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Electricity
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What we will Learn
What is a current?
How to draw electric circuit diagrams
What is potential difference, voltage and E.M.F? What is resistance?
S/E: Circuits arranged in series versus parallel
S/E: Fixed resistors versus variable resistors
Effects of electric current
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Types of Sources used tomake Electricity
Thermal
Geothermal
Nuclear
Hydroelectric
SolarWind
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Life without Electricity---!!!
How would you prepare your lunch?
How would you wash clothes?
Think about all the luxuries the discovery ofElectricity has brought us! No Television
No Radio
No Lights
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Inventors and Inventions
1752 Lightning Rod 1879 Light Bulb Ben Franklin Thomas Edison
1800 Electric Battery 1888 AC Power Count Alessandro Volta Nikola Tesla
1805 Refrigerator 1910 Flashlight
Oliver Evans Conrad Hubert 1876 Telephone 1920Traffic
Light
Alexander Graham Bell Garrett Morgan
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More Inventors and Inventions
1927 Television 1973 Internet Philo T. Farnsworth Vinton Cerf
1945 Computer Mauchley and Presper
1954 Microwave 1955 TV Remote
Percy Spencer Eugene Polley
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Where does Electricity come from?
Mainly 2 sources:
1) Power Stations
- Supply a lot ofelectricity
- Used in many electricalappliances
2) Electric Cells (batteries)
- Supply a little electricity
- Portable
- Safe
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Batteries are e pumps.
a. They provide a voltage
difference to a circuit.b. Types: wet-cells & dry-cells
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How does an Electrical Appliance Work?
To make an electrical appliance work,electricity must flow through it.
The flow of electricity is called an electriccurrent.
The path along which the electric current
moves is called the electric circuit.
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What is an Electric Current???
Definition:
An electric current is the rate offlow of electric
charges in a circuit.
connecting
wire
electric
cell
filament
flow of electrons
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Electric Charges
Electric charges are made up ofpositive
charges (protons) and negative charges
(electrons).
When these charges flow in a circuit, a
current is produced.
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How does electricity flow?
The battery in a circuit gives energy to
the electrons and pushes them around
a circuit, from the negative terminal of
the cell, round the circuit and back tothe positive terminal of the cell.
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The SI unit for electric current is ampere (A).
Smaller currents are measured inmilliamperes (mA).
Different electrical components andappliances require different sizes of currentto turn them on.
How to Measure Current?
1 A = 1,000 mA
1 mA = 0.001 A
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Instrument to Measure Current
An ammeteris an instrument used for
measuring electric current.
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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.
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Electric Circuits
Electric circuits aremade up ofelectricalcomponents
These componentsmust be joinedtogether without anygap in between to
form a closed circuit.
connecting wires
electric cell
circuit boardlight bulb
Note:
Components refer to the
light bulb, wires, battery
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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.
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An electric current flows only when there is:
a source ofelectrical energy and
a closed circuitconnecting wires
electric cell
circuit boardlight bulb
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How to draw Circuit DiagramsComponent Symbol Component Symbol
An electric cell Battery
Light bulb (lamp) Switch
Switch (open)
Switch (closed)
Connecting wires(not joined)
Connecting wires(joined)
+ +
Symbols are used to represent the various electrical
components in circuits.
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Examples of circuit diagrams:
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Switches
A switch is used to open or close a circuit.
Main switch used in buildings
Switches used on circuit boards
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Circuit diagrams for open and closed circuits
Open circuit.Bulb does not light up whenthe switch is open.
Close circuit.Bulb will light up when theswitch is closed.
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Series and Parallel
There are 2 ways in which an electric circuit can be
arranged:
1. Series
2. Parallel
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Series Circuit
A series circuit connects the components oneafter the other
A single loop is formed
A break in any part of a series circuit stops theflow of current in the whole circuit.
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Parallel Circuit
A parallel circuit divides into two or more branches.
The current divides and flows through each parallelbranch.
If a component breaks or is removed, the other
components remain on.
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Which of the following is a series circuit?Which is a parallel circuit?
Series Circuit Parallel Circuit
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Draw the circuit diagram for the following set up and
state whether it is a series or parallel circuit.
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Draw the circuit diagram for the following set up and
state whether it is a series or parallel circuit.
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Which is brighter?
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Which of the following is a series circuit?Which is a parallel circuit?
