Page 107 - Physics 10_Float

CURRENT ELECTRICITY

The amount of energy delivered by a power of one kilowatt in
one hour is called kilowatt-hour.

Self Assessment
One kilowatt-hour1 kWh= 1000 W ×1 hour A light bulb is switched on for
=1000 W × (3600 s) 40 s. If the electrical energy
5
= 36 × 10 J=3.6 M J consumed by the bulb during
The energy in kilowatt-hour can be obtained by the following this time is 2400 J, find the
formula: power of the bulb.
The amount of energy in kilowatt-hour
watt x time of use in hours
= Remember
1000 To work out the energy
The electric meter installed in our houses measures the transferred, the time must be
consumption of electric energy in the units of kilowatt-hour in seconds and the power in
according to which we pay our electricity bills. If the cost of watts.
To work out the cost, the
one kilowatt-hour i.e., one unit is known, we can calculate
power must be in kilowatts and
the amount of electricity bill by the following formula: the time must be in hours.
Cost of electricity = number of units consumed × cost of one unit
watt x time of use in hours
= x cost of one unit
1000
Example 14.8: Calculate the one month cost of using 50 W
energy saver for 8 hours daily in your study room. Assume +2
that the price of a unit is Rs. 12. +1
Solution: Given that, Power = 50 W = 0.05 kW, time = 8 hours Voltage 0
Number of units consumed = 8 × 30 × 0.05 =12 units -1
-2
Therefore, total cost = 12 × 12 = Rs. 144 Time
Fig.14.14: variation of
14.12 DIRECT CURRENT AND ALTERNATING
voltage with time.
CURRENT

The current derived from a cell or a battery is direct current
(d.c.) - since it is unidirectional. The positive and negative
terminals of d.c sources have fixed polarity, therefore, level of +200
d.c remains constant with time (Fig.14.14). On the contrary, Voltage (V) +100
0
there is also a current which changes its polarity again and -100
again. -200
0 0.02 0.04 0.06
Time (s)
Such a current that changes direction after equal intervals of
time is called alternating current or a.c (Fig.14.15). This type of Fig. 14.15: Variation of voltage
with time.
current is produced by AC generators.
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