CHAPTER – 12
ELECTRICITY
Think life without “electricity” in this modern society. Is
it possible to survive without electrical energy in world of technology. Since
we are science student, so it is necessary to understand the basic concept
behind the word “electricity”
Chargefi
(q)
It is a very small particles present in an atom it can be
either negative (electron) or positive (proton)
“Coulomb” is the SI unit of charge, represented by C.
Net charge (Q)– Total charge
IC Net charge is equivalent to the charge contained in
nearly electrons 6· 1018
Q = ne (n is no: of electron)
If Q = 1C, e = 1.6 ·
10–19C
(negative charge on electron)
Q 1
n
= =
e 1.6 ·
10–19
100 18 18
= 
· 10 = 6.2 ·
10
16
n = 6 ·
1018
electron
Current (I)
Rate of flow of net charge is called current. Denoted by
(I)
|
Q = |
tt is time
SI unit of current is “Ampere” rep. by A.
Ampere fi Defined as one coulomb of
charge following per second.
|
1C 1A = 1s |
In an electric circuit the electric current flow in the
opposite direction of the flow of electron (–ve charge) conventionally. It
flows from the +ve terminal of battery or cell to –ve terminal.
Small quantity of current are expressed in
mA (milli Ampere) = 10–3
A uA (micro Ampere) = 10–6 A
Ammeter– It is
an instrument used to measure the electric current in a circuit.
It is always connected in series m a circuit
It is represented
by the symbol ––––––––+ A
––––––– in an electric circuit. It has – low resistance.
Electric Circuit–
It is a closed path along which an electric current flow.
The arrow
shows the direction of electric current in circuit.
In this
circuit ammeter is connected in series.
The electron can
only flow when there is difference of electric pressure. For example “water
flowing through a tube” It is only possible when there high pressure at one
side and low at another side, then it will move from high pressure to low
pressure.
In case of electric current, the flow of charge is made
possible due to chemical action with in a cell that generates the potential
difference across the terminals of the cell.
8. Electric potential Difference– It is defined as the work
done in carrying a unit charge from one point to another between the two points
of an electric circuits.
|
W = Q |
V – Potential Difference W – Work Q – Net Charge
SI unit of potential difference – Volts rep. by “V”
One Volt fi When 1 Joule of work is done
to carry one coulomb (1C) of charge from one point to another of a current
carrying conductor then the potential difference is send to be IV.
|
IV = |
1J 1C |
Voltmeter fi It is an instrument, used to
measure the potential difference and represented by the symbol + V –
in an electric circuit. It
is always connected in parallel across the points between which the potential
difference is to be measured. It has high resistance.
Symbols for some commonly used instrument in circuit diagrams
(1) 
Cell
(2) Battery
(3) Key
(switch) open
(4) Key
(Close)
(5) Joint
wire
(6) Wires
with no join
(7) Bulb
(8) Ammeter
(9) Voltmeter
Georg Simon Ohm (physicist) 1787 – 1854
Found the relationship between the current (I) flowing
through a conductor and potential difference (V) across the terminals of a
conductor using the circuit diagram.
Ohm’s Law fi He stated that the electric
current flowing through a conductor is directly proportion at to the potential
difference across its ends, provided the temperature remain constant
V a
I
V = IR
Where “R” is the proportionality constant for the given
metal at given temperature and is said to be resistance, the graph between V
and I is always straight line.
Resistance– It
is the property of a conductor that opposes the flow of current. It is represented
by ‘R’ and symbol is 
SI unit of resistance “Ohm” OR W
1 Ohm – The
resistance of a conductor is said to be one Ohm, when the potential difference
across the conductor is 1V and the current flowing through it is 1A. V = IR
\
R = V
I
1 Ohm or 1 W
= 1V 1A
Rheostate–
As we know that V = IR
\ 1 = V { Shows that current throughconductor
resistor is inversely}
R
proportional
is its resistance
So to increase or decrease the current accordingly in the
circuit a component is used is called “Rheostat”, that regulates the current
without changing potential difference. Represented by “Rh”
Its symbol is 
OR 
it a conductor
has less Resistance, then more current will flow through it.
