SI and Prefix

1
SI and Prefix
Engineers communicate using standard language of
measurement International System of Units (SI)
adopted in 1960
Basic Units
2
SI and Prefix
Engineers communicate using standard language of
measurement International System of Units (SI)
adopted in 1960
Derived Units
3
Charge, Current and Voltage
Charge the most basic quantity of electric
circuit measured in Coulomb (C)
Elements of an atom electrons, protons and
neutron
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 1018 electrons
4
Charge, Current and Voltage
When electrons move in an electric circuit, they
cause the Current to flow
Direction of electron flow
Direction of current flow movement of positive
charge
Current time rate of change of (positive)
charge
Mathematically,
, measured in amperes (A)
5
Charge, Current and Voltage
e.g. 1 A 1 coulomb of charge flows in 1
second
6
Charge, Current and Voltage
7
Charge, Current and Voltage
Two common types of current flow
Direct current - DC
- constant with time
Alternating current - AC
- varies sinusoidally with time
we will discuss more on this later in the course
8
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) 
1 V 1 J/C
9
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. 
Vab Electrical energy gained by a unit positive
charge when it moves from b to a
Point a is at potential of Vab higher than point
b
Potential at point a with respect to point b is
Vab
As a unit charge moves from a to b it looses
electrical energy. Where does the energy go ?
10
Charge, Current and Voltage
??
Point a is at potential of 10 V higher than
point b
Point a is at potential of -10 V lower than
point b
Point b is at potential of 10 V lower than point
a
Point b is at potential of -10 V higher than
point a
11
Charge, Current and Voltage
Two common types of voltage
DC Voltage
- constant with time
AC Voltage
- varies sinusoidally with time
12
Power an Energy
Power and energy is related mathematically
p power, w energy
the time rate of expanding or absorbing
energy
power is measured in watts (W)
13
Power an Energy

• Power of an element is the product of voltage
  across it and the current through it

• Use the Passive Sign Convention when calculating
  power

p vi
p -vi
Absorbing power
Supplying power
14
Power an Energy

• Using passive sign convention, power can either
  be positive or negative

Examples
Using passive sign convention,
Power absorbed , p 2 x 3 6 W
15
Power an Energy

• Using passive sign convention, power can either
  be positive or negative

Examples
Using passive sign convention,
Power absorbed , p -4 x 3 -12 W
OR, Power supplied , p 12 W
16
Power an Energy

• Using passive sign convention, power can either
  be positive or negative

Examples
Using passive sign convention,
Power supplied , p 6 x -4 -24 W
OR, Power absorbed , p 24 W
17
Power an Energy

• Using passive sign convention, power can either
  be positive or negative

Examples
Using passive sign convention,
Power absorbed , p 6 x -4 -24 W
OR, Power supplied , p 24 W
18
Power an Energy

• Using passive sign convention, power can either
  be positive or negative

Examples
Using passive sign convention,
Power absorbed , p -6 x -4 24 W
19
Power an Energy
For any electric circuit ,
Sums of power absorbed and supplied in a circuit
always equal to ZERO
20
Power an Energy
We paid bill to TNB based on the amount of
electric energy we consumed energy is measured
in Joules (J)
Unit for energy used by TNB is Wh or kWh (1 Wh
???? J)
21
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.