Page 77 - Physics_Form_2

Current electricity
Task 2.3
(a) Identify the electrical appliances
found at your home; list them and
discuss their power ratings.
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(b) Suppose the appliances you listed
are used for 1 hour, what will be
the total energy used?
Commercial unit of electrical energy
You have learnt that, the unit of
electrical power is watt, and thus, unit of
electrical energy should be watt-second.
However, watt-second unit is too small
as an electrical energy unit for everyday
commercial purposes.
Thus, to resolve this challenge, a larger
unit, the kilowatt-hour (kWh), is used.
One kWh is the energy consumed in one
hour by an appliance working at the rate
of 1 kW. Commercially, 1 kWh is named
as a unit of electricity. For example, if
the user consumes 5 units of electricity,
it means that, electrical energy of 5 kWh
has been consumed.
1 kWh 1 kilowatt  1 hour
 1 000 W  3 600 s
=36 000 000 watt-second or 3.6 10 J 6
=3.6 MJ
The unit of kWh is used to determine
the cost of electrical energy consumed
by electricity users. Normally, electricity
cost is calculated by taking the price of
a unit of electricity times the consumed
electrical energy in kWh (total units).
Physics Form 2 Final.indd 71
That is;
cost  price  total units
In Tanzania, electricity users buy units
of electricity before using the energy.
Therefore, the Tanzania Electric Supply
Company Limited (TANESCO) distributes
special electricity meters that enable one
to pay before consuming. These meters are
famously known as “Lipa Umeme Kadiri
Utumiavyo” abbreviated as LUKU. Figure
2.39 shows a picture of LUKU meter
distributed by TANESCO.
kWh
Figure 2.39: An example of a LUKU meter
with 3.35 kWh (units)
Example 2.16
A television set (TV) rated 40 W is
switched ON for 5 hours every day.
(a) How much electrical energy in kWh
does this TV consume in 30 days?
(b) If the price of a unit of electricity
is Tsh 229.60, what is the total
electricity cost of watching this TV
in 30 days?
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