Fourth Class Notes on Work, Heat and Energy (Principles of Heat and Fluids)

Fourth Class Notes on Work, Heat and Energy (Principles of Heat and Fluids)

 

Energy

Energy is the capacity or the ability to do work. It can be categorized into two forms:

  • Transient Energy (Mechanical Work, Heat)
  • Stored Energy (Kinetic, Potential)

Mechanical Work

The product of the displacement (distance traveled) of the body multiplied by the component of the force in the direction of the displacement.

Work\quad =\quad Force\quad \times \quad Distance

Imperial VS SI units for Work
 Imperial  SI Unit
 W = Work  ft-lbf  N-m or Joule
 F= Force applied lbf Newton (N)
d = Distance traveled ft m

Example 1. Find the amount of Work being applied when a weight of 1000 lb is lifted through a distance of 124 inches and express it in imperial unit standard as ft-lbf.

W = F x d
W = 1000 x (124 in x (1ft/12in))
W = 1000 x 10.33
W = 10330 ft-lbf

 

Example 2. A ventilating fan having a mass of 165 kg is hoisted 96m from the ground to the roof of a building. Neglecting friction and other losses, compute the work done. Express the work in SI unit.

W =F x d = m x g x d

W = (165 kg) x (9.8 m/s^2) x (96 m)

W = 155,343 J

 

Power

 Power is the time rate at which work is done or work per unit time.
Power\quad =\quad Work\quad /\quad Time

imperial SI
P = Power ft-lbf /sec Watt (J/sec)
W = Work ft-lbf N-m or Joule (J)
T = Time sec sec

The horsepower (hp) is a widely used unit in the Imperial system

1 hp = 550 ft-lbf /sec = 746 watts

Example 1. Determine the power required in kilowatts and horsepower to move an elevator weighting 2000 lbf vertically through 40 ft in 10 seconds.

In Imperial

P=\frac { W }{ T } =\frac { (F\times d) }{ T } =\frac { ({ 2000 }{ lb }_{ f }\quad \times \quad 40ft) }{ 10sec } =\quad 8000\quad ft-{ lb }_{ f }/sec

 

In Horsepower

P\quad =\quad \frac { 8000\quad ft-{ lb }_{ f } }{ sec } \div \frac { 500\quad ft-{ lb }_{ f } }{ sec } hp\quad =\quad 14.5\quad hp

 

In Kilowatts

P\quad =\quad 14.5\quad hp\quad \times \quad .746\frac { kW }{ hp } \quad =\quad 10.8\quad KW

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