Electric current, potential difference and electric current. Ohm’s law; Resistance, Resistivity,
Factors on which the resistance of a conductor depends. Series combination of resistors,
parallel combination of resistors and its applications in daily life. Heating effect of electric
current and its applications in daily life. Electric power, Interrelation between P, V, I and R
1. Electric current, potential difference and
electric current. Ohm’s law; Resistance,
Resistivity,
Factors on which the resistance of a conductor
depends. Series combination of resistors,
parallel combination of resistors and its
applications in daily life. Heating effect of
electric
current and its applications in daily life. Electric
power, Interrelation between P, V, I and R
Revised syllabus -CORONA
PANDEMIC
2020
Effects of Current
2. What is Current Electricity?
Any motion of charges from one section to
another is current. When two bodies at
different potentials are linked with a wire,
free electrons stream from B to A, until
both the objects reach the same potential,
after which the current stops flowing. Until a
potential difference is present throughout a
conductor, current runs. The division of
physics that deals with charges in motion is
termed as current electricity.
14. 3a) Electric circuit :-
Electric circuit :- is a continuous and closed path of an electric
current.
A schematic diagram of an electric circuit comprising
of a cell, electric bulb, ammeter and plug key.
A
+
-
+-
bulb
cell
ammeter
plug key
15. b) Symbols of components used in electric
circuits :-
An electric cell A battery or combination
of cells
Plug key or switch Plug key or switch
(open) (closed)
Electric bulb A resistor of
resistance R
Variable resistance or
or rheostat
Ammeter Voltmeter
A wire joint A wire crossing over
without joining
A v+ - -+
+ - + -
16.
17.
18. Types of Electric Charges
• There are two types of electric charges :
positive charges and negative charges.
• (i) Opposite charges (or Unlike charges) attract
each other. For example, a positive charge
attracts a negative charge.
• (ii) Similar charges (or Like charges) repel each
other. For example, a positive charge repels a
positive charge; and a negative charge repels a
negative charge.
20. 1) Electric current :-
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. I= Q
t I – current
Q – quantity of charge
Here (Q=ne)
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).
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.
21. Calculate the
number of
electrons
constituting one
coulomb of charge.
• Solution.
• We know that the charge of an
electron is 1.6 × 10–19 coulomb
(or 1.6 × 10–19 C). [ hint: use Q=ne]
• Now, If charge is 1.6 × 10–19 C,
No. of electrons = 1
• If charge is 1 C, then No. of
electrons =Q/e
• = 6.25 × 1018 electrons
(NCERT
Book
Question)
Q1
22. An electric bulb
draws a current of
0.25 A for 20
minutes. Calculate
the amount of
electric charge that
flows through the
circuit
Solution.
Here, Current, I =0.25 A
Charge, Q = ? (To be calculated)
And Time, t = 20 minutes
t= 20 × 60 seconds = 1200 s
We know that, I =Q/t
So, 0.25 =Q/1200
Q = 0.25 × 1200 C
Q = 300 C
Thus, the amount of electric
charge that flows through the
circuit is 300 coulombs
(Lakhmir
singh Book
Question)
Q1
24. Electromotive Force (EMF) and
Voltage:
Just as a liquid in a horizontal tube does not
flow, no current runs in a copper cable by
itself. Water will flow out of the other end
at a steady rate if one end of the tube is
attached to a tank with water such that
there is a pressure difference amidst the two
ends of the horizontal tube.
25. There is a pressure
difference amidst
the two ends of
the horizontal
tube.
There is a
difference of
potential between
two ends of the
cell
26. emf [ electro motive force]
an electric motor pumps water from high pressure to low pressure through
water tank
similarly an electron motor pumps the electron from -ive chamber to +
chamber throgh bulb
27. Electric potential
and
Potential difference
• The electric potential (or
potential) at a point in an electric
field is defined as the work done
in moving a unit positive charge
from infinity to that point.
• Potential is denoted by the
symbol V and its unit is volt.
