SSC,CBSE chapter on Electricity

1. Standard/ Grade/ Class 10
Electricity
Gurudatta K Wagh
Standard/ Grade/ Class 10
Electricity
Gurudatta K Wagh

2.  Electric current and circuit
 Electric potential and potential difference
 Ohm's Law
 Factors responsible for the resistance of a conductor
 Resistance of a system of resistors
• Resistors in series
• Resistors in parallel
 Heating effect of electric current
• Practical applications of Joule's heating
 Electric power

3.  Electric current and circuit
If the electric charge flows through a conductor (a
metallic wire) we say that there is an electric
current in the conductor.
A switch makes a conducting link between the cell
and the bulb. A continuous and closed path of
an electric current is called an electric circuit
If the circuit is broken anywhere (or the switch of
the torch is turned off), the current stops
flowing and the bulb does not glow

5. Electric current The amount of charge flowing
through a particular area in unit time, i.e. the
rate of flow of electric charges
In metallic wires, electrons constitute the flow
of charges
Electrons were not known at the time when the
phenomenon of electricity was first observed. So,
electric current was considered to be the flow
of positive charges and
the direction of flow of positive charges was
taken to be the direction of electric current

6. Conventionally, in
an electric circuit
the direction of
electric current
(positive to
negative) is taken
as opposite to the
direction of the
flow of electrons
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

7. Electric circuit

8. Current formula
If a net charge Q flows across any cross-section of
a conductor in time t, then the current I through
the cross-section is
current I = net charge Q/ time t
SI unit electric charge = coulomb 'C' equivalent
to the charge contained in nearly 6 x 1018
electrons
An electron possesses a negative charge of 1.6 X
10-19
C

9. Electron/s Charge (C)
1 1.6 x 10-19
6 x 1018
6 x 1018
x 1.6 X 10-19
= 9.6 x 10-1
Charles-Augustin de Coulomb

10. The chemical action within a
cell generates the potential
difference across the
terminals of the cell, even
when no current is drawn from
it
In order to maintain the current
in a given electric circuit, the
cell has to expend its chemical
energy stored in it

11. SI unit electric current = ampere 'A'
One A is constituted by the flow of one C of
charge per second
1A = 1C/ 1s
André-Marie Ampère
Small quantities of current
milliampere 1mA = 10-3
A
microampere 1µA = 10-6
A

12. Ammeter measures
electric current in a
circuit; always
connected in series in
a circuit
•YouTube video explain working of
ammeter. Add
Instruments used to measure smaller currents,
in the milliampere or microampere range, are
designated as milliammeters or microammeters

13. Flow of charges inside a wire
Electrons are able to travel through a perfect
solid crystal smoothly and easily, as if they were
in a vacuum. The motion of electrons in a
conductor is different from that of charges in
empty space.
When a steady current flows through a
conductor, the electrons in it move with a certain
average drift speed. The drift speed of electrons
for a copper wire carrying a small current is
small, 1mm s-1

14.  Electric potential and potential difference
What makes the electric charge to flow?
The electrons move only if there is a difference
of electric pressure – potential difference –
along the conductor
The potential difference may be produced by a
battery consisting of one or more electric cells

15. Definition Electric potential difference
between two points in an electric circuit
carrying some current = work done to move a
unit charge from one point to the other
Potential difference between two points (V) =
Work done (W)/ Charge (Q)
SI unit volt (V)

16. One volt is the potential difference between two
points in a current carrying conductor when 1
joule of work is done to move a charge of 1
coulomb across one point to other
1 volt = 1 joule/ 1 coulomb
The potential difference is measured by
means of an instrument called the voltmeter.
The voltmeter is always connected in parallel
across the points between which the potential
difference is to be measured

17. Alessandro Giuseppe Antonio
Anastasio Volta
A voltmeter is an
instrument used for
measuring electrical
potential difference
between two points in an
electric circuit.

