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CLB10402 IAMK
Energy and Power
When there is current through a resistance, electrical energy is converted to
heat or other form of energy, such as light. The light bulb then becomes too
hot to touch, the current through the filament that produce light also produce
unwanted heat, because the filament has resistance.
Energy – is the ability to do work.
Power is the rate at which energy is used.
Power (P)

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CLB10402 IAMK
Energy and Power
When there is current through a resistance, electrical energy is converted to heat or other form of energy, such as light. The light bulb then becomes too hot to touch, the current through the filament that produce light also produceunwanted heat, because the filament has resistance.
Energy – is the ability to do work.
Power is the rate at which energy is used.
Power (
P
) is a certain amount of energy(
W
) used in a certain length of time(t) , expressed as :
t W P
P
= power in watts (
W
),
W
is energy in joules (J) and t is time in seconds(s).
One watt (W) – is the amount of power when one joule energy is used in one second.
Power in an Electric circuit
.When current through a resistance, the collisions of the electrons produce heat as a result of the conversion of electrical energy. The amount of power dissipated in an electric circuit is dependant on the amount of resistance and on the amount of current, expressed as follows:
R I P
2
Where
P
is power in watts (W),
I
is current is amperes (A), and
R
is resistance in ohms (). Equivalent expression for power in terms of voltage
Ω
and current by substituting
V
for
IR
(ohm’s law) :
VI P I IR IR I R I I R I P
)()()(
2
Another equivalent expression can be obtained by substituting
V/R
for
I.
RV P RV V VI P
2
The relationship between power and current, voltage and resistance expressed in the preceding formulas are known as Watt’s law. In each case I must be in amps, V in volts and R in ohms. To calculate the power in resistance you can use any one of the three power formulas depending on the information you have. For example you have the values of current and
JUL09
CLB10402 IAMK
voltage in a circuit, so in this case to calculate power we can use
P=VI
. If you know
I
and
R
, use the formula
R I P
2
. If there only V and R then use
RV P
2
JUL09

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