that causes two particles to be attracted
to, or repelled from, each other.
– Two types – positive and negative
– Atom – proton (positive), electron (negative),
neutron (electrically neutral)

6. • Attraction and Repulsion - Like charges repel
each other and opposite charges attract each other
• Ions
– Outside force can cause an electron to leave its
orbit -atom is referred to as a positive ion
– Outside force can cause an atom to gain an
electron -atom is referred to as a negative ion
• Free Electrons
– An electron that is not bound to any particular
atom
– Can neutralize a positive ion

8. Current

9. Current
• Current – the directed flow of charge
through a conductor
– Thermal energy (heat) is sufficient to free
electrons in copper
– Free electron motion is random unless outside
force is applied

10. Current
• Represented by the letter i (for intensity)
• Measured in charge per unit time

11. Current
• Coulomb (C) – represents the total charge of
approximately 6.25 x 1018 electrons
• Unit of Current – Ampere (A) = 1
coulomb/second
• Example: 3 coulombs of charge pass a point in a
wire every two seconds. Calculate current.
A
1.5
C/s
1.5
s
2
C
3




t
Q
I

12. 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).

13. Electrical Current

14. Electrical Current
• Electron Flow Versus Conventional Current
Insert Figure 1.10

15. 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.

16. .

18. Figure 4.17,
4.18
Different Wave Forms

19. Current
Insert Figure 1.11

21. Voltage

22. Voltage
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).
(OR) Pressure required for the flow of electrons.

23. Voltage
• Voltage – a “difference of potential” that
generates the directed flow of charge
(current) through a circuit
Insert Figure 1.12

25. Voltage
• Often referred to as electromotive force
(emf)
• Unit of Voltage – volt (V) = 1
joule/coulomb
• Volt – the difference of potential that uses
one joule of energy to move one coulomb of
charge.

27. Various representations of an electrical system

28. Ohm’s Law and Power

29. Ohm’s Law
• German Physicist – George Simon Ohm
– Found that current is inversely proportional to
resistance for a given voltage
– Known as Ohm’s law
• The Relationship Between Current and Voltage
• The Relationship Between Current and Resistance

30. Basic Circuit Calculations
• Using Ohm’s Law to Calculate Current
where
R = the circuit resistance
V = the applied voltage
R
V
I 

31. Resistance Element

32. Power
• Power – the amount of energy used per unit
time
• Unit of Power – Watt (W) = 1 joule/second
• Calculating Power
where
P = the power used, in watts (W)
V = the applied voltage, in volts (V)
I = the generated current, in amperes (A)
P = IV

33. Power
• Other Power Equations
– Use IR in place of V
– Use V/R in place of I
R
I
I
IR
VI
P 2




R
V
R
V
V
VI
P
2





34. Power
• Power and Heat
– Resistors and other components convert energy
to heat (transducer)
– If power rating is exceeded, the component will
keep getting hotter and be destroyed
– Common guideline – select a component with
twice the required power-dissipation capability

35. Power
• Efficiency – the ratio of a circuit or
components output power to its input power
where
 = the efficiency, as a percentage
Po = the output power
Pi = the input power
100


i
o
P
P


36. Circuit Fundamental

37. • Circuit Loads
– Source – supplies the power
– Load – absorbs (uses) the power
– Full Load – one that draws the maximum
current

38. A Circuit
• Current flows from the higher voltage terminal of the source
into the higher voltage terminal of the transducer before
returning to the source
+
Source
Voltage
-
I
+ Transducer -
Voltage
The source expends
energy & the transducer
converts it into
something useful
I

40. Three-Phase Systems
• So far, our discussion of AC systems has been
restricted to single-phase arrangement
– as in conventional domestic supplies
• In high-power industrial applications we often
use three-phase arrangements
– these have three supplies, differing in phase by 120

– phases are labeled red, yellow and blue (R, Y & B)

41. • Relationship between the phases in a three-
phase arrangement

42. • Three-phase arrangements may use either 3 or 4
conductors