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# ELECTRICITY

advertisement ```Electricity
What is Electricity???
Definition:
Electricity is a form of energy that can
be easily changed to other forms.
Where does Electricity come from?
Mainly 2 sources:
1) Power Stations
-Supply a lot of electricity
-Used in many electrical
appliances
2)
Electric Cells (batteries)
- Supply a little electricity
- Portable
- Safe
How does anElectrical Appliance
Work?
• To make an electrical appliance work,
electricity must flow through it.
• The flow of electricity is called an electric
current.
• The path along which the electric current
moves is called the electric circuit.
Current???
electric
cell
connecting
wire
filament
flow of electrons
Definition:
An electric current is the rate of flow of electric
charges in a circuit.
Electric Charges
• Electric charges are made up of positive
charges (protons) and negative charges
(electrons).
• When these charges flow in a circuit, a
current is produced.
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 to
the positive terminal of the cell.
How to Measure Current?
• The SI unit for electric current is ampere (A).
• Smaller currents are measured in
milliamperes (mA).
1 A = 1,000 mA
1 mA = 0.001 A
• Different electrical components and
appliances require different sizes of current
to turn them on.
Instrument to Measure Current
• An ammeter is an instrument used for
measuring electric current.
What is Electric Current?
Electric current is a measure of the rate of flow of electric
charge in a circuit.
current = charge
time
I =
q
Coulomb, C
t
seconds, s
Andre-Marie Ampere
Image credit: https://scientibulous.wordpress.com/2012/06/07/poster-on-andre-marie-ampere/
Unit of current
I =
q
Coulomb, C
t
seconds, s
1A=1C
s
A current of 1 A is 1
coulomb of charge flowing
every second.
So, a current of 5 A is 5
coulombs of charge flowing
every second.
Electric Circuits
connecting wires
•electric
Electric
circuits are made up
cell
electrical components
light bulb
• These components must be
joined together without any
in between to form a closed
circuit board
circuit.
Note:
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”.
• 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.
Examples of circuit diagrams:
Switches
A switch is used to open or close a circuit.
Main switch used
in buildings
Switches used on circuit
Circuit diagrams for open
and closed circuits
Open circuit.
Bulb does not
light up when
the switch is
Close circuit.
Bulb w ill light
up when the
switch is
Series andP a r a l l e l
There are 2 ways in which an
electric circuit can be
arranged:
1. Series
2.Parallel
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.
P a r a l l e l 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.
Which of the following is a
series circuit?
Which is a parallel circuit?
Which is brighter?
What is Voltage?
Voltage (also known as Potential difference) is the work
done (or energy transferred) between two points in a circuit,
per coulomb of charge. It is the “push” to the charges in the
circuit.
Voltage is measured in volts
(V).
work done
charge
1V=1 J
C
A 1 V cell gives 1 joule (J)
of energy to each coulomb
of charge.
voltage =
So, a 12 V battery gives 12
J of energy to each
coulomb of charge.
Alessandro Volta
Image credit: https://pbs.twimg.com/media/C6HwLIrWUAEiev6.jpg
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.
1.5VDry Cell
12VCar Battery
9 VDry Cell
How t o Measure Voltage?
• The SIunit for voltage is volt
(V).
• Avoltmeter is an instrument
used for measuringvoltages.
Voltmeter
• Voltmeters must
be connected in
parallel to thecircuit.
• The positive side of voltmeter is
connected to th e positive
terminal of the ce l l , andvice
versa.
Recall: Ammeter!!!
• It must be connected in series in the
circuit.
• Positive side of ammeter mustbe
connected nearest to the positive
t erminal of the battery (electr ic
cell), and vice versa.
Electromotive Force (e.m.f)
• Electromotive force is the
same as voltage.
• E.m.f refers to the amount of
energy supplied by the electric
source (eg. battery) to each unit
of electric charge.
• E.m.f is also measured by a Vm
Electromotive Force (e.m.f)
Potential Difference (P. )
d
.
Potential Difference
•
(p.d)
Remember Diffusion?
• High  Low
• Similarly, electric charges
will flow from a point of
higher potential (energy) to
lower potential
electric
cell
connecting
wire
filament
flow of electrons
•This difference in electric potential between 2 points in a
circuit is known as the potential difference. It is the same
as VOLTAGE also.
What is Resistance?
Resistance (R) is a property of a material that prevents the
flow of charges.
Resistance is measured in
ohms (Ω).
resistance =
1Ω=1
voltage
current
V
A
Resistance is measured
using an ohmmeter.
