1/R
p
= 1/R
1
+ 1/R
2
+ 1/R
3
v) Equivalent resistance is less than the smallest individual resistance.
25. Joule’s law of heating:
It states that the heat produced in a conductor is directly
proportional to (i) the square of the current I through it (ii) proportional to its
resistances R and
(iii)
the time t for which current is passed. Mathematically, it can be expressed as
H = I
2
Rt
joule = I
2
Rt/4.18 cal
or
H = VIt
joule = VIt/4.18cal
26. Electric energy:
It is the total work done in maintaining an electric current in an
electric circuit for given time.
Electric energy, W = VIt = I
2
Rt joule
27. Electrical power:
Electrical power is the rate at which electric energy is consumed by
an appliance.
P = W/t = VI = I
2
R = V
2
/R
28. Watt:
It is the S.I. unit of power. The power of an appliance is 1 watt if one ampere of
current flows through it on applying a potential differences of 1 volt across its ends.
1 watt = 1 joule/1 second =1 volt x 1 ampere
or
1 W = 1 Js
-1
= 1 VA
1 kilowatt = 1000 W
1Mega watt= 10
6
watt
1Gigawatt=10
9
watt
29. Kilowatt hour:
It is the commercial unit of electrical energy. One kilowatt hour is the
electric energy consumed by an appliance of 1000 watts when used for one hour.
1 kilowatt hour
(kWh) = 3.6 x 10
6
J
33

SUGGESTED ACTIVITIES
1.
To verify ohm’s law.
2.
To observe the flow of current is different for different components.
3.
To study the factors on which the resistance of conducting wire depends.
4.
Resistors in series and resistors in parallel
HOTS QUESTIONS (SOLVED / UNSOLVED)
Q.1.Why is the tungsten metal more coiled in the bulb and not installed in straight parallel wireform?
Q.2.Why are fairy decorative lights always connected in parallel?
Q.3.
What will happen when -
a)Voltmeter is connected in series?
b)Ammeter is connected in parallel?

In radio & stereo speakers, (ii)
In refrigerator doors, (iii) in
audio & video cassettes players, (iv)
On hard discs & floppies of computers & (v) in
children’s toys.

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- Fall '14
- Chemical reaction, N.C.E.R.T Text book