Electric PowerPhysicsMrs. Coylehttp://www.cnas.missouristate.edu/assets/cnas/Wind_Power.jpg

Part IElectrical PowerJoules LawThermal Energy Expended by a Resistor

Wind Generator

Generator

Generator

Niagara Falls Hydroelectric Planthttp://www.niagarafallslive.com/images/NY_Hydro_Electric_Plant.JPG

Hydroelectric Power Plant

Remember:P= W / t (Power=Work/time)

Remember: W= Vq and I = q/t

So: P= I V

Electric Power, P= I VKnown as Joules LawP: is the power consumed by a resistor, R.

Unit: Joule/s= Watt

Derive the following using P=IV and Ohms Law:

P=V2/R

P=I2R

Problem 1The current through a car motor is 150A. The battery used is 12V. How much energy is supplied by the battery in 5s? Answer: 9,000J

kWhkiloWatt hour

What does the kWh measure, a) Energy or b) Power ?

Problem 2A small desktop radio has a resistance of 8,000 . The voltage is 120V. How much current does it draw?How much power does it use?How much does it cost to run the radio for 12 hours, if 1kWh costs $0.15? Answers:a)0.015A, b)1.8W, c) $0.00324

Resistors Expend Thermal EnergyWasted heat energy is called Joule Heatingor I2 R loss.

Why is long distance power transmitted at high voltages?Hint: P = I V

Answer: For a given P, keep the current, I, low to minimize I2 R loss in the transmitting wires, so increase V.

Electric heaters(Coil Heaters)

P= V2/R

The lower the R the greater the heat given off by the resistor for a given voltage.

Problem 3An electric heating element has a resistance of 9 . The voltage is 120V.Calculate the current through the element.Calculate the thermal energy supplied by the element in 10min. Answer:a)13.3A, b) 9.6x105 J

Part IIBrightness of a LightbulbSome Safety Issues

Light Bulb

Brightness of a Light bulb and PowerThe greater the power actually used by a light bulb, the greater the brightness.

Note: the power rating of a light bulb is indicated for a given voltage and the bulb may be in a circuit that does not have that voltage.

Wattage and Thickness of FilamentFor a given V, (P = IV) the higher the wattage of a light bulb, the larger the current and therefore the smaller the resistance of the filament (V=I R).

Thus, the higher wattage bulb will have a filament of lower resistance and therefore a larger cross-sectional area (R= L / A).

QuestionWhich light bulb has a thicker filament, 60W or 75W?

Answer: 75W

Problem 4A 60W light bulb is 22% efficient. How much energy is lost to heat in 5min?

Answer: 1.4x104 J

Safety IssuesWhy is it not safe to handle electric appliances with wet hands?Hint: A current of 15mA can cause serious injury.The resistance of a dry human body is in the range of 100,000. The resistance of a wet person can drop to the range of 5,000.

Problem 5A current of 1mA can be felt by a person. Will a person feel the current if the person has dry hands (a resistance of 100,000 ) and touches a 9V batterys terminals?

Answer: I=0.09mA, No.

Problem 6The same 9V battery is touched by a person with wet hands. Will this person feel the current if the resistance is 5,000 and the person touches the batterys terminals?

Answer: I=1.8mA, Yes