18. One joule is the equivalent of one watt of power
radiated or dissipated for one second
James Prescott Joule

19.  Ohm's Law
Definition The potential difference across the
ends of a given metallic wire in an electric
circuit is directly proportional to the current
flowing through it, provided its temperature
remains same
V I, V/ I = constant (R),∝ V = IR

20. Relationship between the potential
difference across a conductor and the
current through it
V-I graph is a straight line passing through the
origin. V/I is a constant ratio
Georg Simon Ohm

21. R is a constant for the given metallic wire at a
given temperature and is called resistance.
Resistance The property of a conductor to resist
the flow of charges through it
SI unit = ohm Ω
Ohm's law R = V/ I
•If the potential difference across the two ends of
a conductor is 1 V and the current through it is 1
A, then the resistance R of the conductor is 1 Ω
•1 ohm = 1 volt/ 1 ampere

22. The current through a resistor is inversely
proportional to its resistance. If the
resistance is doubled the current gets halved
A component used to regulate current without
changing the voltage source is called variable
resistance. A rheostat is often used to change
the resistance in the circuit

23. The word rheostat was coined
about 1845 by Sir Charles
Wheatstone. It is a two-terminal
variable resistor. The term
"rheostat" is becoming
obsolete, with the general term
"potentiometer" replacing it
The most common
way to vary the
resistance in a
circuit is to use
a rheostat

24.  Factors responsible for the resistance of a
conductor
The current is different for different
components. Certain components offer an
easy path for the flow of electric current
while the others resist the flow.
The ammeter reading decreases to one-half
(half) when the length of the wire is
doubled.

25. The ammeter reading increases when a
thicker wire of the same material and of the
same length is used
The ammeter reading changes when a wire of
different material of the same length and the
same cross-section is used

26. On applying Ohm's law, it is observed that,
The resistance of the conductor depends
1)on its length,
2)on its area of cross-section, and
3)on the nature of its material
Resistance (R) of a uniform metallic conductor
is directly proportional to its length (l) and
inversely proportional to the area of cross-
section (A)

27. R l and R 1/ A∝ ∝
Hence R l/ A = p l/ A∝
ρ (rho) = constant of proportionality =
electrical resistivity of the material of the
conductor
SI unit = Ω m
It is a characteristic property of the material

28. Metals and alloys have very low resistivity,
range 10-8
– 10-6
Ω m = good conductors of
electricity
Conductor tungsten (ρ = 5.20 x 10-8
) is used
almost exclusively for filaments of electric bulbs,
whereas copper (ρ = 1.62 x 10-8
) and aluminium
(ρ = 2.63 x 10-8
) are generally used for electrical
transmission lines

29. Resistivity (ρ) of an alloy is generally higher
than that of its constituent metals. Alloys do
not oxidise (burn) readily at high temperatures.
For this reason alloys are commonly used in
electrical heating devices like electric iron,
toasters, etc.
Nichrome (ρ = 100 x 10-6
) = Alloy of nickel,
chromium, manganese, and iron
The above values are at temperature 20 °C

30. Insulators
rubber, glass – high resistivity
1012
– 1017
Ω m
Resistance (R) and resistivity of a material (ρ)
vary with temperature

31.  Resistance of a system of resistors
Current through a conductor depends upon its
resistance and the potential difference
across its ends.
Application of Ohm's law to combination of
resistors
Two methods (series and parallel)

32. Series
Parallel

33. • Resistors in series
Having resistances R1, R2 and R3 connected end to
end in series
Value of the current in the ammeter is the same,
independent of its position in the electric
circuit
In a series combination of resistors the
current is the same in every part of the
circuit or the same current through each
resistor

34. Potential difference is equal to the sum of
potential differences V1, V2 and V3
The potential difference across a
combination of resistors in series is equal to
the sum of potential difference across the
individual resistors,
V = V1 + V2 + V3

36. It is possible to replace the three resistors joined
in series by an equivalent single resistor of
resistance R, such that the potential difference
across it, and the current I through the circuit
remains the same,
V = IR
Applying Ohm's law to the three resistors
separately,
V1 = IR1 V2 = IR2 V3 = IR3

37. Hence IR = IR1 + IR2 + IR3
OR
Rs = R1 + R2 + R3
When several resistors are joined in series,
the resistance of the combination Rs equals
the sum of their individual resistances R1, R2,
and R3 and is greater than any individual
resistance

38. • Resistors in parallel
Total current I = sum of the separate currents
through each branch of the combination
I = I1 + I2 + I3
Rp is the equivalent resistance of the parallel
combination of resistors
I = V/ Rp
Applying Ohm's law to each resistor
I1 = V/ R1, I2 = V/ R2, I3 = V/ R3
V/ Rp = V/ R1 + V/ R2 + V/ R3

