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