Watts and Horsepower

As we explained in the previous section, power is defined as the time rate of doing work, or stated another way, work or energy divided by time. The units of power in SI units are defined in the following mannen 

Note the following 1N m is called 1 joule (J), and 1 J/s is called 1 watt (W). In U.S. Customary units, the units of power are expressed in Ibf fr/s and horsepower (hp), in the following mannen 

Customary units of power are related to the SI unit of power, watt (W), in the following manner  

Another common unit used in the United States in air-conditioning and refrigeration systems is ton of refrigeration or cooling. One ton of refngetation represents the capacity of a refri ra-tion system to freeze 2000 lb or 1 ton of liquid water at 32°F into 32°F ice in 24 hours. It is 

To get a feel for the relative magnitudes that watt and horsepower physically represent, consider the following examples. 

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When the automobile became preeminent in the early twentieth century, it did so with good reason. Wliether the energy to power a bicycle is anaerobic or aerobic in nature, it is still minuscule in comparison to what an automobile s internal combustion engine can deliver. In the United States, almost all subcompact cars are equipped with engines that can generate 100 or more horsepower (74,600 watts), and can sustain this output all day long. 

In the cgs system, power has the units of ergs per second in the mks system, units of joules per second lor watts) and in the English system, units of foot-pounds per second. A common engineering unit is the horsepower, defined as 550 foot-pounds per second or 33.000 footpounds per minute. The SI unit of power is the wall. I watt = I joule per second. < I joule is the work done by I newton acting through a distance of I meler.l I joule = watt-second = I07 ergs= I07 dyne-centimeters. The SI unil of force is the newton. (I newton = 10s dynes). See also entry on Units and Standards. 

Figure 6.59 shows the extension of the problem about capacity of the human heart which displays the file Wheart= = in two capacity units, in watts and in horsepower. 

Power, P, defiaed as the rate at which work is performed, is expressed ia terms of energy divided by time and is most commonly given in units of horsepower, as for the power suppHed by mechanical devices such as diesel engines, or in the SI units of watts, especially when measuring electrical power. One horsepower is equivalent to the amount of power needed to lift 33,000 pounds (14,982 kg) one foot (30.5 cm) in one minute. One watt is equivalent to the power required to perform one joule of work per second. In a simple direct-current circuit where potential is represented by E ... 

NOTE To convert horsepower to watts, multiply ky 746 to convert pound-force-feet to newton-meters, multiply ky 1..356 and to convert revolutions per minute to radians per second, multiply ky 0.1047. 

The human engine cannot match this power output, yet the mechanical efficiency of the bicycle helps tremendously because a vei y small amount of horsepower can generate great speed. For example, 0.4 horsepower (298 watts) of output can result in 25 niph (40 kph) speeds or better. One set of calculations shows that if a cyclist rode on level ground, with no rolling resistance, and aided by a 25 mph tailwind, it would require only around 0.2 horsepower (150 watts) to sustain a 25 mph pace. 

HORSEPOWER (hp). The horsepower is considered an anachronism in science and technology Use of the SI unit of power, the watt, is preferred, When used, 1 horsepower equals (1) 42,44 Btu/minute, (2) 33,000 foot-pounds/minute or (3) 550 foot-pounds/second. 

Power 1 kilowatt 137.56 foot-pounds forqnsr second 1 kilowatt 56.87 Btu per minute 1 kilowatt 1.341 horsepower 1 horsepower 550 foot-pounds forqnsrsecond 1 horsepower 0.707 Btpersecond 1 horsepower 745.7 watts Heat, energy, and work equivalents ... 

Iherc follows a remark lha l the di ffcrencc i n a tea enclosed by the last line and the first shews the quantity of steam saved. Assuming that the engine was running at the same speed as Lee s engine, the area enclosed by the Iasi line shows the power developed to be 9.4 horsepower. (Boulton Watt Collection.)... 

Power is the rate at which energy is transferred, and therefore it has dimensions of [ML2/P]. One watt (W) is defined as 1 J/sec. Other common units are horsepower and Btu/hr. (Curiously, the metric cheval-vapeur is equivalent to only 735 W, slightly less than the U.S./British horsepower of 746 W.) See Table A-9 for interconversion factors for these units. 

Another way to treat the loss problem is to compute the power which is lost as the product of friction force and the shearing velocity at the moving boundary. If the force is in newtons and the velocity in meters per second, the power loss is in watts. In English engineering units, if the force is in lbs and the sliding speed in inches per second, the loss expressed as horsepower is ... 

Power - Energy that is capable or available for doing work the time rate at which work is performed, measured in horsepower. Watts, or Btu per hour. Electric power is the product of electric current and electromotive force. 

Watt - The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of Voltage and Current (amperage). 

Energy and power values for biological systems are often expressed in several different sets of units. Among these are watts, calories, kilocalories, British Thermal Units (BTU), ergs, horsepower, electron-volts, and others. Unless the same units are used on both sides of the equation, and... 

Start with 1 Ibf-ft/s and convert it to N-m/s and show that 1 Ibf ft/s is equal to 1.36 watts. Knowing that 550 Ibf ft/s is equal to 1 horsepower, how many kilowatts is that ... 

Chapter 13 presents energy and power and explains the disdncdon between these two topics. The importance of understanding what is meant by work, eneigy, power, watts, horsepower, and efficiency is emphasized in Chapter 13. 

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