Lesson 13j: Power
Remember from earlier in the course we had a formula for relating electricity to energy changing
forms...
V
=
Δ
E
q
→Δ
E
=
qV
(
equation 1
)
•
But from way back in Science 10 and Physics 20 we had a formula to determine the power as
energy is changed from one form to another over a period of time...
P
=
W
t
=
Δ
E
t
(
equation 2
)
P = Power measured in Watts (W), the equivalent of J/s
W = ∆E = change in energy measured in Joules (J)
t = time measured in seconds (s)
•
This formula tells you that power is the rate at which energy is changed from one form to
another.
◦
For example, a 60W bulb changes electrical energy into light and thermal energy at a rate of
60 Joules per second.
We can now play around a bit with these two formulas to get...
P
=
Δ
E
t
=
qV
t
(
equation 3
)
•
Recall that we have a formula for the current flowing through a circuit...
I
=
q
t
(
equation 4
)
•
So we can simplify the equation 3 by subbing out the parts of equation 4...
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 Fall '08
 Staff
 Energy, Power, electrical energy, Joule, Kilowatt hour

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