Energy, Work and Power

The terminology in steam power has specific technical meanings. Understanding these terms will greatly help to understand the material that will follow

Energy
Energy comes in different forms. Heat is the energy due to the vibration of the molecules in a substance. Kinetic Energy is the energy contained in a moving mass. Potential Energy is energy stored that can be released, for instance, in a mass that can fall from some height, a compressed spring that can be released, or chemicals that can react.

There are a number of units that energy can be measured in:

BTUs (British Thermal Units) The amount of energy needed to cool or heat one pound of water by one degree Fahrenheit.

Foot Pounds The energy used to apply a force of one pound through one foot, for instance, by lifting a one pound weight one foot. 1BTU = 778 ft-lb.

Newton Meters (Nm) The metric unit for applying a force of one Newton (= xxx pound) through one meter (= 3.xx feet).

Joule (J) The metric unit for energy, usually applied when measuring heat. 1 J = 1 Nm. 1 BTU = 1055 J.

Calorie (cal) An obsolete metric unit of the heat required to raise one gram of water (1/454 lb) by one degree Celcius (=1.8 F)

Work
Although some technical hair-splitting can be done here, Work and energy are the same thing and have the same units. For our use work means mechanical work, that is, what happens when a force acts through a distance, or a torque acts through an angle.

Work has to be done to a spring to compress it. This work is converted to potential energy stored in the spring. When the spring is released the energy (minus fricition loses) can be reclaimed as work applied to another object.

Similarly, if a torque is applied to accelerate a flywheel, work is converted to kinetic energy stored by the flywheel.

A key point to remember is that work only occurs if a force acts through a distance. You can push against a boulder until you're completely worn out, but unless it moves you haven't done any work. (This is one of those cases where the terminology can be important.)

Power
Power is the  rate  at which energy is transfered or work is done. It is work or energy divided by time. For instance, James Watt defined the Horsepower as 33,000 ft-lbs of work expended per minute. If a horse pulling on a rope could lift a 100 pound weight 330 feet in one minute, it would working at a rate of one horsepower (i.e., 100 lbs x 330 ft = 33,000 ft lbs). If our horse lifted a 200 pound weight 330 feet in one minute, it would working at a rate of two horsepower.

The metric unit for power is the Watt (W). 1 hp = 746 W.

There is often confusion over BTUs and power. The BTU is a unit of energy, not power. A "100,000 BTU" furnace is actually a 100,000 BTU/hour furnace. That is the rate at which it supplies heat, so much heat energy per hour. If this furnace runs for 20 minutes, it has produced 33,000 BTU of heat energy (i.e., 1/3 hour x 100,000 BTU/hour).

Energy is power multiplied by time. Watts and kilowatts (1000 watts) are power. Kilowatt-hours (kWh) is the unit of energy your electrical company charges you for.