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Electricity :It is the presence or flow of charges
Charge:
It is the property of an atom
It can be (+)ve or (–)ve
Like charges repel each other and unlike charges attract
each other
Its S.I unit is coulomb
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Charge on one electron is e=-1.6x10-19C
No of electrons in 1 coulomb of charge
n=6.25x1018
Current:
It is rate of flow of charge.
Current I=Charge/time
S.I unit of current is Ampere
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Ammeter:
Current is measured by ammeter
It has low resistance and
It is always connected in series
•1Ampere:Defined as one coulomb of
charge flowing per second.
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Electric circuit:
A continuous and closed path of electric current is
called electric circuit.
Electric current flows from the +ve teminal of the cell to the –ve
terminal of the cell through the ammeter and bulb.
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Potential difference:
Work done to move a unit charge from one point to
another point of the conductor
Potential difference V=Work/charge
S.I unit of potential difference is Volt
Potential difference is measured by Voltmeter.
One Volt: 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 I Volt.
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Voltmeter :
It is an instrument, used to measure the potential
difference .
It is always connected in parallel across the points
between which the potential difference is to be
measured.
It has high resistance.
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Ohm’s Law: Ohm’s Law states that the potential
difference between two points is directly proportional to the
electric current, at a constant temperature.
This means potential difference V varies as electric current.
V ∝ I
V = RI
I = VR
R = VI
Where, R is constant for the given conductor at a given
temperature and is called resistance.
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Resistance:
Resistance is a property of conductor due to which it resists the
flow of electric current through it.
A component that is used to resist the flow of electric current in a
circuit is called a resistor.
In practical application, resistors are used to increase or decrease
the electric current.
Cause of Resistance in a Conductor:
Flow of electrons in a conductor is electric current. The positive
particles of conductor create hindrance to flow of electrons,
because of attraction between them, this hindrance is the cause of
resistance in the flow of
conductor
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Variable Resistance:
The component of an electric circuit which is used to regulate the
current, without changing the voltage from the source, is called
variable resistance.
Rheostat:
This is a device which is used in a circuit to provide variable
resistance.
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Factors on Which Resistance of a Conductor Depends:
Resistance in a conductor depends on nature, length and area of
cross section of the conductor.
(i) Nature of Material: Some materials create least hindrance and hence, are
called good conductors. While some other materials create more hindrance in
the flow of electric current,
(ii) Length of Conductor: Resistance (R) is directly proportional to the
length of the conductor.
R ∝ l …(i)
(iii) Area of Cross Section: Resistance R is inversely proportional to the area
of cross section (A) of the conductor. This means R will decrease with an
increase in the area of conductor and vice versa.
R ∝ l/A ….(ii)
From equations (i) and (ii)
R ∝ l/A
R = ρ l/A ……. (iii)
Where, ρ (rho) is the proportionality constant. It is called the electrical
resistivity of the material of conductor.
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Resistivity:
It is characteristics property of the conductor to resist the flow
of current. the resistance offered by a wire of unit length and
unit cross sectional area is called resistivity
ρ = R A/l ..(iv)
Resistivity is also known as specific resistance.
Resistivity depends on the nature of the material of the
conductor.
Materials having a resistivity in the range of 10-8 Ωm to
106 Ωm are considered as very good conductors.
The resistivity of materials varies with temperature
S. I. unit of resistivity (ρ) is Ωm.
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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
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Resistance in Parallel(Minimum Effective Resistance)
Let us take three R1 , R2 and R3 , that are connected in
parallel in the electric circuit.
Now, I1 = V/ R1 , I2 = V /R2 , I3 = V/ R3
Total current (I) = I1 + I2 + I 3 substitute the value of I 1, I 2, I3
and I =V/ R =V/ R1 + V /R2+ V/ R3
V/ Reff =V ( 1/R1 + 1/ R2+ 1/ R3)
1/ Reff = 1/R1 + 1/ R2+ 1/ R3
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Disadvantage of series connection in on electric circuit :–
If one appliances stop working due to some defect then all
the other appliances stop working
All the appliances have only one switch due which they
cant be turned on and off separately
All the appliances do not get same voltage as voltage gets
divided in series combination
The equivalent resistance increases too much due which the
amount of current flowing becomes very small.
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Heating effect of Electric Current : when an electric current passes through a
conductor (resistor) the conductor becomes hot after some time and produces
heat. This is called heating effect of electric current.
• The current is same in all
the resistors
• The net voltage gets
divided among resistors
• Eqvivalent resistance is
given by Reff =R1+R2+R3
• Resistors are connected in
series
• The current is divided in
individual resistor
• The voltage is same
across all the resistors
• Eqvivalent resistance is
given by
1/Reff=1/R1+1/R2+1/ R3
• Resisors are connected
in parallel
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Joule’s Law of Heating Effect Of Electricity :
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
• H = I2 Rt
Application of Heating Effect of Electric Current :–
Used in electric iron, toaster, oven, heater etc.
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)
It is also used in the “fuse connected in an electric circuit
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Electric 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 Aluminum Copper, Iron lead etc. The alloy should be of low
melting point 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.
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Electrical power:
Electrical power is the rate at which electric energy is consumed by an
appliance.
P = W/t
P = VQ/t
P =VI
P = I2R
P = V2/R. SI unit of electric power is “Watt” (W).
1 Watt: 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 X1 Ampere
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Electrical Energy–: It is the total work done in maintaining an
electric current in an electric circuit for given time.
P = E / t
E=P X t =Vit
SI unit of electrical energy = Ws or J
Commercial unit of electrical energy = KWh or One unit
E = P X t
1KWh = 1000W X 3600 s
= 36 X10 5Ws
1KWh= 3.6 X106 J (SI unit Ws = J)
One horse power = 746W