• A potential of 1 volt at a point
means that 1 joule of work is
done in moving 1 unit positive
charge from infinity to that
point. Since the unit of charge is
coulomb,
• The difference in electric
potential between two
points is known as
potential difference.
• The potential difference
between two points in an
electric circuit is defined
as the amount of work
done in moving a unit
charge from one point to
the other point.
28. If W joules of work has to be done to move Q
coulombs of charge from one point to the other point,
then the potential difference V between the two
points is given by the formula :
• Potential difference, V =W/Q
where W = work done and Q = quantity of charge moved
• The SI unit of potential difference is volt which is denoted by
the letter V. The potential difference is also sometimes
written in symbols as p.d.
• The potential difference between two points is said to be 1
volt if 1 joule of work is done in moving 1 coulomb of electric
charge from one point to the other.
• 1 V = 1 J C–1
29. How much work is done
in moving a charge of
2 coulombs from a
point at 118volts to a
point at 128 volts ?
• V =W/Q
Here, Potential difference, V
= 128 – 118 = 10 volts
• Work done, W = ? (To be
calculated)
Charge moved, Q = 2 C
• Putting these values in the
above formula, we get :
10 =W/2
W = 10 × 2
Thus, Work done, W = 20
joules
(Lakhmir
singh Book
Question)
Q2
32. 4) Ohm’s law :-
Ohms law is a relationship between the potential difference across a conductor
and the current 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.’
I α V or V α 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
K
A B
( )
33.
34. 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 )
35. 5a) Resistance :-
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.
38. RESISTANCE AND RESISTIVITY
factors on which resistance depends:-
• Length of the conductor.
• R∝ l
• Cross-sectional area of conductor. R∝(1/A).
• Nature of the material of conductor .ρ
• Temperature of the conductor.
39. b) Factors on which the resistance of a conductor
depends :-
The resistance of a conductor depends upon its:-
i) Length
ii) Area of cross section
iii) Material of the conductor.
Resistance is directly proportional to the length of the conductor and
inversely proportional to the area of cross section of the conductor.
R α l
R α I /A
or R α l
A
or R = ρ l
A
Where ρ (rho) is a constant of proportionality called Resistivity 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.
Insulators like rubber, glass etc. have high resistivity 1012 Ωm to 1017 Ωm.
40. Resistance and Resistivity
If the physical conditions i.e. temperature,
mechanical strain, etc. remain constant It was
discovered through experimentation that the
current travelling through a conductor is directly
proportional to the potential difference V in
between its two ends.
Inside the conductor, the electric charge is nil
everywhere, and thus, there is no current in
electrostatic situations. This does not necessarily
convey that all charges inside the conductor are
at rest.
48. 6a) Resistors in series :-
When three resistors R1, R2 and R3 are connected in series across AB
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
R1 R2 R3
+ +--
V1 V2 V3
A B
V
+ -
49.
50. b) Resistors in parallel :-
When three resistors R1, R2 and R3 are connected in parallel across AB,
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
R3
R2
( ) A
I1
I2
I3
+-+ -
A B
V
+ -
52. NUMERICALS
• Sample Problem 1. Potential difference
between two points of a wire carrying 2
ampere current is 0.1volt. Calculate the
resistance between these points.
• Solution- From Ohm’s law we have :
R = V/I
Resistance, R = 0.05 ohm
53. Sample Problem 2. A simple electric circuit has a 24 V
battery and a resistor of 60 ohms. What will be the
current in the circuit ? The resistance of the connecting
wires is negligible.
Solution - Using Ohm's law
V=IR
I = V/R
I=24/60
I= 0.4 A
54. 7) Electrical energy and Electric power :-
i) Electrical energy :- is the work done to maintain the flow of
current in a conductor.
W = Q X V I = Q / t Q = I X t
W = I X t x V V = IR
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 t t
Power = I2R 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.
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.
55. 8) Heating effect of electric current :-
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 Q = I or P = V X I
t t t
or Heat energy supplied = P X t = V X I X t
According to Ohm’s law V = IR
Heat produced H = I2Rt
( ) A
V
R
A B
I I
+ -
+ -
+-
