Download Current Electricity

lskandhar School
Grade
I
Physics
Current. El_eetricity
Circuit: the conducting path through the bulbs,
wires and battery is called a circuit. The
diagram has been drawn using symbols.
Ammeter
bulb
Current
charges' For any neutral obiect, there will be no
net charges as it contains equal number
of positive and negative charges.
moving from positive to negative and they marked
arrows in this \,r/ay on their circuit
diagrams.
should remember that, in a circuit, the electrons
are really moving in the opposite
direction to the conventional current.
Conventional
Electron flow
current flow
phVstcs
*v2an
Notes/ Term2,2O3.Z
Page L of 6
Grade 8 Physics
lskandhar School
F
.The more
is'
the charges passing through the wire in one second, the bigger the current
o
I =L , where I
is
current in AmPeres
t
Q
is
the charge in Coulombs
T
is
the time is seconds
An Ammeter is an instrument used
to
measure electric current (rate of flow of electric
charges).
It is to be connected in series with the circuit. The unit is amperes A'
Circuit symbol
current flows
The ammeter has red (positlve) and a black (negative) terminal, Note that the
in to the ammeter through the red terminal and leaves through the black terminal. So
positive pole of the battery should be connected to the positive terminal of the ammeter
and negative pole of the battery should be connected to the positive terminal of the
ammeter.
below
lf the connections are interchanged, the pointer on the ammeter will deflect slightly
the zero mark.
ln figure left, three ammeters have been included in the circuit. The ammeters measure the
current at different points around the circuit but all three read the same'
The current is sarne at all points in a simple'circuit'
Notes/ Term?,?0a2
Page 2 of 6
Grade 8 PhYsics
lskandhar School
like resistance, lamP or heater of
circuit.Whenthechargeflowsthroughthelampsin
a
a circuit, their energy is converted
to other forms such as heat and light'
points can be defined as the amount of work
Potential difference (p'd) between two
from a place of lower potential to a place of
done in bringing one coulomb of charge
higher Potential. Or
potential difference (p'd), across the component'
potential difference (P.d) =
V
=Y where
enersv converted to other forms
i'th999*99191!-
V is potentialdifference (p'd)'
0
W is work done & Q is charge
Measurine potential difference {pdl
Voltmeter is an instrument used
to
measure potential difference between
two points'
out through
must frow into the positive terminar and
simirarry to the ammeter the current
thenegativeterminal.Sopositivepoleofthebatteryshouldbeconnectedtothepositive
pole of the bqttery should be connected to the
terminal of the voltmeter and negative
positive terminal of the voltmeter'
Circuit sYmbol
---&is Volt (V)
The Sl unit of Potential difference
Page 3
NotePJ TermZ,2Ot2
of 6
lskandhar School
Grade 8 Physics
Electro motive force (e.nr.fl or {gl
The e.m.f of a cell can be measured by a voltmeter connected directiy across the
terminal of the cell.
E.m.f is the amount of energy carried by one couiomb of charge rrvhen pass through the
power supply.
The e'm.f of the power suppiy must be equal to the sum of potential difference across
all circuit components.
For example, in
the circuit shown e.m.f {e} = V1 + V2
Some of the circuit symbols:
Symbo!
ln-
Name
Symbol
Name
Bulb (old symbol)
---f-_l-_--
resistor
cell
-,gr-_.-_
voltmeter
I
Switch (open)
bulb
Notes/ Terrn2,2O1,2
ammeter
-l'jF
battery
Page 6
of6
Physics Grade 9
lskandhar School
RESISTANCE
Students should be oble to..
*
.t
#
,[-
*
f
f
.l
I
.
.
.
.
.
.
o
.
State that resistanss = p.d/current and use the equation
resistance= voltage/current in calculations
describe an experiment to measure the resistance of a metallic conductor using
a voltmeter and an ammeter and make the necessary calculations
discuss the temperature limitation on Ohm's law
use quantitatively the proportionality between resistance and the length and
the cross-sectional area of a wire.
Calculate the net effect of a number of resistors in series and in parallel
Describe the effect of temperature increase on the resistance of a resistor and a
filament lamp and draw the respective sketch graph of current/voltage
Draw circuit diagrams with power sources (cell, battery or a.c mains), switches
(closed and open), resistors (fixed and variable), lamps, ammeters, and
voltmeters
State that the current from the source is the sum of the currents in the separate
branches of parallel circuit
no calculations on the whole circuit, recalling and using formula including
R = V/l and those for p.d in series, resistors in series and resistors in parallel
cl potential
Iltu)L be
us a
LiletE must
d conductor,
LUiluuLLUr, there
t[c]w through
LIltuugil a
currenL flow
maKe a current
To
lo make
PULE,rLrar
difference or vottage across it.
