SI and Prefix
Engineers communicate using standard language of measurement:
International System of Units (SI) adopted in 1960
Basic Units
Quantity Basic unit Symbol
Length
Mass
Time
Electric current
Thermodynamic temperature meter kilogram second ampere kelvin m
Kg s
A
K
Luminous intensity candela cd

SI and Prefix
Engineers communicate using standard language of measurement:
International System of Units (SI) adopted in 1960
These are derived from basic units
Derived Units

Charge, Current and Voltage
Charge – the most basic quantity of electric circuit – measured in Coulomb (C)
Elements of an atom : electrons, protons and neutron negative charge positive charge neutral
1 electron carries 1.602 x 10 -19 C of (negative) charge i.e. - 1 C consist of 1/(1.602 x 10 -19 ) of electrons
= 6.24 x 10 18 electrons

Charge, Current and Voltage
When electrons move in an electric circuit, they cause the Current to flow i
Direction of electron flow
Direction of current flow = movement of positive charge
Current : time rate of change of (positive) charge
Mathematically, i  dq dt
, measured in amperes (A)

Charge, Current and Voltage i  dq dt
 q  t
 t o i dt
- Charge transferred between t o and t e.g. 1 A = 1 coulomb of charge flows in 1 second i (A)
1 t (s) q (C)
1
1 t (s)

Charge, Current and Voltage q (C) q (C)
1 i (A)
1
-1
1 2 t (s) i (A) t (s) t (s) t (s)

Charge, Current and Voltage
Two common types of current flow i (A)
Direct current - DC
- constant with time t (s) i (A) t (s)
Alternating current - AC
- varies sinusoidally with time we will discuss more on this later in the course

Charge, Current and Voltage
Voltage (potential difference) between two points, being equal to the electrical energy gained by a unit positive electric charge moving from one point to the other.
Voltage : The amount of energy needed to move a unit positive electric charge from one point to the other - measured in volts (V)
Mathematically, v  dw dq
1 V = 1 J/C

Charge, Current and Voltage
Voltage (potential difference) between two points, being equal to the electrical energy gained by a unit positive electric charge moving from one point to the other.
V ab
: Electrical energy gained by a unit positive charge when it moves from b to a a + V ab
 b
“Point a is at potential of V ab than point b” higher
“Potential at point a with respect to point b is V ab
”
As a unit charge moves from a to b it looses electrical energy. Where does the energy go ?

a
+
10 V b

Charge, Current and Voltage
 a

- 10 V b
+
“Point a is at potential of 10 V higher than point b”
“Point b is at potential of 10 V lower than point a”
“Point a is at potential of -10 V lower than point b”
“Point b is at potential of -10 V higher than point a”

Charge, Current and Voltage
Two common types of voltage v (V)
DC Voltage
- constant with time t (s) v (V) t (s)
AC Voltage
- varies sinusoidally with time

Power an Energy
Power and energy is related mathematically : p  dw dt p = power, w= energy
:: the time rate of expanding or absorbing energy :: power is measured in watts (W) p  dw dt
 dw dq
 dq dt
 v i p  v i

Power an Energy p  v i
• Power of an element is the product of voltage across it and the current through it
• Use the Passive Sign Convention when calculating power: i i
+ + v
 v
 p= vi
Absorbing power p= -vi
Supplying power

Power an Energy
• Using passive sign convention, power can either be positive or negative
ABSORBED SUPPLIED
Examples
2 A
+
3 V

Using passive sign convention,
Power absorbed , p = 2 x 3 = 6 W

Power an Energy
• Using passive sign convention, power can either be positive or negative
ABSORBED SUPPLIED
Examples
- 4 A
+
3 V

Using passive sign convention,
Power absorbed , p = -4 x 3 = -12 W
OR,
Power supplied , p = 12 W

Power an Energy
• Using passive sign convention, power can either be positive or negative
ABSORBED SUPPLIED
Examples
- 4 A

6 V
+
Using passive sign convention,
Power supplied , p = 6 x -4 = -24 W
OR,
Power absorbed , p = 24 W

Power an Energy
• Using passive sign convention, power can either be positive or negative
ABSORBED SUPPLIED
Examples
- 4 A

6 V
+
Using passive sign convention,
Power absorbed , p = 6 x -4 = -24 W
OR,
Power supplied , p = 24 W

Power an Energy
• Using passive sign convention, power can either be positive or negative
ABSORBED SUPPLIED
Examples
- 4 A

-6 V
+
Using passive sign convention,
Power absorbed , p = -6 x -4 = 24 W

Power an Energy
For any electric circuit ,
 p  0
Sums of power absorbed and supplied in a circuit always equal to ZERO

Power an Energy
We paid bill to TNB based on the amount of electric energy we consumed – energy is measured in Joules (J)
Since p  dw dt
, energy absorbed or supplied by an element from to t to t o is : w  t t   o p dt t
 t o v i dt
Unit for energy used by TNB is Wh or kWh (1 Wh = ???? J)

Power an Energy
The amount of electric energy consumed depends on power ratings of the electric devices we use: e.g. a 100W bulb consumed 100 x 60 x 60 = 360, 000 J of energy in
1 hour OR 0.1 kWh of energy.