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ELECTRICITY
Electricity
and
its Effect
(notations)
Physical
Quantity
Symbols
SI
unit
Voltage (potential difference)
V
Volt
(V)
Power
P
Watt
(W)
Charge
Q
Coulomb (C)
Work or
Energy
W
Joule
(J)
Resistance
R
Ohm
(
)
Current
I
Ampere
(A)
Resistivity
Ohm metre
(
m)
Laws
of
electric
forces:
(i)
Like
charges repel and
unlike
charges
attract each
other.
(ii)
Charges
of
a
conductor reside
on
its outer
surface.
Current:
The
rate
of
flow
of
charges
(Q)
through
a
conductor is
called current
(I) and
is
given
by.
Current =
charge
or
I
Q
. The
SI unit
of
current is ampere
(A).
Time
t
1
Ampere
1
coulomb
1
second
The
current
flowing
through
a
circuit
is
measured
by
a
device
called
ammeter
.
Ammeter
is
connected
in
series
with
the
conductor
.
The
direction
of
the
current
is
taken
as
the
direction
of
the
flow
of
positive
charge
and
opposite
to
the
flow
of
electrons
through
the
conductor
.
Electric cell:
It is
the
simplest form
of
arrangement
to
maintain
a
constant potential
difference
between two
points.
Electromotive force:
The potential difference
at
the
terminals
of
cells
in
an open
circuit is called electromotive force (emf)
and
is denoted
by
letter
E.
Potential
difference
is
the
work
done
in
bringing
a
unit
charge
from
one
place
to
another.
Potential Difference
charge
1Coulomb
(C)
work
1Joule
(J)
,
1
Volt
(V)
Ohms
law:
At
any
constant temperature
the
current (I) flowing
through a conductor
is
directly
proportional
to
the potential difference
(V)
across
it.
Mathematically,
I
V
vice

versa
V
I
I
R
or
V =
RI
R
V
,
I
V
where
R
–
Resistance,
V
–
Voltage (P.D.),
I
–
Current
Symbols
of
a
few
commonly used
components
in
Circuit
Diagrams
Component
Symbol
Component
Symbol
An
electric
cell
Electric
bulb
Battery
of
cells
A
resistance
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Plug
key or
switch
(open)
or
Variable
resistance
(Rheostat)
or
A
closed
plug
or
switch
or
Ammeter
+
A
–
A
wire
joint
Voltmeter
+
V
–
Wires
crossing
Galvanometer
+
G
–
Resistance
:
Resistance
is
a
property
of
a
conductor
by
virtue
of
which
it
opposes
the
flow
of
electricity
through
it
.
Resistance
is
measured
in
Ohms
(
)
.
Resistance
is
a
scalar
quantity
.
Conductor
:
Low

resistance
material
which
allows
the
flow
of
electric
current
through
it
is
called
a
conductor
.
All
metals
are
conductors
except
Hg
and
Pb
etc
.
Resistor
:
High