Copper connecting wire is a good conductor and a current passes through
it easity where as a similar piece of nichrome wire is not so good and less
current ftows for the same potentiaI difference
i.€., nichrome wire has more resistance than the copper wire.
Resistance is the opposition to the flow of electrons.
unit of resistance is Ohm (O)
Larger units are Kilo ohm (kO) and Mega ohm (MCI)
Sl
1kC)=1000f)
1MQ = 106Q
The resistance of a wire depends on its dimensions as wetl as on the
conducting abitity of the material from which it is made.
Page
1
Notes/Term 212013
Physics Grade 9
lskandhar School
Factors affectinq resistance
Length:
Doubting the length of a wire doubtes its resistance,
CfOSS-SeCtiOna[
afea:
resistance
I
R".
R."L
Doubting the 'end on' area of a wire hatves
it
A
L
ROC
7
The fitament of the butb is made of very thin tungsten wire because
tungsten has a higher metting point
than a copper wire. So
heating etements are normatty made of nichrorne. Higher
resistance offers higher heating effect
Whenever a current ftows through a resistance, heating effect occurs. This
principte is used in heating etements and atso in the fitaments of butbs.
Material: A nichrome wire has more resistance
The heating effect occurs because electrons collide with atoms as they
pass through a conductor. The electrons lose energy and atoms gain
energy and vibrate faster. Faster vibrations mean higher temperature.
Connecting wires used in circuits have a low resistance as possible so that
energy wasted as heat is kept to a minimum
TempefatUfe:
{
For metal conductors, resistance increases with
temPerature
For semiconductors, resistance decreases with
temPerature
ln metals conductors, as temperature increases atoms gain energy and
viUraie faster. This opposes the free movement of etectrons. 5o
resistance increases
Constantan and manqanin, both copper-based attoys, is often used in
standard resjstors because their resistances change very littte unless they
are heated strongty.
f
Semiconductors tike siticon and germanium are ihsulators at room
temperature. As temperature increases more and more etectrons
become free to move. So it conducts as its resistance decreases
ThefmiStOIS have a high resistance when cotd but a much [ower
resistance when hot. They contain semi conducting materiats. Some
etectricat thermometers use a thermistor to detect a temperature
change.
Carbon is not ctassed as a semi conductor, but its resistance decreases
with temperature
Page 2
Notes/Term 212013
lskandhar School
Physics Grade 9
RESISTORS
Devices speciaity made to provide resistance are calted rc51lt!_tE_.
curren6 and p.d at the levets for other circuit components to function
property.
.
.
A length of thin nichrome wire makes a simple resistor.
ln some, the resistance is provided by a thin layer of carbon, white
other contains a tong thin attoy wire coited to take up less space.
Circuit svmbols
resistor
variable resistor
Variable resistors {Rheostats}
It is used for varying current. The resistance of the resistor can be changed.
ln hi-fi equipment, rotary (circutar) variabte resistors are used as votume
controts.
siid tng
contaol
.<---.-->
resrstance
coii
oi
resistance wii"e
,rJ:;:;;;;,.
Above figure
resisror used to contror the brighrness of a
butb
lf the connections are between A and C it acts as a fixed resistor and
lf connected between A and B, resistance can be varied
Moving the sliding control of the voriable resistor to the right increases
the tength of resistance wire in the circuit. This increases the resistance
in the circuit which reduces the current in the circuit and dims the bulb
Page 3
Notes/Term 212A13
Physics Grade
lskandhar School
I
ln 1826, Georq Ohm carried out experiments with different metal wires to
discover how the current through each depended on the potentiat
difference apptied across its ends. These can be summed up in a law known
as Ohm's law which states
The current flowing through a metallic conductor is directly proportional
to the potential difference across its terminals, provided temperature
and other physical conditions remain constant.
Materiats',if,tct', obeys Ohm's taw is catted Ohmic conductor.
Materials which does not obeys Ohm's law is catled Non-ohmic conductor.
"
Resistance is catculated using the equation
Resistance (R)
=
V
R=7
Resistance of a metal conductor is defined as the ratio bet\ /een the
potential difference across the conductor to the current through the
conductor
lf potentiat difference is expressed in Vott($ and current in Amper-e(A),
then resistance is expressed in Ohm { A )
'{rok(V}
i.e., Ohrn {er) =
Ampere(A)
A conductor has a resistance of 1O if a current of 1A flows through
when a potential difference of tY is applied dlcross its ends.
Page 4
it
Notes/Term 212013
Physics Grade 9
lskandhar School
E.AI'E.KIMtrN I I U'
F
rnr
^ -r
nrclcT
'\NEA)UI(tr
A \l/^r
]I.trJIJ I AI'(LL
rl. eos i.r t
resrsianctl
,t?
Connect the circuit as shown in the above diagram.
Keep the variabte resistor at high yatue and ctose the switch.
