Post on 05-Jan-2016
Aim: How can we Aim: How can we explain electrical explain electrical
power and energy?power and energy?
Do Now:
Which is brighter:
60 W light bulbs in series or 60 W light bulbs in parallel?
Electrical Power (P)Electrical Power (P)•The time rate at which The time rate at which electrical energy is usedelectrical energy is used
•Measured in Watts (W)Measured in Watts (W)
VIP
FormulasFormulasOhm’s Law
V = IR))(( IIRP RIP 2R
VI
R
VVP )(R
VP
2
Electrical Energy Electrical Energy (W)(W)
•The work done (or the The work done (or the energy expended)energy expended)
•Measured in JoulesMeasured in Joules
Substitute P = VI
Substitute P = I2R
R
VPSubstitute
2
ResistorResistor RR
ΩΩVV
VoltsVoltsII
AmpsAmpsPP
WattsWatts
11 2222 100.76100.76 4.584.58 461.4461.488
22 1717 77.8677.86 4.584.58 356.6356.6
33 99 41.2241.22 4.584.58 188.8188.8
TotalTotal 4848 220220 4.584.58 1007.1007.66
44 7575 110110 1.471.47 161.3161.3
55 330330 110110 0.330.33 36.6736.67
66 5050 110110 2.22.2 242242
TotalTotal 27.527.5 110110 44 440440
Seri
es
Para
llelComplete the Chart
Calculate the energy dissipated through resistor 2 in:
1 s
W = Pt
W = (356.6 W)(1 s)
W = 356.6 J
1 minute
W = Pt
W = (356.6 W)(60 s)
W = 21,396 J
1 hour
W = Pt
W = (356.6 W)(3,600 s)
W = 1,283,760 J
Path of Least Path of Least ResistanceResistance
When given the When given the opportunity, current opportunity, current will always take the will always take the path of least resistancepath of least resistanceExample: Students and homeworkExample: Students and homework
Kirchoff’s LawsKirchoff’s Laws1. The current 1. The current
entering a entering a junction must junction must equal the equal the current leaving current leaving a junctiona junction
Junction
Gustav Robert Kirchhoff 1824-1887
5 A 3 A
8 A
6 A 2 A
2. For any loop in a 2. For any loop in a circuit, the sum of circuit, the sum of the voltage drops the voltage drops must be equal to must be equal to the voltage gains.the voltage gains.
8 V 12 V4 V
24 V
Voltage GainsVoltage Gains Voltage DropsVoltage Drops
VVTT = 24 V (battery) = 24 V (battery) VV11 = 8 V = 8 V
VV22 = 12 V = 12 V
VV33 = 4 V = 4 V
= 24 V