# ELECTRICITY-converted.pptx

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ELECTRICITY-converted.pptx

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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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