# BHARDWAJ CLASSES

## Wednesday, October 25, 2017

The chapter Electricity in class X PHYSICS covers the topics like electric charges, electric current,electric potential,ohm's law, resistances ,electric power and electric energy. Here we are going to provide Important definitions and short notes of each topic and  a sheet of questions for practice which would cover each and every topic of the chapter.

ELECTRICITY - IMPORTANT DEFINITIONS

Current- Rate of flow of charge is called Current.S.I. unit of electric current is Ampere. Conventionally, direction of current is opposite to the direction of flow of electrons.
I = Q / t
One Ampere - The current is said to be 1 ampere when one coulomb of charge is passed per second through a conductor.
Ammeter- It is the device used to measure electric current in the electric circuit. It is always connected in series in the circuit.
Electric circuit -  a closed path of electric current is called electric circuit.
Potential difference - It is defined as the work done to move a unit charge from one point to another. Its S.I.  unit is volt (V).
Potential difference (V) = work done(W) / charge(Q)
One Volt - 1 volt is the potential difference between two points in a  current carrying conductor when 1 joule of work is done to move the charge of 1 coulomb from one point to another.
1 V = 1 J/C
Voltmeter -  It is a device used to measure potential difference. It is always connected in parallel across the points between which the potential difference is to be measured.
Galvanometer - It is a device used to detect current in the circuit. It is always connected in series.
Ohm's law -  It states that “ the potential difference across the conductor is directly proportional to the electric current, provided physical conditions( temperature, pressure etc) remains same.
V ∝ I
V = I R , where R is proportionality constant called Resistance.
Resistance -  It is the property of a conductor to resist the flow of charges through it. Its S.I. unit is Ohm(Ω).
One Ohm - The resistance of a conductor is 1 Ohm if 1 ampere current is passed across the two ends of conductor having potential difference 1 volt.
Factors on which resistance of conductor depends-
(i) Resistance is directly proportional to the length of the wire . R∝ l
(ii) Resistance is inversely proportional to the area of cross section i.e. a  thin wire will have more resistance then a thick wire.  R ∝ A
(iii) When we increase the temperature, resistance of conductor increases.
R ∝ l/A
R = ρ l / A
Where ρ is called resistivity or specefic resistance of material.
Resistivity -  It is defined as the  resistance of a wire having length 1 m and area of cross section 1 m 2. S.I. unit of resistivity is Ω m.
Combination of Resistors:
(i) Series Combination - If two or more resistors are connected in series combination,equivalent resistance of combination - R = R 1 + R2 + R3 ……
(ii) Parallel Combination - If two or more resistors are connected in parallel combination, equivalent resistance of combination - R = 1/R1 + 1/R2 + 1/R3 …….
Joule’s law of heating- It states that heat produced in a resistor is -
(i) directly  proportional to the square of current
(ii) directly proportional to resistance
(iii) directly proportional to time for which current flows through a resistor.
H = I2Rt
Practical applications of heating effect of current -

•  In bulb, heating effect of current in filament produces light.
•  Fuse in electric circuit is most important application of heating effect it protects circuit from over voltage and high current.
•  The electric iron, electric toaster, electric kettle, electric heater all are based on heating effect of current.
Electric power -  The rate of doing work is called power. s i unit of power is watt (W).
P = V x I
P = V x (V/R) = V2 / R
P = (IR) x I = I2 R
Watt - It is the power consumed by a device that carries 1 A of current when operated at potential difference of 1 V.
Commercial unit of Electrical Energy - Commercial unit of electrical energy is kilowatt hour (KWh).
1 KWh = 1 KW x 1 hour
1 KWh = 1000 W x 3600 seconds
1 KWh = 3.6 x 106 J