Resistance
• When an electric current f l o w s
through a circuit, there w i l l be some
resistance that opposes it. (similar
to friction)
• It can be measured by voltage
over current
Resistance
• Good conductors o f electricity
have
LOWRESISTANCE. (Eg. Metal objects)
–Electricity is able to f l o w through
them very easily
• Poor conductors o f electricity have HIGH
RESISTANCE. (Eg. Wood, cloth)
–Electricity is not able toflow
Resistance
• The SI unit f o r resistance is ohm ()
• Different electrical components
have different resistance
• For example, nichrome wires have a
higher resistance than copper wires.
So should we use
nichrome o r copper
to make wires?
Example 1
• An electric rice cooker
operates at 240 V and uses a
current o f 8 A. What is the
Vroelstiasgtean(Vc)eo f the rice cooker?
=240 V
Current (I)
=8 A
R=
V
I
240
R=
8
= 30 
Resistor
A resistor is a component designed to reduce the current.
Resistors are often used with
components that require less
current than is being supplied
to the whole circuit.
Most electronic circuits need to
have resistors to work properly.
Resistors can be produced that reduce the current by
different amounts.
Describe how does this system works?
How will you vary the flow of water?
Describe how does this system works?
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The Unfortunate Siblings of Benjamin
Ohm’s law
Ohm’s law is a formula that links voltage, current and
resistance. Resistance is measured in ohms (Ω).
voltage = current × resistance
V=I×R
Depending on the values given,
the formula can be used to
calculate voltage, current and
resistance in a circuit.
What two other equations
could you write from this
Ohm’s law formula triangle?
What does Ohm’s law mean?
These three equations can be derived from the Ohm’s law
formula triangle. But what do they actually mean?
•
•
•
V=I×R
If the voltage increases, then either the
current flowing through a material or the
resistance will also be increased.
R = V/I
The resistance of a material is constant for a
given temperature, which means that current
and voltage must be proportional.
I = V/R
For a low resistance material, more current is
allowed to flow for a given voltage. For a high
resistance material, less current will flow at the
same voltage.
A bulb has a current of
20 A and a voltage of
100 V
What is the resistance?
R = V
I
= 100 V
20 A
= 5Ω
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.
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
resistors are
useful in light
dimmers and
variable resistor symbol
S/E:Resistors
• Resistors can be connected in
series or parallel
extra resistor in
series results in
dimmer bulb
Resistors connected
in SERIES
single resistor
extra resistor in
parallel results
in brighter bulb
Resistors connected
in PARALLEL
S/E: Resistors in Series
• When resistors
are connected in
series, the
resistance w i l l
add up and
increase.
2
3
• R = RT1 h+eRt2ot+alRr3e…sistance for this circuit is:
2+3=5 
S/E: Resistors in P a r a l l e l
• When resistors are
connected in parallel, the
resistance will decrease
• This is because they will
provide alternate routes for
the current to flow.
S/E: Resistors in P a r a l l e l
1
=
1
+
1
R2
R1
R
+
1
R3
What is the
resistance of the
cir1cuit1? + 1
R
=
6
6
6
2
1
=
6
R
6
=
1
3
R = 3
The final
resistance is
WhatElectrical
is Electrical
Power?
Power (also known
as Wattage)
• is the rate at which an electric appliance converts electrical
energy to other forms. (rate of energy transfer)
• is expressed in terms of “watts”. Watt is equivalent to Joule
per second.
• is a product of the voltage and current passing through the
component.
power (W) = voltage (V) × current (A)
P = IV
We can express the relationship between current,
voltage and power mathematically using the equation:
Power = Current x Voltage
P=IV
Voltage measured in Volts (V)
Current measured in Amps (A)
Power measured in Watts (W)
Electrical Power
power (W) = voltage (V) × current (A)
What is the power of a bulb which uses a 230 V mains supply
and has a current of 0.44 A passing through it?
P = V × I = 230 × 0.44 = 101.2 W
What is the voltage across a microchip if it has a normal
operating power of 0.5 W and draws a current of 0.1 A?
V = P ÷ I = 0.5 ÷ 0.1 = 5 V
Other equations for Power
Electrical power can also be calculated using resistance.
The equations linking power to resistance are found by
substituting the equation V = I × R into the power equation:
P=V×I
P=V×I
and…
V=I×R
and…
I=V÷R
Therefore, using substitution:
Therefore, using substitution:
P=I×R×I
P=V×V÷R
P = I2 × R
P = V2 ÷ R
Power calculations
If you have a filament bulb and it has a potential
difference of 200V across it and a current of 0.2A running
through it. At what power is the bulb operating at?