FACTORS ON WHICH RESISTANCE OF A CONDUCTOR
DEPENDS–
(1) On
its length (l)
(2) On
its cross sectional area (A)
(3) On
the nature of material
(Resistance) R a
l (Directly
prop. to length)
R
a 1 (inversely
prop to cross-sectional area)
A
R
a l
A
|
|
l A |
Where “
” (rho) is a proportionality
constant known as resistivity of the material of conductor.
11. Resistivity ( ) – the resistance offered by a wire
of unit length and unit crosssectional area is called resistivity.
l
Its
SI unit is W m Since
R = A
R.A W.m2
\
= =
l m
\ SI unit of = Wm.
For a material irrespective of length and area, the
resistivity is a constant.
Resistantly of a material vary with temperature
Resistivity of an alloy (homogeneous mixture of metals) is
generally higher than of its constituent metals. Example Constantan (alloy of
Cu & Ni)
Alloys have high resistivity and do not oxidise (burn)
readly at high temperature, for this reason they are commonly used in
electrical heating devices, like electric iron, heater, toasters etc. For
example “Tungsten” as filament of electric bulb.
Resistance in
Series– (Maximum Effective Resistance)
Let us take three resistance R1, R2
and R3 that are connected in series in a circuit.
Ohm’s low stated
V = IR
The current (I) flowing through the resistance in series
will remain same, where as the potential difference (V) across each resistor
will be different. V = IR
V1 = IR1 , V2 = IR2 , V3 = IR3
Total potential difference (V) = V1
+ V2 + V3
V = IR1 + IR2 + IR3 } Putting
the value of
V,
V1,
V2
& V3 I R = I (R1 + R2 + R3)
Reff = R1 + R2
+ R3
Thus, we conclude that effective Resistance of the several
resistors joined in series is equal to the sum of their individual resistance.
Resistance in
Parallel (Minimum Effective Resistance)
Let us take three R1, R2
and R3, that are connected in parallel in the electric circuit.
Now,
V V V
I1 = , I2
= ,
I3 = R1 R2 R3
Total current (I) = I1 + I2
+ I3 substitute the value of I1 , I2
, I3 and I
V V V V = + +
R R1 R2 R3
V 1 1 1
+ +
R R1 R2 R3
1 1 1 1
= + +
Reff R1 R2 R3
Thus, we conclude that the reciprocal of total effective
resistance of the several resistors connected in parallel is equal to the sum
of the reciprocals of the individual resistance.
Disadvantage of series connection in on electric
circuit :–
1.
In series connection if any of the component
fail to work, the circuit will break and then none of the component (ex. TV,
bulb, fan..) will work.
2.
It is not possible to connect a bulb and a
heater in series, because they need different value of current to operate properly.
Hence, to overcome this problem we
generally use parallel circuit.
Heating
effect of Electric Current :
Explanation® Battery or a cell is a source of electrical
energy.
Battery or
CellElectron
(Chemical
reaction in will come in motion it
will produce potential to flow
current
difference at its through
resistor
two
terminals)
Rest of
energy of source is 
Part of this energy is To maintain this current, converted into heat, that consumed in useful the source has to keep
raises the
temperature work expending its energy of gadget. (like rotating of fan)
This is
known as heating 
This effect is utilized in effect of electric
current devices such as electric
heater, iron etc.
Mathematical
Expression :–
Let us suppose that current (I) is flowing through a
resistor of resistance (R) for the time (t). The potential difference across the
resistance is (V).
W
V =
Q
\
Work done in morning the charge Q will be
W = VQ
W
Then
power, P = [Rate of change of
work done]
t
VQ
=
t
Q
P = VII = – (1)
t
Heat energy supplied by the source for time t will be
Energy or Work
H = P ·
tP =– (2)
time
Put equation (i) in equation (2)
H = VIt
= (IR) It\ V = IR Ohm’s Law]
H = I2Rt
This
is known as Joule’s Law
The law stated that the heat produced in a
resistor is
(i) directly
proportional to square of the current(I)
(ii) directly
proportional to resistance (R) for given current
(iii) directly
proportional to time (t) for which current flow through resistor.
