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2- Electricity
2.3-Energy Transfers
2.3.4 Power Ratings
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Power
The rate at which
energy
is
transferred
or
converted
; tells us how much
energy
is
used
per unit of
time
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Power variables
P
(Power in watts, W)
E
(
Energy
transferred in
joules
, J)
t (
Time
taken in
seconds
, s)
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Watt (W)
1
watt equals
1
joule per second (J/s)
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Power rating
Indicates the
maximum
power an appliance can use when operating at its
highest
setting
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Calculating energy use
E=P×t
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Energy consumption variables
E
(Energy in joules, J or kilowatt-hours, kWh)
P
(Power in watts, W or kilowatts, kW)
t (Time in seconds, s or hours, h)
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Electricity costs
Energy
companies charge for electricity based on
energy
use, usually in
kilowatt-hours
(kWh)
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Calculating cost of electricity
Cost=
Energy
used (
kWh
)
×Cost
per
kWh
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Importance of power ratings
Safety
: Prevents overloading circuits
Efficiency
: Helps choose energy-efficient appliances
Performance
: Indicates performance level of an appliance
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Efficiency
Not
all energy consumed by an appliance is used effectively; some energy is wasted, often as
heat
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Calculating efficiency
Efficiency=
Useful Power Output
/
Total Power Input×100%
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Practical examples of power ratings
Light Bulb
: 60 W
Kettle
: 2000 W (2 kW)
Television
: 150 W
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Understanding
power ratings
is essential for making
informed
choices about
energy use
,
safety
, and
efficiency
in everyday life
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