resistance
materials
are
called
resistors
.
Resistors
become
hot
when
current
flows
through
them
(nichrome
wire
is
a
typical
resistor)
.
Insulator
:
A
material
which
does
not
allow
heat
and
electricity
to
pass
through
it
is
called
an
insulator
.
Rubber,
dry
wood
etc
.
,
are
insulators
.
Equivalent
Resistance
:
A
single
resistance
which
can
replace
a
combination
of
resistances
such
that
current
through
the
circuit
remains
the
same
is
called
equivalent
resistance
.
Law
of
Combination
of
Resistances
in
Series
:
When
number
of
resistances
are
connected
in
series,
the
equivalent
resistance
is
equal
to
the
sum
of
the
individual
resistances
.
V
V
1
V
2
V
3
V
IR
,
V
1
IR
1
,
V
2
IR
2
,
V
3
IR
3
IR
IR
1
IR
2
IR
3
R
R
1
R
2
R
3
.....
R
n
Things
to
remember
in
series
connection
(a)
When
a
number
of
resistances
are
connected
in
series,
the
equivalent
or
resultant
resistance
is
equal
to
the
sum
of
individual
resistances
and
resultant
resistance
is
greater
than
any
individual
resistance
.
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(b)
If
n
resistances
each
of
value
R
are
connected
in
series,
the
equivalent
resistance
R
e
is given
by:
R
e
=
R
+
R
+
R
..........
n
times
R
e
=
nR
R
e
=
Number
of
resistors
×
resistance
of
each
resistor
(c)
Equal
current
flows
through
each
resistance
and
it
is
also
equal
to
the
total
current
in
the
circuit
.
This
is
because
there
is
no
other
path
along
which
the
current
can
flow
.
(d)
The
potential
difference
across
the
ends
of
the
combination
is
distributed
across
the
ends
of
each
of
the
resistances
.
The
potential
difference
across
any
one
of
the
resistances
is
directly
proportional
to
its
resistance
.
(e)
The
equivalent
resistance
when
used
in
place
of
the
combination
of
resistances
produces
the
same
current
with
the
same
potential
difference
applied
across
its
ends
.
(f)
When
two
or
more
resistances
are
joined
in
series,
the
result
is
the
same
as
increasing
the
length
of
the
conductor
.
In
both
cases
the
resultant
resistance
is
higher
.
(g)
In
a
series
combination,
the
equivalent
resistance
is
greater
than
the
greatest
resistance
in
the
combination
.
Law
of
Combination
of
Resistances
in
Parallel
:
If
resistance
R
1
,
R
2
,
R
3
,
.....
etc
are
connected
in
parallel
then
the
equivalent
resistance
(
R
)
is
given
by
I
I
1
I
2
I
3
I
V
,
I
1
V
,
I
2
V
,
I
3
V
R
R
1
R
2
R
3
1
R
n
1
.....
V
V
V
V R
R
1
R
2
R
3
1
1
1
R
R
1
R
2
R
3
Things
to
remember
in
parallel
connection
(a)
When
a
number
of
resistances
are
connected
in
parallel,
the
reciprocal
of
the
equivalent
or
resultant
resistance
is
equal
to
the
sum
of
reciprocals
of
the
individual
resistances
and
is
always
smaller
than
the
individual
resistances
.
This
is
because
there
are
a
number
of
paths
for
the
flow
of
electrons
.
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(b)
If
there
are
n
resistances
connected
in
parallel
and
each
resistance
has
a
value
of
R
1
1
1
..........
n
times
1
R
e
R
R
R
1
n
R
e
R
n
R
e
R
R
e
Resistance
of each
resistor
number
of
resistors
(c)
The
potential difference
across each
resistance is
the
same
and
is equal
to the total
potential difference
across the
combination.
(d)
The
main current divides itself
and a
different current flows
through each
resistor.
The
maximum
current flows through
the
resistor having minimum resistance
and vice
versa.
(e)
If
an
equivalent resistance
R
e
is connected in place
of
combination, it produces
the
same current for
the same
potential difference applied
across
its
ends.
(f)
In a
parallel combination,
the
equivalent resistance is lesser
than the
least
of
all
the
resistances.
(g)
If
two resistances
R
1
and
R
2
are connected
in parallel
then
R
e
R
1
R
2
R
1
R
2
1
1
1
R
1
R
2
R
1
R
2
Sum
of
two
resistances
R
1
R
2
Product
of
two
resistances
R
e
(h) If
there are
n
resistors each
of
resistance
R
.
Let
R
S
be the
resultant resistance
of
series
combination
and
R
p
be the
resultant resistance
of
parallel
combination.
Then,
R
S
=
nR
n
R
p
=
R
R
p
R
/
n
n
2
.
R
S
nR
Electrical
energy:
Capacity
of the
flowing electricity
to do
work is called its electrical
energy.
R
V
2
t
Electrical
energy
(work)
=
V
I t
I
2
Rt
Pt
The
SI
unit
of
electrical
energy
is
Joule
.
One
Joule
is
the
amount
of
energy
consumed
when
an
electrical
appliance
of
one
watt
rating
is
used
for
one
second
.
The
commercial
(practical)
unit
of
electrical
energy
is
kilowatt

hour
(kWh)
.
R
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t
V
2
2
Power,
P
W
VI
I
R
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The
SI unit
of
electric
power
is
watt (W).
The
power
of
a
machine doing work
at
the
rate of 1
Joule
per
second
is equal to one
watt.
Electrical
energy =
Electrical power
×
Time.
Important
Formulae:
1.
Coulomb’s
law
F
K
q
1
q
2
(
k
is constant
of
proportionality)
r
2
q
1
and
q
2
=
two electric
charges
r
=
distance between two
electric
charges
F
=
Force
2.
V
W
;
W
V
Q
;
Q
W
Q
V
V
p
.
d
.
W
=
work done,
Q
=
Quantity
of
charge
transferred
3.
V
R
I
;
R
V
;
I
V
I
R
V
=
pd
;
R
=
Resistance,
I
=
current.
4.
R
I
;
R
A
A
l
R
=
Resistance;
l
=
length;
A
= Area of
cross section;
=
rho,
a
constant known
as
resistivity
5.
Series
combination
R
R
1
R
2
R
3
.....
R
n
6.
Parallel
combination
1
R
n
R
R
1
R
2
R
3
1
.....
1
1
1
For
equal
resistances
Rs
nR
(For
series
connection)
n
Rp
R
(For
parallel
connection)
Rs
n
2
Rp
Rs
=
Effective resistance
in
series
Rp
=
Effective
resistance in
parallel
n
=
number
of
resistors
R
=
Resistance
of
each
resistor
7.
P
W
;
Power
work
Energy
consumed
t
time
Time
W
=
V
×
I
×
t
;
Power
=
potential difference
×
current
×
time
8
.
2
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R
(
W
I Rt
)
W
V
2
t
9.
P
=
V
×
I
;
Power
=
potential difference
×
current
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10.
P
I
2
R
;
Power
= (current)
2
resistance
11.
V
2
R
resistance
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(potential
difference
)
2
P
;
Power
12.
Electric energy
=
P
×
t
;
electric energy
=
power
×
time
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