Electric Power Notes

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William Dwight Whitney The Century Dictionary and Cyclopedia: An encyclopedic Lexicon of the English Language. (New York: The Century Co., 1889)4499
Copyright: 2009, Florida Center for Instructional Technology. See license.

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Fill In the Blank Power Notes

C) Power - rate of doing work or expending energy

JamesWatt

James Watt -
1736 - 1819

'... radically improved the power, efficiency, and cost-effectiveness of steam engines'. 'He developed the concept of horsepower,[4] and the SI unit of power, the watt, was named after him.' - Wikipedia

P	=	W/t	=	Fd/t	=	Fv
(watt)		= (J/s)		(N)m/s

Scalar

Ex 1) A force of 40 N is exerted on a mass, causing it to move with a constant speed of 5 m/s.

What power is used?

Ex 1) A force of 40 N is exerted on a mass, causing it to move with a constant speed of 5 m/s. What power is used?

P	=	W/t	=	Fd/t	=	Fv
(watt)		= (J/s)		(N)m/s

= Fv = (40 N)(5 m/s)

200 Nm/s

or 200 watts

Ex 2) A girl weighing 500. newtons takes 50. seconds to climb a flight of stairs 18 meters high.

Her power output vertically is

Ex 2) A girl weighing 500. newtons takes 50. seconds to climb a flight of stairs 18 meters high. Her power output vertically is

F = 500. N

t = 50. secs

d = 18 m

P = ?

P	=	W/t	=	Fd/t	=	Fv
(watt)		= (J/s)		(N)m/s

P = Fd/t

P = 500. N(18. m)/50. sec

P = 180 Nm/sec = 180 watts

Ex 3) A 95-kg student climbs 4.0 m up a rope in 3.0 seconds.

What is the power output of the student?

Ex 3) A 95-kg student climbs 4.0 m up a rope in 3.0 seconds. What is the power output of the student?

m = 95 kg

d = 4.0 m

t = 3.0 sec

P = ?

P	=	W/t	=	Fd/t	=	Fv
(watt)		= (J/s)		(N)m/s

P = Fd/t

First find F (weight)

F = mg = 95 kg(9.81 m/s²)

= 931 N

P = [931 N(4.0 m)]/(3.0 sec)

P = 1.2 x 10³ W

(Rounded to 2 significant digits)

Conservation of Energy