Series Circuit Parallel Circuit
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l
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Voltage
An electric cell gives energy to the electrons andpushes them round a circuit. Voltage is a measureof how much energy the electrons receive.
Different voltages are supplied by different cells andbatteries.
12 V Car Battery
1.5 V Dry Cell
9 V Dry Cell
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How to Measure Voltage?
The SI unit for voltage is volt (V). A voltmeter is an instrument used for measuring voltages.
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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.
R ll A t !!!
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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.
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Electromotive Force (e.m.f)
Electromotive force is the same as voltage.
E.m.f refers to the amount of energy supplied by theelectric source (eg. battery) to each unit of electric charge.
E.m.f is also measured by a voltmeter
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Electromotive Force (e.m.f)
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Potential Difference (P.d.)
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R i
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Resistance
When an electric current flows through a circuit, therewill be some resistance that opposes it. (similar to friction)
It can be measured bydividing voltage by the current.
R = VI
R = ResistanceV = VoltageI = Current
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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
Wi i t t f
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Wire resistance greater for:
1) Longer wires
2) Thinner wires
3) Higher temperatures
R i t
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The SI unit for resistance is ohm () Different electrical components have different resistance
For example, nichrome wires have a higher resistance than copper
wires.
Resistance
So should we use nichrome or copper to
make wires?
Ohms law
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Ohms law
Voltage DifferenceCurrent =
Resistance
I
VR
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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 =V
I
R =240
8
= 30
Resistors
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Resistors
An electrical component that is specially made to have a
certain resistance is called a resistor. They can be connected in a circuit to resist the current
flow.
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Fixed Resistors and Variable Resistors
Fixed resistors have only one resistance value Variable resistors can be adjusted to change
the resistance.
fixed resistor symbol variable resistor symbol
Variable resistors are usefulin light dimmers and otherelectric appliances
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S/E: Resistors
Resistors can be connected in series or parallel
single resistor
extra resistor in
series results in
dimmer bulb
extra resistor in
parallel resultsin brighter bulb
Resistors connected
in SERIES
Resistors connectedin PARALLEL
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S/E: Resistors in Series
When resistors are connected
in series, the resistance will
add up and increase.
R = R1 + R2 + R32 3
The total resistance for this circuit is:
2 + 3 = 5
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When resistors areconnected in parallel, theresistance will decrease
This is because they will
provide alternate routes forthe current to flow.
S/E: Resistors in Parallel
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S/E: Resistors in Parallel
6
6
1
R=
1
R
1
R
1
R+ +
1 2 3
What is the resistance of thecircuit?
1
R =
1
6
1
6 +
1
R=
2
6
1
3= R = 3
The final resistance issmaller
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Heating Effect of Electric Current
When an electric current flows through a wire, the wire heats up.
Electrical energy has been converted into heat energy.
The greater the resistance of the wire, the greater the amount of heat
produced. This heating effect is used in common electrical appliances.
Iron Kettle Hair Dryer
A k ttl b th d i h i
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A kettle uses both copperand nichrome wires.
Copper has low resistance while nichrome has high resistance.
Which material, copper or nichrome, should be used for the heating
element, and for the external wire?
Use copper wirefor the external
wire as it has low
resistance &
produces less heat
Use nichrome wire for the heating
element as it has high resistance&
produces a lot of heat
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Household circuits
a. Mostly parallel.
b. 120 V in the U.S.
c. More devices plugged in a
circuit mean more current.d. More current means more heat
in the wiring.
Electric Power and Energy
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Electric Power and Energy1. Electrical power is the rate at which
electricity is converted into anotherform of energy.a. Power = current x voltage
b. Unit is the watt or kilowatt.2. Electrical energya. Energy = power x time
b. Unit is the kilowatt-hour.(1000 watts for 1 hour)
c. This is what we buy from the
electric company
Light Bulbs
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Light Bulbs
1. Incandescenta. Have a tiny
filament that resists theflow of electrons.
b. The filament gets
hot and glows toproduce light.
c. Very hot and
inefficient
2 Fluorescent
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2. Fluorescent
making it give off U-V rays.c. Phosphor absorbs U-V rays and
glows to make light.d. Very cool, efficient, and last a
long time.
a. Filled with gas
and coated withphosphor.b. Electricity
excites the gas
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Electrical safety devices
a. Fuses - one time use.
b. Circuit breakers - can be resetand reused.