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WEL COME
TO
COE RAC SECTOR
MODULE 2
BASIC ELECTRICITY
AND
BASIC ELECTRONICS
AND
ELECTRO-MECHANICS
LESSON 16
REALTIONSHIP BETWEEN V,I & R
IN A DC CIRCUIT
OBJECTIVES
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•
•
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•
Define EMF & P.D. & their units.
Define current & its units
State the method of connecting Voltmeter
State the method of connecting an Ameter
Draw & Describe a simple circuit.
E.M.F.
Eletro Motive force
It is now necessary to consider the reason for the motion of
electrons in an electrical circuit.
In order to move free electrons in one direction & so produce a
current, a source of electrical energy is required. The torch battery is a
source of electrical energy.
The battery is said to have an E.M.F. which is available to drive
the free electrons in the closed path in electrical circuit. The diff. in the
distribution of electrons between the two terminal of the battery produce
this E.M.F. Unit of E.M.F. is watt. E.M.F. of an electrical source referred
to as its Voltage.
Potential Difference
The voltage across the mains when the load
is put on is known as potential difference and
its unit is volt.
An electrical voltage (V) is measured in volts
(V).
The following expressions are used for
decimal, multiple and sub multiples of 1volt
1 kilovolt = 1 kv = 1000 v
= 1 x 103 v
1 milivolt = 1 mv = 1/1000 v
= 1 x 10-3 v
1 microvolt = 1 µv = 1/1000000v = 1 x 10-6 v
Electric Current
•
•
•
Fig. shows a simple electric
circuit. When the switch is
closed, the lamp lights because
the electrical current flows from
the source of electrical energy
(battery) electrons can move
through a conductor at different
rate and electric current will,
therefore have different values.
The current symbol (I) is
determined by the count of
electrons that pass through a
cross section of a conductor in
one second.
The current of 1 amp. Means
that in one second about 6.24 x
= 1 x 103 v
1018 electrons moves through a 1 kilovolt = 1 kv = 1000 v
1 milivolt = 1 mv = 1/1000 v
= 1 x 10-3 v
cross section of a conductor.
1 microvolt = 1 µv = 1/1000000v = 1 x 10-6 v
State the method of connecting Voltmeter
Electrical voltage is
measured with a volt
meter in order to
measure the voltage
source. The terminal
of the source must be
connected to the
terminal of a volt
meter. Positive to the
positive terminal &
negative to the
negative terminal as
shown in figure.
State the method of connecting Ameter
In order to measure
current-measuring
instruments called
current meter or
Ameter. An ameter is
used to measure
current for this the
current must flow
through the
measuring
instruments. Hence
Ameter is connected
in terms with load
SIMPLE ELECTRIC CIRCUIT
A simple electrical circuit
shown in figure is the
complete path way of the
current flow from the
battery via the switch and
the load back to the
battery
An electrical circuit
consist of :
voltage source
connecting wires
load
switch
Types of simple circuit
Open circuit
Short circuit
Closed circuit
Open circuit
• In an open circuit
there is an
infinitely high
resistance in the
circuit provided
most of the time
by the open
switch there fore
no current can
flow.
Short circuit
• The other important
extreme condition is
the short circuit will
occur,for example
when two terminal of
a torch battery are
joined
• A short circuit may
also occur if the
insulation between
the two cores of the
cable is defective
Closed circuit
When the switch is
closed, the lamp
lights because the
electrical current
flows from the
source of electrical
energy (battery)