Artirrcr rha
rrerialrla
rpcictnr
lrheostat)
and
keeo
.'--r
.
vsr tsvle
Llrv
\','-_-_*-/
-'_-
nuJvJL
the ammetgf feadinq tO
a
constant vatue and note the ammeter reading (l) and voltmeter reading (V)
Then R is calcutated using the formuta
R-
voltmeterreading
ammeterreading
Repeat the experiment for different I vatues and the corresponding V is
noted. For each trial find
R
Draw a graph of current against potentiat difference is ptotted as shown
betow. The vatue of xly gives the unknown resistance
R
C
0-)
O
pd
The circujt shown is not suitabte for measuring high resistances- lf the
resistance is high, the current through the resistor is smatt, and the smatl
current drawn by the vottmeter adds its effect to the reading on the meter
Page 5
Notes/Term 212A13
Physics Grade 9
lskandhar School
lilamenl
of bulb
is varied by adjusting the
The current through the fil.ament of the tight butb
temperature of
rheostat. As the current is increased from a low vatue,- the
is at a temperature of
the fiLament rises. when white hot, the fitament
more than 30000C
given in the figure {a}
Typicat potentiaL diffei-ence and cui'rent values are
ril grapl.l in figure (b) shows that current is not proportional to the
potentia[ difference
current
p.d.
j
---
30
I
60
90
'i2
14
p,d.
current
()
21.
I
i
l1
,tr.
26
an
A
40
-^^l^.^^= ie5lSld
-
-:i1---=
Fig (a)
:
a
o
f
()
1
68
pdiv
Fig (b)
of the filament
The catculations of p.d/current show that the resistance
becomes qreater as the temperaturg rises'
Page 6
Notes/Term 212013
Physics Grade 9
lskandhar School
Voltaee-cu rr.ent grap h
V
Resistance
increases as
Obeys Ohm's
law-Resistance
Resistance
decreases as
temperature
temperature
increases
remaining
Constant
(Filament lamp)
{Fixed resistor)
(Thermistor)
increases
Resistprs in series and Parallel
Combined Resistances
o
of resistofs in series
tf two ar mare resistars are connected in series, they give a higher
resistance than any of the resistors by itself.
The effect is the same as joining several tengths of resistance wire to form
longer tength
lf resistors
in series, their combined resistance
Rr and R2 dre
the equation
R is given by
ft.-Rr+R,
For three of more resistors, the above equation can be extended by adding
R:.... and so on.
equivalent resistance rt
r
, I
R1
Rz
Al-. lr
rtl
ltl
v1
v"
v3
total potential difference '= V
Page 7
Notes/Term 212013
Physics Grade 9
lskandhar School
Combined Resistances
of resistars in parallel
. lf twa or more resistors are connected
in parollel, they give a lower
itself.
resistance than any of the resisfors by
The effect is the same as using a thick piece of resistance wire instead of a
thin one. There is a wider conducting path than before"
lf two resistors Rr and Rz are in parattet, their combined resistance R is given
by the equation
I
RRr
I
R2
For three or more resistors, the equation can be extended by adding 1/R3,
...and so on.
lf the above equation for two resistors is rearranged,
it becomes
RrxR,
R
R1+R2
equivalent resistance ft
I
-n
_ {*"-':7,
I
potential
Page
I
difference:v
-+!.-;_
\*____ri
:
f.-7r
..1 'l-+
-:. \;1-V
I
Notes/Term 212013
Physics Grade 9
lskandhar School
Bulbs in Series and Parallel
ln series
voltmeter reads 1.5 V
threel5Vbulbs
three 1.5 V cells
voltmeter reads 4.5 V
The butbs are connected in series.
o
o
The butbs share the potential difference (vottage) from the battery, so
each gtows dimty
lf one butb is removed, the other goes out because the circuit is broken
ln parallel
A
C
The butbs are connected in paral[el.
.
o
the futt PD from the battery because each is connected
directty to it. So each gtows brightty
lf one butb is removed, the other keeps working because it is stitt part of
an unbroken circuit
Each gets
Page 9
Notes/Term 212013
Physics Grade 9
lskandhar School
Circuits and Switches
lf two or more butbs are to be powered by one battery, as in a car lighting
system, they are normatty connected in paratle[. Each butb gets the futt
battery p.d. Also, each can be switched ON and OFF independently
Buibs ai'ranged in paraliel
can have inCepenceni sv/iiches
These two show two atternative ways of drawing
exactty the same circuit as that shown in the first
figure
Basic
circuit ruies
There are some basjc rutes for a[[ series and parattel circuits. The particutar
current vatues depend on the resistances and potential differences.
The equation V = lR atways appties to every resistor.
When resistors or other cornponents are
When resistors or other components are in
in series:
. The current through each of the
components is the same
. The total PD (vottage) across alt the
components is the sum of the PDs
across each of them
parallel:
Page 10
.
.