Application of Heating Effect of Electric
Current :– GL
(1) Used
in electric iron, toaster, oven, heater etc.
(2) It
is also used in bulb to produce light.
(Filament of bulb is made of strong metal with high melting
point such as tungsten (m.pt = 3380°C). This filament can retain as much of the
heat generated as possible, to become very hot and emit light)
(3) It
is also used in the “fuse connected in an electric circuit {Fuse a safety
device, protect the circuits and appliance by stopping the flow of high
current. The wire of fuse is made of an alloy of metals for ex Aluminium
Copper, Iron
lead etc. The alloy should be of low m.pt and high
resistivity, fuse is always connected in series circuit. When large current
flow through the circuit, the temperature of fuse wire will increase. This
melts the fuse wire and break the circuit.
“ Fuses” used for domestic purposes are rated as 1A, 2A,
3A, 5A, 10A etc. for various operation depending upon the power of appliance
using.
Example- let us
consider an appliance “electric Iron” which consume 1KW electric power, at 220V
P = VI
I
= P = 1KW
= 1000W V 220V 220V
I = 4.54A
In this case a 5A fuse is required.
Electric Power :–
In case of electricity, it is defined as the rate of change electrical energy
dissipated or consumed in an electric electrical energy dissipated or consumed
in an electric circuit.
|
P = VI |
|
|
or P = I2R V2 or P = R |
( ( R |
or P
= Electrical Energy (E) time (t)
SI unit of electric power is “Watt” (W).
1 Wattfi Defined as the power consumed
by a device, when 1A of current passes through it at the potential difference
of 1V.
P = VI
1 Watt = 1 Volt · 1 Ampere
29 Electrical
Energy–E – Electrical Energy
P = Et
– time
t
\
E = P · t
SI unit of
electrical energy = Ws or J
Commercial unit
of electrical energy = KWh or One unit
E = P · t
\
KWh = 1KW ·
h
= 1000W ·
3600 s
= 36 ·
105Ws
=
3.6 · 106J (SI unit Ws = J)
\
1 KWh = 3.6 ·
106J
One horse power = 746W
EXERCISE
(Question
Bank)
Very Short Answers (1 Mark)
1.
Define the SI unit of (one mark each)
(a)
Current
(b)
Potential Difference
(c)
Resistance
(d)
Electric Power
(e)
Electric Energy (Commercial)
2.
What is the conventional direction of flow of
current?
3.
Define the term resistivity?
4.
On what factors does the resistance of a
conductor depend?
5.
How is the voltmeter and ammeter connected in
the electric circuit.
6.
Heating effect of current carrying conductor is
due to –
(Ans : loss of
kmectic energy of electron)
7.
Why the filament of bulb has high melting point?
8.
How does use of a fuse wire protect electrical
appliance?
9.
What is the relationship between power, current
and potential difference (Ans : P =
VI)
10.
How many joules are there in 1KWh?
Short Answer (2-3 marks) type Questions
1.
Draw a schematic diagram of a circuit consisting
of a battery of six cell of
1.5V each, three resistor each of 3W in series and a plug key.
2.
State Ohm’s law. Draw the graph between V&I?
3.
What is joule’s Heating effect of current,
derive its expression?
4.
A wire of length L and R is stretched so that
its length’s doubled and the area of cross section is halved. How will its
(i)
Resistance change
(ii)
Resistivity change.
5.
An electrical appliance of power 2KW works at
potential difference of 220V. Does it require fuse of 5A, give reason?
6.
Calculate the total effective resistance between
points A and B
Long Answer (5 Marks) type Questions
1.
On what factor the resistance of conductor
depends give its mathematical expression. Give the SI unit of resistivity?
Calculate the resistivity of a metal of length 2m at 20°C
having the resistance of 20W
and diameter 0.3mm?
2.
In a circuit below, calculate
(1)
Calculate total effective resistance
(2)
The total current through the circuit.
(3)
Potential difference across 4W and 2W.
3.
Three resistance of 2W , 3W and 5W are connected in the electric
circuit.
Calculate the
(1)
Maximum effective resistance
(2)
Minimum effective resistance
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