for CBSC and State students

2. CONTANT:
1. ELECTRIC CURRENT
2. ELECTRIC CIRCUIT
3. ELECTRIC POTENTIALAND POTENTIAL DIFERENCE
4. SYMBOLS OF COMPONENTS USED IN ELECTRIC
CIRCUITS
5. OHM’S LAW
6. RESISTANCE
7. ELECTRICAL ENERGYAND ELECTRIC POWER
8. HEATING EFFECT OF ELECTRIC CURRENT

3. Electric current :- is the flow of electrons through a conductor.
The device which causes the flow of electrons through a conductor is
called a cell.
Electrons flow from the negative terminal to the positive terminal.
Electric current flows from the positive terminal to the negative
terminal.
This is called conventional current.
Electric current is expressed as :- The rate of flow of charges through a
conductor or the quantity of charges flowing through a conductor in
unit time. Q
I =
I – current
t Q – quantity of charge
t – time
The SI unit of electric charge is coulomb (C). It is the charge contained
in 6x10 18 electrons.
The SI unit of current is called ampere (A). By Andre-Marie ampere
(1775-1836) . Its scalar quantity.
One ampere is the current flowing through a conductor if I coulomb of
charge flows through it in 1 second.
1coulomb
1ampere =
1 second
Electric Current is measured by an ammeter. It is always connected in series
in a circuit.

4. • Small quantities of current are expressed in milliamperare (1mA=𝟏𝟎−𝟑
𝑨)
or in microampere (1 μA=𝟏𝟎−𝟔
A)
• An instrument called ammeter measures electric current in circuit.
• The electric current flows in the circuit from the positive terminal of the
cell to the negative terminal of the cell through the bulb and ammeter. As
shown in below figure.
• A body said to be negatively charged, if it gains electrons.
• A body said to be positively charged, if it loses electrons.
• The total charge acquired by a body is an integral multiple of charge on a
single electron. This principle is called quantisation of charge.
1. Magnitude of charge on one electron 𝒆 = −𝟏. 𝟔 × 𝟏𝟎−𝟏𝟗
C.
2. Charge on 𝒏 electrons, 𝒒 = 𝒏𝒆 = 𝒏 × 𝟏. 𝟔 × 𝟏𝟎−𝟏𝟗
C.
𝒏=( …..-2,-1,0,1,2……)
3. If q amount of charges flows through a conductor in t time, then
𝑰 =
𝑪𝒉𝒂𝒓𝒈𝒆(𝒒)
𝑻𝒊𝒎𝒆(𝒕)
since 𝒒 = 𝒏e , where n is number of electron
flowing through conductor.
4. Magnitude of charge on one proton + 𝟏. 𝟔 × 𝟏𝟎−𝟏𝟗
C.

5. 2. ELECTRIC CIRCUIT
A closed and continuous path through which electric current flows
known as electric circuit.
A schematic diagram of an electric circuit comprising of a cell,
electric bulb, ammeter and plug key. All connected through
connecting wires. These wires are generally made of copper.
1. When key open----circuit is called open circuit----current not flow through circuit
2. When key Closed --- circuit s called closed circuit----- current flow through circuit
A
+
-
- +
bulb
cell
ammeter
plug key

6. *. The direction of electric current is taken as opposite to the
direction of the flow of electrons (negative charges). In an electric
circuit the current flows from positive terminal of the cell to
negative terminal.
* The electron moves in a drift speed (constant speed) is the order
of 𝟏𝟎−𝟒 𝒎
𝒔

7. Electric current will flow through a conductor only if there is a difference in
the electric potential between the two ends of the conductor. This difference in
electric potential between the two ends of a conductor is called potential
difference.
The potential difference in a circuit is provided by a cell or battery. The
chemical reaction in the cell produces a potential difference between the two
terminals and sets the electrons in motion and produces electric current.
Potential difference :- between two points A and B of a conductor is the
amount of work done to move a unit charge from A to B.
Work done W
Potential difference = or V =
Charge Q
The SI unit of potential difference is volt (V). By Alessandro Volta, It’s a
scalar quantity.
One volt is the potential difference when 1 joule of work is done to move a
charge of 1 coulomb from one point to the other.
1 joule 1J
1 volt = or 1 V =
1 coulomb 1C
Potential difference is measured by a voltmeter. It is always connected in
parallel across the two point between which the potential difference is to be
measured.