The PD (vottage) across each component
is the same
The total current in the mains circuit is
the sum of the currents in the branches
Notes/Term 212013
Physics Grade
lskandhar School
9
Resistor cotour codes
The colour code
You 'r'ead' lhe flngs ct.l the resistor like this
Each coiour
stands lor a
i5L
number:
Black
Brown
Fed
Orange
Yellolv
Green
Blue
Violet
Grey
White
ligure
number of
2nd
rigure
0
1
Z
3
A
5
6
7
B
I
orange
000
reC
2
resistance vaiue = 27 000 ()
= 27 kA
The lourth ring givgs:he tolerance This tells you by how much the
resistance may ciiler f rom the marked value:
Golci
5%
Silver
10%' Nc colour
20%
\{rhen onc of the lights on a Christmas ree breaks, the others go out as rvell.
\Vhat cjoes this tell you about the rvay the lights are connected?
2 Give ra,o a<ivantages of connecting bulbs to a batter-v in parallel.
3 Redrarv either of the circuits on the risntso that it has a single switch which
rurns Ljoth bulbs on and off t!)gether.
;4 'I-his quesrion is about dre circuit on tJ.re right:
1
reaclings on two of the ammeters are shorvn. What are the readings on
i
a)'fhe
:
ammetcrs X and Y'i
b) iltne IrD across the bamcry is 6\i rvhat is the PD
i
i
across each of the bulbs?
(Note: ,r,ou can neglect the PI) scross an amlneter.)
In circuit A on the righr:
a) \\rhat does the arnmeter reacl?
b) Wlrat is tlrc PD across each of thc rcsistrlrsl
In circuir I) on the right:
a) What docs Lhe arnnleter rcail rvhcn thc >',i jlch
is
opcn ( OI:Ir\ )
b) Whar is lhe cltrrent through cach oi tlle '1 o
rcsistors u'hen the slvitch is closerl (ON)?
c) What does the arrurreter reaci rvhen thc srvitch
is
ckrsccl?
is^ the combined resistatrce oi'dre trvtl
rvhcn
thc switch is closcd?'
resistors
Wl'rich resistor arrangement, C or 11. on the right
ha-s tbe lc.r,ver resistancc? C-heck your answer b1:
d) What
c ___-.t:--.--r-1012
caiculation.
Page
990
11
I
10 ()
Notes/Term 212013
Physics Grade 9
lskandhar School
Catcutate the effective resistance in each of the fottowing
4.
#
20
4()
a
5.
Calcutate the ammeter reading in each of the foltowing circu'its
a) ln Fig (a)
(i) what is the vatue of current ftowing when the switch is open?
(ii) What current ftows
when the switch is ctosed?
b) ln Fig (b)
(ii What current flows through the ammeter when.the switch is ctosed
and the variabte resistor set to zero?
(ii) What current ftows when the switch is ctosed and the variable
resistor set at 20 ohm?
c) Using fig (c) exptain why no current witt ftow when the switch is ctosed?
What effect witt there be on the circuit if the switch remains open but A
is joined to B? What effect witt there be on the circuit with the switch
ctosed with A joined to
Page 12
B?
Notes/Term 212A13
6
For
Teach er's
(c) Determine the number of compressions produced in one second*
Use
(d) calculate the speed of the wave when the wavelength of the wave is 2 cm.
Speed = ...........................t2I
5. Fig.5.1 showsanelectricalcircuitcontaining a12V powersupplyandanumberof
resistors.
12V
rl
.. t'
la 't'
6A
.t
(l
- - L
i.-&'
2A
I
Fig
5.1-
.
,,,,
(a) Calculate the combined resistance of
(i) the 2O and 4O resistors in series,
a*, &t* *L
(ii) the 3O and 6O resistors in paraltel.
;: ., .' ;:. .
,l
i t
(,',' '
l
,.' g
---t'-t*'
r,_
resistance = .......3...:
-rL
......................... t2I
(b) Calculate the reading of the ammeter in Fig. 5.1.
'T - 'Y
L - -1:
*t-.
_J
)'
l€
.i-)
d-
j\
ammeter reading = .............'.
:.
:....:.......,................
l2l
(c) Determine the potential difference across the 4O resistor.
,.)
j-
{-.,
olsK 2014
,*
lskandhar School
Grade 9 Physics
PRACT!CAL ELECTRCITY
4.!@---..
_
-, ,
r 1s$d*#
+,.@.
. w&,
Students should be able to
*
Describe the use of electricity in heating, lighting and motors
Do calculations using the equations power = voltage x current, and energy = voltage x
current x time
'i'
*
*
Calculate the cost of using electricat appliances where the energy unit is kWh
State the hazards of damaged insulation, overheating of cables and damp conditions
*
Explain
settings
the use of fuses and circuit breakers and fuse ratings and circuit
breaker
'i. Explain the need for earthing the metal cases and for double insulation
* State the meaning of the terms live, neutral and earth
+ Describe how to wire a mains plug
* Explain why switches, fuses and circuit breakers are wired into the live conductor
Electrical Power
Electrical power is the rate of electrical energy conversion. An electric fire is more powerful
than a light bulb because [t converts electrical energy into heat energy more quickly.
