POWER

POWER
POWER
In physics, Power is an important quantity. First I will give you a pratical example
and then I will define it.
Suppose we have two people, named Luigi and Maria, that do the same work, that is carryng 10
boxes from one place to another (the distance is the same).
Let’s suppose that Luigi carries two boxes at a time, while Maria, being a girl, carries only one
box at a time. It is evident that Luigi will do the same amount of work as Maria in less time.
Another example
Climbing the stairs quickly (picture 1) takes the same physical work as
when you go to a normal pace (picture 2). In both cases, in fact, the
muscles of the legs must develop a force that contrasts the gravity for
the difference in level. What changes is the speed with which you
perform the work.
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Now we will give the definition:
POWER is the physical quantity that takes into consideration both work and time;
That is
It is called Power the physical quantity that considers the work done and the time needed.
In symbols: P 
W
t
If we measure the work in Joule and the time in seconds, Power is measured in Watt or in its
multiples as Kilowatts (KW):
1Watt 
1 joule
1sec ond
1 KW = 103 Watt = 1.000 Watt
The watt (W) is the SI unit of power: it is equal to one joule (J) of work performed per second (s)
Another unit of misure of the power, we don’t use anymore, is the horse power (in symbo H P)
1 HP = 735 Watt = 0,735 KWatt
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Example
What is the Power, if a force produces a Work of 60 J in a time of 10 sec?
In symbols:
W  60 J

t  10sec
P  ?

P
W
60 J

 6W
t 10sec
Example
Consider two engines A and B, the former produced work of 40 J in a time of 10 seconds, the latter
a work of 20 J in a time of 5 seconds.
What can you say about the power of the two engines?
a) PA > PB
b)
PA = P B
c) PA < PB
Solution:
PA 
WA
40 J

 4W whereas
t A 10sec
PB 
WB 20 J

 4W
t B 5sec
so, the answer is: PA = PB
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Typical power outputs
Washing machine motor
250 W
Athlete
400 W
Large jet engine
75∙106 W
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Some information about James Watt’s life
James Watt was born on 19 January 1736 in Greenock, in Scotland. Both his parents were
Presbyterians: his father was a shipwright, while his mother, Agnes Muirhead, came from a
distinguished family. Initially Watt was mostly schooled at home by his mother but later he
attended Greenock Grammar School. He exhibited great manual dexterity, engineering skills
and an aptitude for mathematics, while Latin and Greek failed to interest him.
In 1764, Watt married Margaret Miller, with whom he had five children. His wife died and
in 1776 he was married again, to Ann MacGregor, with whom he had two other children.
He died on 19 August 1819 at his home Heathfield.
Watt began to be interested in steam engines and in order to reduce the waste of energy he
invented a regulator ( it is still called James Watts regulator) to maintain uniform the speed
of the steam engine and found a way to transform the alternating rectilinear motion of the
piston in the continuous rotary motion.
The Watt, SI unit of Power; takes the name after him .
http://it.wikipedia.org/wiki/James_Watt
http://en.wikipedia.org/wiki/James_Watt
http://www.youtube.com/watch?v=MoNvWJyBp0Q
http://www.youtube.com/watch?v=l-UW8vV7LD0
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