Units of Work and Energy

The SI unit of energy (and work, since energy is the ability to do work and thus must have the same units) is the joule (J). This unit is named in honor of the English physicist James Prescott Joule (1818-1889) whose work clarified the concepts of work and energy. A joule combines the units of force and distance:

We can illustrate this unit with two examples. It requires about 1 J of work to lift a baseball from the ground to your chest, whereas it takes about 10J of energy to pick up an average physics textbook from the bottom shelf of a bookcase and stand up to read it.

When dealing with the energies of atoms or electrons, though, the joule is too large a unit. For these purposes, another unit, the electron volt (eV), is used. The conversion factor between electron volts and joules is

A frequently used multiple of the eV is the MeV, which equals one million eV.

The Concept of Energy

The concept of work is very useful in understanding the concept of energy. As we stated at the beginning of this chapter, energy is the capacity to do work; that is, energy allows us to perform tasks, to do work. We can also think of energy as the result of doing work. It is the chemical energy stored in the man’s body in Figure 4.4 that enables him to do the work on the car as he pushes it,

Energy is the capacity to do work. The chemical energy in the man’s body enables him to do work on the car

FIGURE 4.4 Energy is the capacity to do work. The chemical energy in the man’s body enables him to do work on the car.

converting chemical energy into energy of motion of the car and into heat (thermal energy) as the tires rub against the pavement.

This idea of energy as something stored that can do work was called first vis viva (Latin for “living force”) by the German philosopher Gottfried W. Leibniz (1646-1716), because he thought that only living things could have the capacity to do w'ork. The English scientist Thomas Young (1773-1829) realized that inanimate objects, like the wind, can do work—by moving a windmill or a ship, for example. He proposed the name energy, a name he fashioned from Greek words meaning “work within,” for this work stored in bodies.

Of the various types of energy listed at the beginning of this chapter, we will only consider mechanical energy for the moment. Later in this book, most of the other kinds of energy will be studied in some detail. An object may have mechanical energy by virtue of its state of motion, its location in space, or its internal structure.


James Joule, the second son of a wealthy brewer, had a good early education. As a young man, he was taught by the renowned chemist John Dalton and showed a talent for science. At 19, he did several experiments investigating the nature of electromagnets, which resulted in a published paper.

Born in 1818 near Manchester, England, Joule developed an early interest for the machines in his father’s large brewery. This interest made him proficient at designing experiments and building the machines required to run them. He soon developed an almost fanatical zeal for accurate measurements. Such was his dedication that he even took time during his honeymoon to design a special thermometer with which to measure the temperature difference between the top and the bottom of a waterfall that he and his bride visited.

When Joule was 15, his father became ill and retired. Although the young James had to spend time running the brewery, he continued his scientific endeavors. At 22, he calculated the amount of energy produced by an electric current and went on to spend the next 10 years devising experiments to measure energy in every conceivable way.

The initial report of his experiments was met with skepticism and even rejection. The Royal Society did not accept his original paper, and Joule was forced to present his results at a public lecture. His report was finally published in the Manchester newspaper at the instigation of his brother, who was the paper’s music critic (Cardwell 1989).

Eventually, his work caught the attention of other scientists and Joule gained the recognition he deserved. He was elected to the Royal Society in 1850, and years later, he became the president of the British Association for the Advancement of Science. In 1854, his wife died after only 6years of marriage and Joule, deeply distressed, retreated to his w'ork. In 1875, he began to have financial difficulties and Queen Victoria granted him a pension. Toward the end of his life, he became concerned and disturbed about the applications of his work to warfare.

He died in 1889, at the age of 71, after a long illness.

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