39. Resistors in parallel

40. The reciprocal of the equivalent resistance of a
group of resistances joined in parallel = sum of
the reciprocals of the individual resistances
Disadvantages of series circuit
In a series circuit the current is constant
throughout the electric circuit. It is obviously
impracticable to connect an electric bulb and an
electric heater in series because they need
currents of widely different values to operate
properly

41. When one component fails the circuit is
broken and none of the components works,
e.g. fairy lights
Advantages of parallel circuit
•Divides the current through the electrical
gadgets
•The total resistance is decreased
•It is helpful particularly when each gadget has
different resistance and requires different
current to operate properly

42.  Heating effect of electric current
A part of the source energy in maintaining the
current may be consumed into useful work
(like in rotating the blades of an electric fan)
Rest of the source energy may be expended in
heat to raise the temperature of the gadget
An electric fan becomes warm if used
continuously for longer time

43. Heating effect of electric current
If the electric circuit is purely resistive, i.e. a
configuration of resistors only connected to a
battery; the source energy continually gets
dissipated entirely in the form of heat, e.g.
electric heater, electric iron, boiler, geyser

44. Current I, Resistor of resistance R, Potential
difference V, time t, charge Q
Work done P = VQ
Power input in time t, P = V x Q/t = VI
Energy supplied to the circuit by the source in
time t = P x t = VI x t
Energy expended by the source gets dissipated
in the resistor as heat
For a steady current I, the amount of heat H
produced in time t, H = VIt

45. Applying Ohm's law,
H = I2
Rt Joule's law
Heat produced in a resistor is 1) directly proportional to
the square of current for a given resistance, 2) directly
proportional to resistance for a given current, and 3)
directly proportional to the time for which the current
flows through the resistor
H = I2
Rt is used after calculating the current through it,
using the relation I = V/ R
Heat energy is expressed in calories, 4.18 J = 1 calorie
H = I2
Rt/4.18 cal = V2
t/4.18R cal = VIt/4.18 cal

46. • Practical applications of Joule's heating
The generation of heat in a conductor is an
inevitable consequence of electric current. In
many cases it is undesirable as it converts
useful electrical energy into heat. The
unavoidable heating can increase the
temperature of the components and alter their
properties
Heating effect of electric current has many useful
applications. E.g. iron, toaster, oven, kettle,
heater

47. Electrical heating is also used
to produce light, as in a bulb.
The filament must retain as
much of the heat generated as
is possible, so that it gets very
hot and emits light. It must not
melt
A strong metal with high melting point such as
tungsten (melting point 3380 °C) is used for
making bulb filaments. The filament should be
thermally isolated as much as possible, using
insulating support

48. The bulbs are usually
filled with chemically
inactive nitrogen and
argon gases to prolong
the life of the filament
Most of the power
consumed by the
filament appears as
heat, but a small part of
it is in the form of light
radiated

49. Fuse used in electric circuits is a
common application of Joule's
heating. The fuse protects
circuits and appliances by
stopping the flow of any unduly
high electric current. The fuse is
placed in series with the
device.
If a current larger than the specified value flows through
the circuit, the fuse wire melts due to heating
Fuses for domestic purposes are rated 1 A, 2 A, 3 A, 5 A,
10 A etc. For an electric iron which consumes 1 kW
electric power when operated at 220 V, a current 4.54 A
flows in the circuit. Here a 5 A fuse must be used.

50.  Electric power
Rate of doing work is power = rate of
consumption of energy
H = I2
Rt gives the rate at which electric energy is
dissipated or consumed in an electric circuit =
electric power
Power P is given by P = VI
OR
P = I2
R = V2
/ R
SI unit watt (W)

51. Watt Power consumed by a device
that carries 1 A of current when
operated at a potential difference of
1 V
1 W = 1 VA
James Watt
Unit watt is very small
In actual practice a larger unit called kilowatt
(kW = 1000 W) is used

52. Unit of electric energy watt hour (Wh)
One watt hour is the energy consumed when 1
watt of power is used for 1 hour
The commercial unit of electric energy is kilowatt
hour (kWh)
1 kWh = 1000 watt x 3600 second
= 3.6 x 106
watt second
= 3.6 x 106
joule (J)

53. THANK YOU
SSC Std 10th
Textbook
CBSE Std 10th
Textbook
YouTube
Google
Wikipedia
Suggestions and Appreciations welcome
gkwagh@gmail.com