Chapter 11- Electricity
Course Content
• Definition of Electricity
• Circuit Diagrams
– Series and Parallel Circuits
• Calculating total resistances
• Measurement of Electricity
– Ammeters and Voltmeters
– Ohm’s Law
• Safety Devices and Hazards
• Wiring the Plug
What is Electricity
• An atom is a fundamental unit of matter
• made up of
•
•
•
protons (with a positive charge)
neutrons (neutral – no charge)
electrons (with a negative charge)
What is electricity?
• Everything is made of atoms which contain POSITIVE
particles called PROTONS and NEGATIVE particles called
ELECTRONS.
Electron (-)
Proton (+)
Neutron
What is Electricity?
• Electricity – the flow of electrons through a
circuit i.e. the flow of electrical charges
• The path along which the electric current
moves is called the ELECTRIC CIRCUIT
• The device which causes the flow of electrons
through a circuit is called a CELL or POWER
SOURCE (Battery)
What is Electricity?
• Electrons move round a circuit from negative
terminal to the positive terminal of the cell,
producing an electric current. (note: like charges
repel, opposites attract)
• The rate of flow of electric charges is called
ELECTRIC CURRENT
electric cell
connecting wire
filament
flow of electrons
Electric Current
• Conventional Current flow – electric current
flows from positive terminal to negative
terminal
• Alternating Current (AC) – electric current
flows in a clockwise direction and then
changes direction to flow anti-clockwise
• Direct Current – electric current flows only in
one direction
Electric Current
• Electric current is given the symbol, I
• Can also be defined as - the amount of charge
flowing per second
• Electric Current is measured in Amps (A)
• Thus, Current in Amps (A) =
Q
I=
t
Current in Amps (A)
Charge transferred
in coulombs (C)
time in seconds (s)
Electric Current
•This can be rearranged as
Q = It
• or
Q
t=
I
Electric Current
• The SI Unit for Electric Current is Ampere (A)
• Electric Current is measured using an
Ammeter
Ammeter
• It must be connected in series in the circuit.
• Positive side of ammeter must be connected
nearest to the positive terminal of the battery
(electric cell), and vice versa.
Electric Circuit
Electric circuit :- is a continuous and closed path
of an electric current.
A schematic diagram of an electric circuit comprising
of a cell, electric bulb, ammeter and plug key.
cell
-
bulb
+
+
A
plug key
ammeter
Electric Circuits
• Electric circuits are
made up of electrical
components
• These components
must be joined together
without any gap in
between to form a
closed circuit.
connecting wires
electric cell
light bulb
Note:
circuit board
Components refer to the
light bulb, wires, battery
Electric Circuits
• Incomplete circuits are called open circuits.
connecting wire is
missing
no source of
electrical energy
Both the circuits in the diagram are incomplete, hence they are known as
“open circuits”.
An electric current flows only when there is:
• a source of electrical energy and
• a closed circuit
connecting wires
electric cell
light bulb
circuit board
How to draw Circuit Diagrams
Component
Symbol
Component
+
An electric cell
Symbol
+
Battery
Switch (open)
Light bulb (lamp)
Switch
Connecting wires
(not joined)
Connecting wires
(joined)
Switch (closed)
Symbols are used to represent the various electrical components
in circuits.
Summary: Electrical Charge
• Electric charge is given the symbol
Q
• Electrons are the charge carriers that
flow in an electrical circuit from the
negative to positive terminals.
Summary: Electrical Charge
• Charge is measured in
Coulombs
• which is given the symbol
C
Summary: Electrical Charge
• The charge on a proton is
1.6 x
-19
10 C
which is the same size as the charge on an
electron.
Summary: What is electricity?
So electricity is…
 movement of electric charge round a
circuit.
 We call this electric current.
Summary: Charge, Current & Time
• Electric current is given the symbol
I
• Electric current is the movement of
negative charges (electrons) in a circuit
Summary: Charge, Current & Time
• Electric Current is the amount of charge
flowing per second and is given the unit
Amps (A)
Summary: Charge, Current & Time
•If
is of
charge
per second
then in 1 s.
so current
a current
1 A isflowing
1 C of charge
transferred
Q
I=
t
Charge transferred
in coulombs (C)
Current in Amps (A)
time in seconds (s)
Summary: Circuit Diagrams
Component
Symbol
Component
+
An electric cell
Symbol
+
Battery
Switch (open)
Light bulb (lamp)
Switch
Connecting wires
(not joined)
Connecting wires
(joined)
Switch (closed)
Symbols are used to represent the various electrical components
in circuits.
Summary: Circuit Diagrams
Different types of circuit
• There are different ways in which you can
connect cells and electrical components (such as
lamps) to create a circuit:



series
parallel
a mixture of both
Series Circuit
• A series circuit connects the components one after
the other
• A single loop is formed
• A break in any part of a series circuit stops the flow
of current in the whole circuit.
Parallel Circuit
• A parallel circuit divides into two or more branches.
• The current divides and flows through each parallel
branch.
• If a component breaks or is removed, the other
components remain on.
Voltage
• An electric cell gives energy to the electrons and pushes
them round a circuit. Voltage is a measure of how much
energy the electrons receive.
• Different voltages are supplied by different cells and
batteries.
1.5 V Dry Cell
12 V Car Battery
9 V Dry Cell
How to Measure Voltage?
• The SI unit for voltage is volt (V).
• A voltmeter is an instrument used for measuring voltages.
Voltmeter
• Voltmeters must be connected in parallel to the circuit.
• The positive side of voltmeter is connected to the positive
terminal of the cell, and vice versa.
Recall: Ammeter!!!
• It must be connected in series in the circuit.
