Tuesday, January 31, 2012

The unit of work and energy was named after him: James Prescott Joule

About James Joule


James Prescott Joule
 James Prescott Joule (1818–1889), Scotland.

  James Prescott Joule was one of five children in the

family of a well-to-do brewery owner. Since he was a

sickly child with a spinal deformity, both he and his

brother were educated at home until the age of 15 and

later by private tutors.

   The famous English chemist, John Dalton, taught

them chemistry, physics, and the methods of

scientific experimentation
.

  

Later in life Joule acknowledged that John Dalton encouraged him to increase his knowledge of science and of original research.

  When James’ father died, he and his brother ran the

brewery, which prevented him from attending a

university. However, this did not deter him from setting

up a laboratory in his home and continuing his interest

in science after his day at the brewery.

He became proficient in mathematics and learned how

to make accurate measurements in the


brewery.       

   His home experiments resulted in his ability to

measure slight increases in temperature under

various conditions
, which led to his theory for the

equivalence of work and heat energy
.

The unit of work and energy was named after him (the Joule, or the symbol ‘‘J’’)
 
    Now, with his law

Joule’S law

Joule’s law states that:

 
The relationship for heat produced by an electric current in a conductor is related to the resistance of the conductor times the square of the amount of current applied:
 

 H = R I²
 
By experimentation, James Joule established the law

that states that when a current
of voltaic

electricity is sent through a metal or other type

of conductor, the heat given
off over a specific

time period is proportional to the resistance of

the conductor multi
plied by the square of the

electric current.


The equation for this law is: H = R I² , where H is

the rate of the heat given off as watts in joule units, R

is the resistance in the conductor in ohms and    is

the amount of the current (amps) squared.


The application of Joule's law



 
The application of this law is important in all

industries using electricity as a source of energy. The

resistance to an electric current flowing through a

conductor is analogous to the friction of air, the

movement of engine parts, and tires on the road


for a moving car. The electrical, as well as

mechanical, energy is not just ‘‘lost,’’ rather it is

converted to heat, just as is friction.

   
    Joule was interested in improving the mechanical

advantage of electric motors, but
because they were

very primitive during his lifetime, he devoted more of


his work to improving the efficiency of steam engines.

He accurately predicted that electric motors eventually

would replace most other types of mechanical devices.



Another law

:

Law for the mechanical equivalent of heat

: A fixed amount of mechanical work (expenditure of energy)  ends up in a fixed quantity of heat.

An interesing story

 
Earlier in 1798 when Count Rumford was boring out

the brass barrels of cannons,


he noticed that large amounts of heat were generated.

It became obvious that friction


generated by the work of turning the bit in the metal

resulted in heat. Julius von Mayer


also was interested in this relationship and developed

a figure for the mechanical equivalent of heat that was

not very accurate.

  
   James Prescott Joule was the first to consider heat

as a form of energy in his calculation.

 
  He conducted exacting experiments to determine the

amount of heat generated not just by electricity but

also by mechanical work. Joule calculated the amount

of mechanical work needed to produce an equivalent

amount of heat.


   He demonstrated that

41 million ergs of work

produced 1 calorie of heat
, which is now known as

the  mechanical equivalent of heat.

  
   Since 10 million ergs are equal to 1 joule, named

after James Joule, 4.18 joules are then equal to 1

calorie of heat.

Joule’s work enabled others to perfect

the law for the conservation of energy, which states

that energy, like mass, cannot be created or destroyed

but can be changed from one form to another.


I hope that this post was useful.