Energy

Energy

Exploring Engineering

Chapter 4, Part 1

Energy

Adapted From

EnergyEnergy is the capability to do work

Work = force x distance Distance over which the force is applied

Energy Units: SI: joules Mixed SI units: Watt-hours (= 3.6 kJ)English: ft-lbf “foot pound force”

EnergyMixed SI units: Watt-hours (= 3.6 kJ)

PowerHow fast work is done or how rapidly the amount

of energy possessed by an object changed

“Power is defined as time rate of doing work or time rate of change of energy”

Power = work/time

Power Units: SI: watts (joules/sec)English: Horsepower

Kinds of EnergyKinetic EnergyPotential Energy

Some other forms of energy:Magnetic energyElectrical energySurface energy Chemical energy (a form of potential energy)Internal energy etc.

Often mechanical energy

Kinetic Energy

Also known as “Translational Kinetic Energy” (TKE)

TKE = ½ mv2 (SI units)= ½ mv2/gc (English units)

m = mass, v = speed, gc = 32.2 lbm.ft/lbf.s2

Units: ???

Kinetic Energy: ExampleWhat is the translational kinetic energy of an

automobile with a mass of 1X103 kg traveling at a speed of 65 miles per hour (29 m/sec)?

Need: TKE of the vehicleKnow: Mass: 1X103 kg, speed: 29 m/secHow: TKE= ½ mv2

SOLVE: TKE = 4.2 x 105 J

Anything that has mass and is moving in a line has TKE.

Gravitational Potential Energy

GPE is the energy acquired by an object by virtue of its position in a gravitational field-- typically by being raised above the surface of the Earth. In SI, GPE = mgh  in units of joules

In Engineering English units, GPE = mgh/gc in units of ft.lbf

GPE & Power: Example

A person takes 2.0 seconds to lift a 1. kg book a height of 1. meter above the surface of Earth. Calculate the power expended by that person or calculate the energy spent by the person per unit time.Work done = Force x distance = mg x h = 1. x 1. x

9.81 [kg][m/s2][m] = 9.81 [J][m] = 1. x 101 J Power expended = Work done/time = 1. x 101/2.0

[J/s] = 5 Watts

Gravitational Potential Energy

Mt. Everest is 29, 035 ft high. If a climber has to haul him/herself weighing 200. lbm (including equipment) to the top, what is his/her potential energy above sea level when on the summit. Give your answer in both in joules and in ft.lbf.

Gravitational Potential EnergyNeed: GPE in English and SI unitsKnow:

m = 200. lbm = 90.7 kg (“Convert”); h = 29, 035 ft. = 8850. m (“Convert”); g = 32.2 ft/s2 = 9.81 m/s2 & gc = 32.2 lbm ft/s2 lbf (English) and gc = 1 [0] in SI

How: GPE = mgh/gc English

GPE = mgh SI

Gravitational Potential Energy

Solve: English … GPE = mgh/gc

= 200. 32.2 29,035/32.2 [lbm][ft/s2][ft][lbf.s2 /lbm.ft]= 5.81 106 ft.lbf (3 significant figures)

SI … GPE = mgh= 90.7 9.81 8850. = 7.87 106 J

A check direct from the units converter: 5.81 106 ft.lbf = 7.88 106 J …OK

Potential Energy

GPE is NOT the only form of PE.Chemical, nuclear and electromagnetic

are other forms of PEFor us, chemical and electrical energy are

so important that we will reserve extra chapters and lectures to them for later presentation.

Thermal EnergyThermal energy, often referred to as heat, is a

very special form of kinetic energy because it is the random motion of trillions and trillions of atoms and molecules that leads to the perception of temperatureAll higher forms of energy dissipate to thermal energy,

the ultimate energy sink. The laws of thermodynamics state 1) all energy is

conserved and 2) that the thermal energy in the universe, corrected for temperature, always increases.

Energy We have defined energy is the capability to do

workBut energy comes in different guises

• Potential, translational kinetic, rotational kinetic, thermal and others

Energy can be converted from one form to another• The energy in the Universe is conserved• A “control volume” is a subset of the Universe you

construct to isolate the problem of interest. It exchanges energy with the rest of the Universe

Energy Conservation

Energy = F distance is generic equation for energy

Energy is conserved (although it may change form)

System

“The Universe”

System

“The Universe”

: Energy exchanges: Energy exchanges

System energy changes 0Universe energy changes = 0System energy changes 0Universe energy changes = 0

Example of a book lying on a table and then falling on ground

Energy ConservationExample of a control

volumeThe energy in the room

is constant unless we allow exchange with the UniverseE.g., a person could walk

through the door and add energy

A heating duct could also add thermal energy

On a winter day, a window could break and the c.v. would lose thermal energy

Insulated walls

This class room

Door

Control volume example

C.V. boundary

Insulated walls

This class room

Door

Control volume example

C.V. boundary

Application of Control Volumes

The TKE of the vehicle, RKE of the wheels, electrical energy in the lights, thermal energy lost from the radiator, etc.

We deduce that the source of all these energies is exactly equal to the loss in chemical (potential) energy in the fuel.

Summary: Energy

We specifically identified gravitational, potential, and thermal energy

We learned that energy is conserved in the Universe, but not necessarily in a control volume.Deficiencies within a control volume mean that

energy in leaking in or out of the control volume at an exactly compensating amount.