• Positive side of ammeter must be connected nearest to the
positive terminal of the battery (electric cell), and vice versa.
measuring current
Electric current is measured in amps (A) using
an ammeter connected in series in the circuit.
A
measuring current
This is how we draw an ammeter in a circuit.
A
A
SERIES CIRCUIT
PARALLEL CIRCUIT
measuring voltage
The ‘electrical push’ which the cell gives to the current
is called the voltage. It is measured in volts (V) on a
voltmeter
V
measuring voltage
This is how we draw a voltmeter in a circuit.
V
SERIES CIRCUIT
V
PARALLEL CIRCUIT
measuring current
SERIES CIRCUIT
• current is the same
at all points in the
circuit.
2A
2A
2A
PARALLEL CIRCUIT
• current is shared
between the
components
2A
2A
1A
1A
fill in the missing ammeter readings.
3A
?
4A
?
4A
?
4A
3A
1A
?
1A
1A
?
measuring voltage
V
V
V
V
series circuit
• voltage is shared between the components
3V
1.5V
1.5V
parallel circuit
• voltage is the same in all parts of the circuit.
3V
3V
3V
measuring current & voltage
complete the missing current and voltage readings.
a)
6V
4A
V
A
V
A
measuring current & voltage
b)
4A
6V
A
V
A
V
A
answers
a)
b)
4A
6V
4A
4A
3V
6V
6V
2A
3V
4A
4A
6V
2A
The circuit is no longer complete, therefore current can not flow
The voltage decreases because the resistance increases.
The current flow in each bulb is the same.
Resistance
• When an electric current flows through a
circuit, there will be some resistance that
opposes it. (similar to friction)
• It can be measured by dividing voltage by the
current.
R=
V
I
R = Resistance
V = Voltage
I = Current
Ohm’s Law
Ohm’s law states that :
‘The current flowing through a conductor is directly proportional
to the potential difference between its ends provided its
temperature remains constant.’
I=V/R
I
= Current (Amperes) (amps)
V
= Voltage (Volts)
R
= Resistance (ohms)
Resistance
• Good conductors of electricity have LOW
RESISTANCE. (Eg. Metal objects)
– Electricity is able to flow through them very
easily
• Poor conductors of electricity have HIGH
RESISTANCE. (Eg. Wood, cloth)
– Electricity is not able to flow through them
easily
• The SI unit for resistance is ohm (Ω)
Example 1
• An electric rice cooker operates at 240 V and
uses a current of 8 A. What is the resistance of
the rice cooker?
Voltage (V) = 240 V
Current (I) = 8 A
R=
240
8
= 30 Ω
R=
V
I
Resistors in Series
• When resistors are connected
in series, the resistance will
add up and increase.
• R = R1 + R2 + R3…
2Ω
3Ω
The total resistance for this circuit is:
2Ω+3Ω=5Ω
Resistors in Parallel
• When resistors are connected
in parallel, the resistance will
decrease
• This is because they will
provide alternate routes for
the current to flow.
Resistors in Parallel
1
1
1
1
+
+
=
R2
R3
R1
R
What is the resistance of the
circuit?
6Ω
6Ω
1
1
1
+
=
6
6
R
2
1
=
6
R
1
=
3
R = 3Ω
The final resistance is
smaller
Summary:
Series and Parallel Circuits
Voltage, Current and Resistance
Vs
-
+
I3
I1
V1
V2
V3
R1
R2
R3
I2
What type of circuit is this?
Vs
-
+
I3
I1
V1
V2
V3
R1
R2
R3
I2
One electrical path from negative to
positive therefore series.
-
Vs
+
I3
I1
V1
V2
V3
R1
R2
R3
I2
What is the relationship between the three currents?
The current is the same at each point.
I1 = I 2 = I 3
-
Vs
+
I3
I1
V1
V2
V3
R1
R2
R3
I2
What is the relationship between the four voltages?
They add to equal the supply voltage.
Vs = V1 + V2 + V3
-
Vs
+
I3
I1
V1
V2
V3
R1
R2
R3
I2
How do you find total resistance in series?
Add each resistance together.
Rtotal = R1 + R2 + R3
Disadvantages of Series Circuits?
• When one component fails the whole circuit
fails.
• The current is the same at all points and the
voltage is divided between the bulbs. The
more bulbs added the dimmer each one is.
Vs
-
IT
+
V1
R1
IT
I1
V2
R2
I2
V3
R3
I3
What type of circuit is this?
Vs
-
IT
+
V1
R1
IT
I1
V2
R2
I2
V3
R3
I3
More than one electrical path – components connected on
different branches therefore parallel.
Vs
-
IT
+
V1
R1
V2
R
IT
I1
I2
2
V3
R3
What is the
relationship
between the
four
currents?
The four currents
add to give the
total current.
I3
I T = I1 + I 2 + I 3
Vs
-
IT
+
V1
R1
V2
R
IT
I1
I2
2
V3
R3
What is the
relationship
between the
four
voltages?
Each voltage is
equal to the
supply voltage.
I3
VS = V1 = V2 = V3
Vs
-
+
V1
IT
R1
V2
R
IT
The
resistance
in parallel?
I1
I2
2
V3
R3
1
R
T
=
I3
1
R
1
+
1
R
2
+
1
R
3
• If more resistors are connected
in parallel the total resistance
will always decrease
• This is because there are more
branches through which the
electricity can flow.
Advantages of the Parallel Circuit?
• When one bulb fails the rest of the circuit
continues to work.
• The more components, the lower the
resistance. The total current drawn increases.
• Voltage in each branch is the same as the
supply voltage therefore bulbs in parallel will
each be as bright as a single bulb.
NOTE!!
Hazards of electricity +
Safety Devices + Wiring of an
Electric Plug
=
Self Study