8. Note: 1Volt=
1 𝑗𝑜𝑢𝑙𝑒
1 𝑐𝑜𝑢𝑙𝑜𝑚𝑏
1V=
1𝐽
1𝐶
1V= 1J𝑐−1
Smaller units of electric potential,
1 mV = 10−3𝑉, 1μV = 10−6 V
Larger units of electric potential,
1 kV = 103
V, 1MV=106
V

9. circuits :-
An electric cell A battery or combination
of cells
Plug key or switch
(open)
Plug key or switch
(closed)
Electric bulb A resistor of
resistance R
Variable resistance
or rheostat
or
Ammeter Voltmeter
A wire joint A wire crossing over
without joining
A v
+ - -
+
+ - + -

12. Ohms law is a relationship between the potential difference V
across a conductor and the current I flowing through it.
Ohm’s law states that :-
‘The current flowing through a conductor is directly proportional to the
potential difference between its ends provided its temperature remains
constant.’ V V
I α V or V α I or = constant or = R
I I
Where R is a constant called resistance for a given metallic wire at a
given temperature.
Verification of Ohm’s law :-
V
A
+ -
+ -
+ -
R
A B
K
( )

13. Set up the circuit as shown in the circuit diagram. First use one cell
and note the current (I) in the ammeter and the potential difference (V)
in the voltmeter across the nichrome wire AB. Repeat by using two
cells, three cells and four cells and note the readings in the ammeter
and voltmeter. Then plot a graph between the current (I) and potential
difference (V). The graph will be a straight line.
This shows that the current flowing through a conductor is directly
proportional to the potential difference across its ends.
V
I α V or V α I or = R
I
where R is a constant called resistance of the conductor.
Potential difference ( V )
Current ( I )

14. Note: 1. From the above formula, it is clear that
current is inversely proportional to its resistance. If
resistance is doubled, then current gets halved (divided
into two part) and if resistance is halved, then current
gets doubled.
2. The conductor which obey ohm’s law are called
ohmic conductors which do not obey ohm’s law are
called non-ohmic conductors.

15. Resistance is the property of a conductor to resist the
flow of current through it.
V
According to Ohm’s law R =
I
The SI unit of resistance is ohm (Ω).
If the potential difference across the two ends of a wire
is 1 V and the current flowing through it is 1 A then the
resistance R of the conductor is 1 ohm (1 Ω ).
V
Since I =
R
*The current flowing through a resistor is
inversely proportional to the resistance.
So if the resistance is doubled, then the current gets
halved.
1Ω=
1𝑉
1𝐴
= 1 𝑉𝐴−1

16. The resistance of a conductor depends upon its:-
i) Length of the conductor.
ii) Area of cross section of the conductor.
iii) Nature of the Material of the conductor.
Resistance R is directly proportional to the length l of the
conductor and inversely proportional to the area of cross section A
of the conductor, the resistance of a conductor depends on the
nature of the material of which it is made
R α l ---------- 1
R α I /A
or R α l
A------------- 2 from 1 and 2 we get
or R = ρ l
A
Where ρ (rho) is a constant of proportionality called Resistivity or
specific resistance of the material of the conductor.
The SI unit of resistivity is ohm meter ( Ωm).
*Conductors like metals and alloys have low resistivity 10-8
Ωm to 10-6 Ωm. Eg: silver
*Insulators like rubber, glass etc. have high resistivity 1012 Ωm
to 1017 Ωm. Eg: rubber, wood,

17. *Poor conductor material which offers higher resistance
then conductor. Eg; lead, steel, alloy of iron and chromium,
glasses.
• Resistance: A component is an electric circuit which
offers resistance to the flow of electrons constituting
electric current is known as resistor. These are used to
make those electrical devices, where high resistance is
required. It reduces current in a circuit.
• Resistivity: It is defined as the resistance of a conductor of
unit length and unit area of cross-section. Its SI unit Ohm-
meter.
• The resistivity of a material does not depend on its length
or thickness but depends on the nature of the substances
and temperature. It is a characteristic property of the
conductor and varies only, if its temperature changes.
• Rheostat/Variable resistance: it is a variable resistor,
which is used to control the flow of electric current by
manually increasing or decrescring the resistance.