Power =
energy canuerted
tinte
Unit of power is Watt (W) = Joules/second {J/s)
Larger unit is kilowatt (kW)
Notes
-
Term 2,2A13
Page 1 of 8
Grade 9 Physics
lskandhar School
A power of 1 watt means that 1 ioule of energV is being chanEed every second.
Electrical power is also calculated using the equation
Power = voltage x current
P=Vxl
A higher voltage gives more power because each electron carries more energy
A higher current gives more power
because there are more electrons
to lose their
energy every second.
More equations to calculate Power
Alternative equations for calculating power can be found by substituting V = lR and I = V/R in
turn the equation
P = Vl
The formula connecting Power, voitage, current and resistance ai'e given below.
P=lV
P=l2R
P = V2/R
Mains electricitv
Electric current that reverses its direction constantly with respect
alternating current (AC).
to time is called
The current from the main socket is pushed and pulled backwards and forwards through the
circuit 50 times every second. This current is called a.c. The current we are using in our
houses is a.c.
current
or
*
voltage
0
I
_l
Notes
-
Term 2,20L3
Page 2 of .8
Grade 9 Physics
lskandhar School
Direct current (DC)
The unidirectional current from a battery is l<nourn as direct current. i.e, the current flows
only in one direction.
ln power stations, AC is easier to generate than one-way direct current {DC) like that from
a
battery.
The rJiagram below shows an electric kettle connected into a mains socket.
plug with luse
I
switch
luse
rnar r s
;: Supply
ineuiated
cable
lated
healing
elernent
in su
Iive wrre
2S0 V a.c.
50 Hz
neutral wire
?L:
-*gP5*--6
earfh wrre
Live wire
* lt carries current from the rnain socket through the circuit. The potential of this t'rire
changes alternately negative and positive, making the current flow backwards and
forwards through the circuit.
f'
*
wire is at a potential of 230 V or 240 V
The colour of the live wire is red or: btown
Live
Neutralwire
*
lt is a safe wire which completes the clrcuit. The electricity board earths the neutral wire
by connecting it to a metal plate buried in the ground'
* Even though current passes through this wire, it is kept at zei'o voltage, so accidental
touching of this wire does not give electric shock.
.1. lt carries the same current as that in the live wire.
* The colour of the neutral wire is black or blue'
Page 3 of 8
Notes * Term 2,1:013
lskandhar School
Grade 9 Physics
Earth wire
*
This is a safety wire which connects the metal body of the appliance to earth and
prevents it becoming live if a fault develops.
.1. i.e, if the live wire happens to touch the metal casing of the appliance, the current would
*
immediately flow to earth and would blow the fuse.
lf there were no earth wire, the metal casing of the appliance would remain live and
possibly lethal current would flow through anyone who happened to touch it.
of the earth wire is vellow and green (or iust green).
.i. The colour
Switch
r lt is a simple ON and OFF key fitted in the live wire. lt [s used turn ON and OFF the
current in the circuit. lt would work equally well in the neutral wire, but wire in the flex
would then still be live when the switch was turned OFF. This may cause hazards.
Fuse
r
This is short piece of thin wire which overheats and melts if current is too high. lt is
placed in the live wire often as a cartridge inside the plug. lf a fault develops and the
current gets too high, the fuse blows and breaks the circuit before the cable can
overheat and catch fire.
"'{
Fuse values
r
The plug is usually fitted with a 1A, 2A, 34, 54, LOA or 13A fuse. The value tells the
current needed to blow the fuse. Fuse value should be slightly greater than the normal
current through the appliance, but as close to it as possible. So that the fuse will blow as
soon as the current exceeds the rated value.
Double insulation
o
Some appliances-radio for example-do not have an earth wire. This is because their
outer case is made of plastic rather than metal. The plastic acts as an extra layer of
insulation around the wires.
Notes-Term 2,203-3
Page 4 of 8
Grade 9 Physics
lskandhar School
Three-pin Plugs
o
Ptugs are safe and simple way
of connecting appliances to the mains. The diagram below
shows a fused three Pin Plug'
Live wire
Fuso
Outer .lnsulation
Five staees in wirine a Plug
1
wires
Remove the insulating plastic from the three
2.
that the outer insulation
wrap the wires clockwise round the studs, and ensure
is
firmly griPPed.
3.
Tighten the studs
4.
for the appliance
Replace the fuse: check it is of the right rating
5.
Checktherearenoloosestrandsanywhereintheplugbeforereplacingthecover.
