Series, Parallel Differences

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III) Electric Power

Power - time rate of doing work or expending energy 


P = W/t
(watts) (J/seconds)


since V = W/q,



then W = Vq


 P = Vq/t


P = VI

(All equations in reference table)



P = VI   since V = IR …




since I = V/R …


THEN   P = V2/R



P = VI = I2R =  V2/R

P = W/t

Giancoli p. 516) 11



a) R = 20. ohms



P = W/t = VI

W = VIt

= 12V[.60 A]60 sec


b) 430 J




Ex) A series circuit contains two lamps and  a battery with a potential difference of 1.5 volts. If the current in the circuit is 0.10 ampere, at what rate does the circuit use energy? 



Ex) Series circuit w/ two lamps, the battery supplies a potential difference of 1.5 volts. Current in the circuit is 0.10 ampere, what rate does the circuit use energy?

a)15 W b)1.5 W d).15 W c).015W



V = 1.5 V
I = .10 Amps
P = ?

(rate of energy use)



P = VI = 1.5 V(.10 A)


P = .15 watts


or .15 VA





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P, R Relationship????

P = I2R =  V2/R




P,R direct
when resistors
have same current

P = I2R

P,R Inverse
when resistors have same V

=  V2/R




In which kind of circuit would a 5 Ω resistor be brighter (more power) than a 10 Ω resistor?


(Hint: P = V2/R and P = I2R)




Parallel Circuit, P = V2/R

P, R are inverse when
V is constant.


5 Ω would be brighter because it has less R


Review - Series or Parallel?

image of Parallel circuit

Dorling Kindersley Books










AP Physics


p. 517) 33, 35, 37




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