P = IV
P = 0.2A x 200V
P = 40W
Power calculations
If you have a filament bulb and it operates at a power of
60W and it has a potential difference of 240V across it,
what is the current running through the bulb?
P
I
I
I
= IV
= P/V
= 60W / 240V
= 0.25A
Concept Check
Suppose you are buying a refrigerator and you have a
choice between two models with labels shown below.
Which will you take? Why?
Refrigerator A
220 V
0.82 A
60 cycles
Refrigerator B
220 V
0.75 A
60 cycles
What is Electrical Energy
Electrical Energy
• The amount of electrical energy consumed by a certain
appliance can be calculated as the product of it power rating
and the duration of usage.
Electrical Energy = Power x time
• is expressed in terms of “Joules”
• It can also be expressed in Watt.hour (Wh) or kiloWatthour
(kWh)
Prove that Wh is a unit of Energy
1 Wh = (1 W) (1 hr)
= (1 J/s)(3600 s)
1 Wh = 3600 J
1 kWh = 3, 600, 000 J
Sample Problem
1)A 20 w table lamp is used for 10hrs. How much
energy is consumed in kWh?
Given: P = 20 w
t = 10 hrs
E = Pt
= (20 w) (10 hrs)
=
200 Wh
200 Wh x 1 kWh
1000 Wh
=
0.20 kWh
Sample Problem
2) The power rating of a light bulb when use at 220 v is 50 W
a) How much current passes through it ?
b) How much energy is consumed in 10s ?
Given:
A.)
B)
P= 50 w
P= IV
E= Pt
V= 220 v
I= _P_
= (50 W)(0.0028 hrs)
t = 10 s
= 0.14 Wh
= 0.0028 hrs
I= 50 w V
= 0.00014 kWh
220 v
I= 0. 227 A
Electrical Power and Electrical Energy
What do we pay for in
our Electric Bill – is it
Electrical Power or
Electrical Energy?
Heating Effect o f
Electric
Current
• When an electric current f l o w s through
a wire, the wire heats up. Electrical
energy has been converted into heat
energy.
• The greater the resistance o f the wire,
the greater the amount o f heat
produced. This heating effect is used in
common electrical appliances.
Iron
Kettle
Hair Dryer
A kettle uses both copper and 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 wire
for 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
Heating Effect o f Electric Current
ARGON (inert/unreactive gas)
filament wire produces
heat and light
In a light bulb, the
heated filament which is
also a resistance wire,
becomes so hot that
light is also emitted.
Chemical Effect o f
Electric Current
What is ELECTROLYSIS?
Definition:
• Electrolysis is the chemical change that occurs when an electric
current passes through solutions or molten compounds.
Electrolysis
There are 2 uses o f
electrolysis:
1)Electroplating
2)Extraction o f Metals
1.E lectroplating
• Inelectroplating, a key is
covered with a thin layer of
copper when electricity is
passed through the sol- ution.+
copper
wire
key
start
copper
sulphate
solution
copper on
the key
after a few minutes
Uses o f Electroplating
Metal objects canbe
plated with a
thin layer of another
2. Extraction of metals
• Some metals (eg. sodium, aluminium) are obtained
by electrolysis.
• To extract the metal
– heat the solid compound of the metal until it melts
– pass an electric current through the molten
compound
Magnetic Effect o f a Current
• Astraight wire is placed neara
compass. When an electric current
flows through the wire, the
compass needle is deflected. This
shows that an electric current has
a magnetic effect.
current in
wire
compass needle is
deflected
Electromagnet
• An electromagnet is a magnet that is made
by using electricity
• It consist o f a wire coiled around a metal
metal r o d (eg.
Iron)
coil of
wire
iron core
to battery
compass needle is strongly
attracted to iron core
Electromagnet
The magnetic effect of the electromagnet can be
increased by:
1.
increasing the current (by using more batteries)
2.
increasing the number of turns of wire in the coil
3.
Using an iron rod
Differences between an
electromagnet and a magnet:
Electromagnet
Magnet
A temporary magnet
which can be turned
on and o f f using
electric current.
A permanent magnet
which retains
magnetism until it is
purposely
demagnetised
(spoilt).
Magnet can be made
stronger o r weaker
Magnet remains the
same strength
Uses o f
Electromagnets
•
Cranes that l i f t iron/steel
•
Iron/steel separ ators
•
Electric bells
•
Magnetically
crane
Electric bell
Electric motor in fans
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