Notes
-
Term 2,201'3
Page 5.of 8
Grade 9 Physics
lskandhar School
Circuits around the house
I rxia U':,):
Il-.=.-*)
rrurLt1lsif'n
i
hil:lteI
H
\._ t*
')1
-f
i
t 1rrLf -
\1i.rjf
i,i:'*tr!
contains a live and a neutralwire' At
The electricity supply company's cable into each house
several parallel circuits for the
the consumer unit or 'fuse box', these wires branch into
an earth wire'
various appliances. The cable for each circuit also contains
or a circuit breaker'
ln the consumer unit, each circuit passes through a fuse
A
circuit breaker is
obove the specilied volue.
Notes
-
switch which 'trips' (tury off) when the current rises
pressing o button'
can be reset by turning the switch on or
an outomatic
Term 2,7A13
lt
Page 6 of 8
Grade 9 Physics
lskandhar School
Mains sockets
Each main socket is protected by its own fuse or circuit breaker in the consumer unit. The
plugs in the sockets are unfused. The mains sockets are connected to a ring main.
This is a cable which begins and ends at the consumer unit with the live, neutral and earth
wire each forming a long loop or'ring' around the house.
Advontoge is that there ore two conducting poths, sa thinner cobles con be used.
socket is domaged, ihe other socket con still function.
lf one
The ring main is protected with a 30A fuse or circuit breaker in the consumer unit. Each plug
connected to the ring has its own fuse.
Dangers of
\v
electricitv
Main electricity can becorne dangerous if we do not use it properly. Possible hazards include
fires, shocks, burns or even death.
broken strands of wire can mean that a cable has high
resistance at one point. So heat is given off when current flows through. lt may be
enough to melt the insulation and cause a fire
Dirty plug pins give a high resistance where they connect with the socket. When the
* ln old, frayed wiring,
*
*
.!.
current flows through, the plug may overheat.
Too many appliances connected to one socket. lf all the appliances are switched on
at one, the supply cable may become overloaded. A large amount of heat generated
can melt the insulation and start fire.
The wires of electrical appliances are insulated with insulating material such as a
rubber. Due to wear and tear, the rubber can become damaged and the wires
exposed.
lf not replace with new cable, the exposed wires can cause electric
shock
when touching them accidentlY.
Notes
-
Term 2,2OL3
Page 7 of
I
\_./
;l
Grade 9 Physics
'gv calculati0ns
ie convenient way of delivering
energy. ln the home, that energy is delivered
rms using many appliances.
EnergY=Fowerxtime
Uoules) (watt)(second)
The units on an electricity bills are units of energy called kilowatt hour {kWh).
To calculate electrical energy in kilowatt hour
EnergY=Fowerxtime
(kwh) (kw)
(hour)
TkWh is the arnount of electricol enerav that is used bv
\J'
s
7000 W gtr pliances in 7 hour.
The electricity meter in a house is an energy meter. The reading on the meter gives the total
energy supplieei in units. The unit is another name for kilowatt hour.
Notes
-
Term 2,2A13
Page 8 of 8
Physics - Grade 10
lskandhar School
ACTION AND lrsE OF CIRCUIT COMPONFNTS
are pure, especiallY at low
Semiconductors like Silicon and Germanium are insulators if they
!
temPeratures'
temperatures as more and
Their conductivity increases or its resistance decreases at higher
r
more electrons are made free to move'
increases due to
But, in the case of metals, as temperature increases, its resistance
r
increased vibrations of the atoms making electron flow more difficult'
,
adding tiny but controlled
The conductivity of the semi conductors can be increased by
'doping'
amouflts of certain substances called "impurities" by a process called
,
\-/
This can be then used as diodes, transistors and integrated circuits.
DloDEs
pass through it in one direction
Diode is a two terminal, one way device which lets current
r
onlY'
end is the Anode'
The wire nearest to the band is lathode and the one at the other
r
l,
I
fl
,1
ii
I
I
i
."af,oO"
,
I
--]
,-t\
I
ffi
/+L__
\.+\l
circJe
aptianal
\--l
I
I
I
uno,i..- -'-i
i
n
n
it is made of a semi conductor like Silicon or Germanium'
lt conducts when the Anode
is connected
to the positlve terminal and cathode to the
negative terminal of the suPPIY'
r
passes lrr tite
lt is then forward biased, its resistance is smaliand conventionalcurrent
clirection ol the errow on its symbol.
;
l\otes ferm
L, 20 14
Page 1 of 11
Physics -
lskandhar 1icho.,:l
Grrile iC
y''.--..
-tf+ :.-- --t
t\
t'-*-t
I
rrrao{
.]_
-I-r,ll
r
5'l
I
i
A';^
'.-. ),
i
l.>v'
{ii;6
,
,
1'l
I
I
_--_.*__l
Fr,rwqrd bioserJ
Reverse. biased
lf the connections are other way round, it does not conduct, its resistance is large and it is
rev€rse biased.