18. When three resistors R1, R2 and R3 are connected in series acrossAB
i) The current in all the resistors is the same.
ii) The total voltage (PD) across the resistors is equal to the sum of the
voltage across each resistor.
V = V1 + V2 + V3
iii) The eqvivalent resistance is the sum of the resistances of each
resistor.
RS = R1 + R2 + R3
( ) A
R3
+ +
-
-
R1
V1
V3
A B
R2
V2
V
+ -
V K

19. An applied potential V produces current I in the resistors
and R1, R2, and R3 causing a potential drop V1, V2, and V3
respectively, through each resistor
Total potential V= V1+ V2 + V3
By ohms law , V1=IR1, V2=IR2, V3=IR3
Thus V=V1+ V2 + V3 = IR1+IR2+IR3
V= I(R1+R2+R3)
If R is the equivalent resistance and V=IR
Hence, IR= I(R1+R2+R3)
R= R1+R2+R3
some Imp points regarding series combination of resistor
1. The equivalent resistance is equal to the sum of the
individual resistances
2. The current through each resistor is same
3. The potential difference across each resistor is different
4. Main disadvantage of this is: In the combination, if any
of the combination fails to work , then the circuit will
break and none of the components will work.

21. When three resistors R1, R2 and R3 are connected in parallel acrossAB,
i) The voltage (PD) in all the resistors is the same.
ii) The total current in all the resistors is the sum of the current in
each resistor. I = I1 + I2 + I3
iii) The reciprocal of the equivalent resistance is the sum of
the reciprocals of each resistance.
1 1 1 1
= + +
Rp R1 R2 R3
R1
R2
R3
( ) A
I1
I2
I3
+
+
-
K
-
A B
V
+ -
V

22. An applied potential V produces current I1 In
R1, I2 in R2 and I3 in R3.
Total current, I= I1+I2+I3
By ohm’s law I1=V/R1, I2=V/R2 and I3=V/R3
If R is the equivalent resistance, then I=V/R
Thus V/R = V/R1+V/R2+V/R3
V/R = V(1/R1+1/R2+1/R3)
1/R = 1/R1 + 1/R2 + 1/R3
1. The reciprocal of equivalent resistance is equal to the sum
of the reciprocal of individual resistances.
2. The potential difference across each resistor is same.
3. The current from the source is grater than the current
through either resistor.

24. i) Electrical energy :- is the work done to maintain the flow of
current in a conductor.
I = Q / t Q = I X t
V = IR
W = Q X V
W = I X t x V
W = I2Rt
The unit of electrical energy is joule (J).
ii) Electric power :- is the rate at which electric current is used.
Power = Work done P = W W = I2Rt = I2Rt
Time
Power = I2R
t t
R = V = I2 X V = I X V
I I
or Power = I X V
The SI unit of power is watt (W).
One watt is the power when 1A of current flows across a potential
difference of 1V. That is 1W=1Vx1A=1VA
1000 W = 1kW 1kWh = 1000 watt x 3600 seconds = 3.6 x 106 joules
The commercial unit of power is watt hour (Wh) or kilo watt hour (kWh).
One kWh is the power consumed when 1W of power is used for 1 hour.
If I=V/R
then
P=𝑣2/R

25. If a current I flows through a resistor of resistance R and t be the
time for which a charge Q flows through it, then the work done to
move the charge through potential difference V
W = Q X V
P = W = Q X V
t t
Q = I or P = V X I
t
or Heat energy supplied = P X t = V X I X t
According to Ohm’s law V = IR electrical energy consumed is
converted into heat energy.
Heat produced H = I2Rt
Thus, This is known as Joule’s law of heating.
( ) A
V
R
A B
I I
+ -
+ -
- +

26. Practical application of heating effect of electric current.
when electric current is allowed to flow through a conductor, it gets
heated. This phenomena is called heating effect of electric current.

29. GO FOR GREEN ELECTRICITY
References:
1.NCERT text book
2.Google source
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