Numerical - Electricity, CBSE Class 10 Physics Video Lecture

hello friends this video on electricity
part 13 is brought to you by example.com
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factual before going ahead with part 13
here somes problem 3 which says two
lamps one rated hundred watts at 220
volts and the other 60 watts at 220
volts are connected in parallel to
electric mains supply okay
so p1 for the first lamp power is 100
watts for the second lamp power is 60
watts the potential difference for the
first one is 220 volts and the potential
difference for the second one is also 3
20 volts now it says that they both are
connected in parallel to electric mains
supply so what current is drawn from the
line if the supply voltage is 220 volts
so let us see how much current will flow
through this first bulb so the current
through this will be equal to p1 divided
by v1 that is because we know that power
is given as the product of potential
difference and current so this will be
equal to hundred divided by 220 amperes
similarly current drawn by the second
lamp will be equal to p2 by V 2 that is
equal to 60 divided by 220 amperes
therefore total current drawn by these
two bulbs will be equal to I 1 plus I 2
so total current drawn will be I 1 plus
I 2 that is equal to 100 by 220 plus 60
by 220 that is equal to 160 by 220
amperes this comes out to be 0.72 7
amperes so this much amount of current
will be drawn from the line let's look
at the next problem it says which uses
more energy a 250 watt television set in
1 hour
a 120 watt toaster in ten minutes so let
us try to calculate the energy for the
first one for the television set power
is given as 250 watts and time is given
as one hour
so one other mean would mean 60 into 60
seconds for the toaster power is given
as 1,200 watts and time is given as 10
minutes 10 minutes could mean 10 into 60
seconds
now we all know that power is nothing
but work done per unit time that means
energy per unit time therefore we can
say that energy is equal to power into
time taken so power is 250 into 60 into
60 so this comes out to be 9 into 10 to
the power 5 joules again in this energy
will be equal to power into time taken
that is equal to 1200 into 10 into 16 so
this comes out to be 7 point 2 into 10
to the power 5 joules so if you compare
the value of e1 with e 2 we see that e 1
is greater than e 2 that means the
television set uses more energy than the
toaster let us look at problem 5 it says
an electric heater of resistance 8 ohms
so the resistance is given as 8 ohms
draws 15 amperes from the service means
in 2 hours so the resistance of the
heated is 8 ohms and the current it
draws is 15 amperes in how much time in
2 hours so we have to calculate the rate
at which the heat is developed in the
heater so in a way we have to calculate
that rate at which the energy is
consumed by the heater and what water's
rate at which energy is consumed that is
nothing but power so that means we have
2 pi
she let power right now convert this
time into seconds it will be 2 into 60
into 60 seconds now what is how do we
calculate power do you know that power
is given by I square into act so we
really don't need this amount of time
for which it takes this much of energy
so power is I square R so I square is
given as 15 square into R is a tones so
this comes out to be 1800 joules per
second so this is the rate at which heat
is developed in that heater two
conducting wires of the same material
and of equal lengths and equal diameters
at first connected in series and then in
parallel in a circuit across the same
potential difference the ratio of heat
reduced in series and parallel
combinations would be which of these
options now we know that heat produced
is given by v square by R into T because
we know that he could as per Jules law
heating heat produced is nothing but V
into I into T so applying Ohm's law we
can write it as v square by ad into T
now let us first look at the series
formulation what would happen in series
combination heat produce is denoted by
HS this will become v square divided by
RS into T what happens in parallel
combination in the parallel combination
HP will be equal to v square divided by
RP into T now let us try to calculate
the value of R s that is the equivalent
resistance in series combination this
will be equal to R plus R because
resistance of both the wires will be
same because they are made up of same
material they have equal lengths and
they have equal diameters because
resistances give
by Rho L by a so if they are made of
same material that means Rho will be
same they are of equal lengths so L will
be same they have of equal diameters
therefore area will also be the same
therefore the resistance of both the
wires will be same so let us assume that
the resistance of each wire is R so
equivalent resistance in series
combination will be equal to 2r
equivalent resistance in parallel
combination will become 1 by r plus 1 by
r this implies ad T will become R by 2
therefore the ratio of the heat produced
in series combination to the parallel
combination will be equal to v square by
RS into T divided by v square by RP into
T so this V Square and T will get
cancelled so we get RP divided by RS now
RP is R by 2 divided by 2 R so our R
will get cancelled and we get 1 by 4 so
H s by H P will be equal to 1 is to 4 so
if you look at it option C will be the
correct option so with this we will
conclude this lesson on electricity so
what all did we study in this lesson we
talked about what is lecture parent what
causes electric current to flow in a
circuit how do we draw circuit diagrams
to represent electric circuits water
resistors how do we combine resistors in
different types of combinations to get a
desired value of resistance to complete
a circuit so these were some of the
things which we discussed here and we
also talked about the heating effects of
electric current thank you please visit
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