Diodes are used to convert alternating current (a.c) to direct current (d.c), a process called
Rectification.
LTGHT EMTTT|NG
p"lgpEs (LEp)
I
It is a diode made from the semiconductor Gallium arsenide phosphide.
,
When forward biased, the current through it make it emit red, yellow or green light.
F
coloured transl'.,rcent
plartic case
- ---I--J L--___J
-
tl
'
i
,,'ld
.1.!/
r
/
rJ,\
i+/LED
,t
'-11
v
"anodc A
I
rtl
lL
L__---_-l
T
No light is ernitted in the reversed biased conciition, if it exceeds 5V, may cause damage.
I
ln use, LED rnust have a suitable resistor in series with it. (e.9.300 O resistor on a 5V supply)
ro limit the current.
other electronic equipment.
a
LEDs are used as irrdicator lamps on computers, radios and
I
Many clocks, caiculators, video recorders and measuring instrurnent have seven segnleirl
red or green numerical displays. Each segment is an LED and depending on
,,^,rhich have a
voltage across them, the display lights up the numbers 0 to 9.
a
LEDs are sm.:ll, reliable and have a icng iife"
I
Their i";peratirig speecl is high anrJ iheii'current requirernents are very low.
l{otes
- i'erm 1, 2'J14
Pagr: 2
of 11
Physics - Grade 10
lskandhar School
LrsHT pEPENDENT
RESTSTOR
(rDR)
The action of an LDR depends on the fact that the resistance of the semiconductor Cadmium
Sulphide dereases as the intensity of the light falling on it increases,
An LDR and a circuit showing its action are shown below.
LDR
a.\
\,--.
/-\ I }---".r
l\_,
---{-t
AI
6vl
d.c.
Yl
I
I
lz.-I
L--J't, /t I
k:_ i,
6V 0.06A
b
LDR
LDR demonstration circr.rit
When the light from a lamp falls on the 'window' of the LDR, its resistance decreases and
the increased current lights the larnp.
LDRs are used in photographic exposure meters and in series
with a resistor to provide an
input signal for a transistor.
The l-V graph for an LDR is shown below.
t+
I
I
I
.-.,.".}
0
fuotes -
Ierm 1, 101"4
V
Page 3 of
t1
Physics - Grade 10
lskandhar School
THERMISTOR
r
when the
lt contains a semi conducting metallic oxides whose resistance decreases
passing a current through
temperature rises either due to heating the thermistor directly or
it.
.
r
its action. Heating
The diagram below shows a thermistor and a circuit which demonstrates
a
thermistor with a match lights the bulb'
A
oC
thermistor in series with a meter marked in can measure temperatures'
therrntstrlr
r--'*-
t
t--/--i*l
:t
il
GVI
d.c
ol
a
ffi
Tl rernt istor
I
'r
K--r
,_-_r."zi--J
t /\
<_
I
I
)'
/
6V 0.064
b Thernristor demonstration
circuit
the
lf a resistor and thermistor are connected as a potential divider, thevoltage across
resistor increases as the temperature of the thermistor increases'
radiator'
The circuit can be used to monitor temperature, for example in a car
/t
-*-
thernristor'
0
-*-**-----l-
V
I -V 6roph for o l'hermistor
flotes - Term 1, 2014
Page 4
cf
X.J.
Physics - Grade
lskandhar Schcol
1"0
POTENTI4t-DlvlDER
produces an output
A potential divider is a simple circuit with resistors connected in series. lt
voltage (v out) that is a fraction of the output voltage,
r
I
Vt
I
1
Vz
I
I
t
.
.
o
ln the above circuit two resistors Rr and
Rz
are connected in series with a supply of voltage V
The potential differences across the resistors are in the ratio of their resistances.
The current in the circuit is
,
'
Supply voltage
Total resistance
V
(R1 + R2)
So the voltage across Rr is
And the voltage across
Vr =
\r x Rr
I x Rr =
(R1 + R?)
Rz is Vout
111
x *j---
(Rr +
R2)
Ratio of the voltage across each resistor is
vt:
ni
vz
R2
f'lotes - Term 1, .1S14
Page 5 of 11
Phyi,ics -
lskandhar School
Grltje li)
ELECTRONIC SYSTEM
An electronic system can be considered to consist of the three parts as shown in the block diagram.
or
A transducer is a device for converting a anon electrical input into an electrical signal
vice-versa
lnput sensor detects changes in the environment and converts them from their present
form of energy into electrical energy
lnput sensors or transducers include LDRs, thermisters, microphones and switches which
respond, for instance, to pressure changes'
Proces,sor
It is the part of an electronic system, which receives electrical signals from the input sensor and
decides an action like amplification, timing, storing or counting.
