Download Notes of Electricity - Sadhana Group of Institutions

SVPrecis_Notes_10th_Science_Electricity
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Electricity: - The branch of electricity was originated from the Greek words “Electrica” and “Elektron.”
Originally, it was a Greek Philosopher named Thales who observed the attracting capacity of certain
materials when rubbed on other materials. It was Gilbert who classified these materials under two headsVitreous and Resinous. They are named later as positive and negative charge.
Fundamental laws of Electrostatics.
a. There are two kinds of charges namely positive and negative.
b. Like charges repel each other and unlike changes attract each other.
Charge: -Electrons have a negative charge of 1.6 x 10-19C, while protons have an equal positive charge
of 1.6 x 10-19 C.
Current:- The rate at which the charges move is called current, i.e. the quantity of charge in coulomb
crossing a point in unit time is called electric current. We write it as I = q / t , where q – the charge is
expressed in coulomb and t – time is expressed in second.
1 A = 1 coulomb (C) / 1 sec (s) = 1 Cs-1
small units of current are
1 mA = 10-3 A (mA = milliampere)
1 A = 10-6 (A = microampere)
Electricity Potential:- Electrical potential is defined as the work done in carrying a unit positive charge
from infinity to a point against any electric field. Electric potential V is given by V = W / q. It is expressed
in volt or joule / coulomb.
Electric Potential Difference:- Every point surrounding a charge will have electric
potential. When a charge of 1 coulomb is carried from one place to another, the
work done is called as electric potential difference.
It is expressed in volts.
1 volt = 1joule / 1 coulomb or 1 V = 1
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Meters.
a. Voltmeter.
i. Used to find potential difference.
ii. Should be connected in parallel.
b. Ammeter.
i. Used to find current
ii. Should be connected in series.
Ohm’s Law.:- Under similar physical conditions, the current flowing through a wire is directly
proportional to the difference in potential applied across its ends, (i.e.) I  V or V = IR where R is the
resistance offered.
Graphically, it can be shown, as:
Slope (V / I) is a measure of resistance offered.
Ohm:- It is the S.I. unit of resistance. One ohm is that resistance offered by the wire carrying 1A of
current when IV is applied across its ends.
1 ohm = 1volt / 1 ampere (or) 1  = 1 V / 1A.
Resistance:- The opposition caused to the flow of current is called resistance. It is denoted by the
symbol “
in an electrical circuit and is expressed in ohm. It is a property of the wire used in the
circuit. Conductors offer low resistance while insulators offer large resistance.
Factors affecting Resistance:- Resistance of a conducting wire depends on:
a. Nature of material (Resistivity – p)
b. Length of the wire (l) and
c. The cross-sectional area of the wire (A).
Resistivity (p):- The resistance offered by a wire of unit length and unit cross- sectional area is called
resistivity. It is expressed in terms of ohm-metre ( - m). Metals and alloys have low resistivity, while
insulators have high resistivity.
Rheostat:- It is a device which changes the magnitude of current in the circuit, by changing the
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resistance. It is connected in series in the circuit.
Resistances in Series:- When resistors are placed in series:
a. The current through them will be the same.
b. The sum of the potential difference or voltage across them is the total potential difference (i.e)
V = V1 + V2 + V3 = I (R1 + R2 + R3)
c. The equivalent resistance
Rs = R1 + R2 + R3
d. Can be identified when one common point not connected with a third device is seen.
Resistance in Parallel.:- When resistors are connected in parallel:
a. The potential difference across their ends is the same,
b. The sum of the current through them is the current drawn from the source of energy or cell.
c. The equivalent resistance is found by.
1
1
1
1
 
 ......
RP R1 R2 R3
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Electrical Energy:- Since there exists resistance to the flow of current, work has to be done in order to
maintain the flow of current. Since potential difference V is the work done to carry unit positive charge
from infinity to a point, the work done to carry a charge q is given by, Since,
W  qV
q
 q  IT
t
W  ItV
I
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Since
V  IR
W  I 2 Rt  V 2t / R
Also proves the Joule’s law of heating.
Joule’s law of heating: - When a current I flows through a resistor R, heat is produced. The heat
produced H depends directly on the square of the current, resistance and the time t for which the current
is allowed to pass through.
H = I2 Rt, this is called Joule’s law of heating.
Electric Power:- The rate at which electric energy is consumed is called electric power.
Power = Work done / time = W / t.
V 2 qV
VI  I 2 R 

R
t
Power is expressed in joule / second or watt.
When 1 ampere current is passed through a resistor of 1 , the heat energy produced is, 1 joule. If this
flow is maintained for 1 second, the power consumed is 1 watt.
Kilowatt hour:- kWh is the commercial unit for electrical energy.
1 kWh = 1000 Wh
= 1000 x joule x 3600 second
second
= 3.6 x 106 joule
Bulb Rating:- Power and voltage are printed on each electric bulb. If bulb has a 60W – 220V,it means
that when the bulb is operating at 220V, the power dissipated will be 60 watt. i.e. 60 joule of electrical
energy is converted into heat and light per second.