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Charge, Current, Voltage, Power and Energy
SKEE 1023
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
meter
m
Mass
kilogram
Kg
Time
second
s
Electric current
ampere
A
Thermodynamic
temperature
kelvin
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 1018 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,
dq
i
dt
, measured in amperes (A)
Charge, Current and Voltage
dq
i
dt

q

t
to
i dt
- Charge transferred between to 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
1
2
t (s)
i (A)
t (s)
i (A)
1
t (s)
-1
t (s)
Charge, Current and Voltage
Two common types of current flow
i (A)
Direct current - DC
- constant with time
t (s)
i (A)
Alternating current - AC
t (s)
- 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
1 V = 1 J/C
dw
dq
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.
a
+
Vab 
b
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 ?
Charge, Current and Voltage
a
a
+
10 V

b


- 10 V
+
b
“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”
Charge, Current and Voltage
Two common types of voltage
v (V)
DC Voltage
- constant with time
t (s)
v (V)
AC Voltage
t (s)
- 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 dw dq

  vi
dt dq dt
p  vi
Power an Energy
p  vi
• 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
2A
Using passive sign convention,
+
Power absorbed , p = 2 x 3 = 6 W
3V

Power an Energy
• Using passive sign convention, power can either be positive or negative
ABSORBED
SUPPLIED
Examples
-4A
+
Using passive sign convention,
Power absorbed , p = -4 x 3 = -12 W
3V
OR,

Power supplied , p = 12 W
Power an Energy
• Using passive sign convention, power can either be positive or negative
ABSORBED
SUPPLIED
Examples
-4A

Using passive sign convention,
Power supplied , p = 6 x -4 = -24 W
6V
OR,
+
Power absorbed , p = 24 W
Power an Energy
• Using passive sign convention, power can either be positive or negative
ABSORBED
SUPPLIED
Examples
-4A

Using passive sign convention,
Power absorbed , p = 6 x -4 = -24 W
6V
OR,
+
Power supplied , p = 24 W
Power an Energy
• Using passive sign convention, power can either be positive or negative
ABSORBED
SUPPLIED
Examples
-4A

-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
dw , energy absorbed or supplied by an element from
p
dt
to t to to is :
w
t
t
to
to
 p dt  
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.