Output device
lamPs,
It converts the electrical energy supplied by the processor into attother form. These include
LEDs, [oudspeakers,
motors, heaters, relays and CRO etc.
lJotes - Term 1, .1014
Pag,e 6
of 11
lskandhar School
Physics - Grade
1.0
Feedback
ln many systems, all or part of the output (energy or information) is fed back to the input.
This process of returning the output energy of a device to the input is called Feedback
Feedback is positive if it acts in the same direction as the input and increases the output.
r
lt is used to produce a.c in electricaloscillators and in bistables (basic memory elements in
computer)
.
Too much positive feedback can cause instability in a system (lf sound from the speaker is
picked up by microphone and fed back to amplifier, the speaker will produce 'howls')
n
.
.
Feedback is negative if it acts opposite to the input and reduces the output.
ln the amplifier, it decreases the gain and makes it more stable, better controlled and less
distorted
Applications
.
.
r
Controlof body temperature
Control ofa heater
Control of a car exhaust
ijotes - Ierm i,1014
Page 7
of L1
Physics - Grade 10
lskandhar School
CAPACITORS
They cannot conduct charge in the way
capacitors are devices connected in electronic circuits.
to store charge for a short interval of
resistors and other components do., but it has the ability
time.
lts ability to store charge is called capacitance
'
.
r
lts Sl unit is farad (F)
have much smaller value
ln practical terms a farad is a very Iarge unit and most capacitors
than 1 farad.
r
Common sizes of capacitors are made in small units'
Capacitance of a capacitor depend on
,/
,/
,/
Area of the Plates
Separation between the Plates
lnsulating medium between the plates
CaPacitance
t.t
=;ffi
The circuit symbol for a capacitor is given below
iF
of charge on one plate produces
capacitors do not allow electron to flow through it, but any change
plate'
an equalcharge in the induced charge on the opposite
ruotes - Term L,
20i4
Page 8 of 11
Physics -Grade 10
lskandhar School
CharginE a caPacitor
insulator' called dielectric'
parallel metal plates separated by an
A capacitor consist$ of two
co(to*cll(rrl$
to Plntlls
m$llr! Pllrlgo
tal
P'rrallel'Ptatr'r e:aPacttor
as polyesters and
oxide, Mylar, mica and plastic such
Dielectric material includes aluminum
polYProPYlene'
a cell or battery'
is to connect its two leads to
The simplest way of charging capacitor
negative terminal onto
two prates, erectrons frow from the
when the battery is connected to the
the battery at its
leave the other plate and flow onto
one plate, making it negative. Electrons
plates depends on the
positive' The amount of charge on the
positive terminal.'Ihis plate becomes
vottage of the batterY'
rr !fltiil
7' jrlille X
€,
rn
CJ
E
a)
t
{}
Ll)
(s
E
#--*-_-
slsctran lltrrv (irr wire)
(a|
Cltarglng
Charging
Durtngthecharging,thereisabriefflowofelectronsrounrlthecircuitfromXtoYibutnotthrough
by sensitive meter'
i,voul6 be detected at any point in the circuit
current
brief
a
and
the dielectric)
Charging stop when tr"ro
Y and opposes the battery voltage'
The voltage build up between X and
so the charging current is zero'
flow ln the clrcuit
voltage are equal and no electrons
i;1gg g i:f 11
I'lctes - Ierrn
l,
2014'
Physir.s -
lskandhar School
Gtadt l0
Disclrarging a capacitors
Removing the charge form the capacitor plates is called discharging.
When a conductor is connected across a charged capacitor, there ls a brief flow of electrons form
the negatively charged plate to the positively charged plate. {i.e.) form
Y
or X. the charge stored by
the capacitors falls to zero, as does the voltage across it.
e
(tr
E
o
B
o
tinls
slsctron llovtr Io
discharge caPacilor
{bi
Discharging
DischargJng
D*emonstrq$on
ln the circuit below, a two way switch charges the capacitor, C in position 1 and discharges it in
position 2
100kil
lf the'ralue of
R
and C
i-q
iarger, capacitor wiil take ionger tirne to charge and discharge.
The direction of Centre zero rnilli ammeter reverse for each process.
Notes --fernr 1, ,?014
Page L0 of tr1
Physics - Grade
lskandhar School
1.0
Effect of capacitors in d.c and a.c circuits
Direct current
ln the circuit shown below, the supply is d.c but the lamp does not light because capacitors block
d.c
I
000p
F
2.5V
0.34
Alternating current
ln the circuit below, the supply is a.c and the lamp lights, suggesting that a capacitor passes a.c
0O0p.F
2V
ll.c.
?.5V
o.3A
No current actually passes through the capacitor since its plats are separated by an insulator.
As
the a.c reverse direction, the capacitors charge and discharge causing electrons to flow to and
fro rapidly in the wires joining the plates.
So a.c flows the circuit lighting the lamp.
f{otes - Term 1, 201-4
P,rge 11 i-rf 11