Transcript 4. Basic concepts of energy

Prepared for BIO/EES 105
Energy in our World
Kenneth M. Klemow, Ph.D.
Wilkes University
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Overview
◦ Energy defined
◦ Forms of energy
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The physical nature of energy
◦ Energy and Newtonian Laws of Motion
◦ Units of measure
◦ Conversions
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Terminology pertaining to energy
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Ability to do work
Physicists distinguish between kinetic
and potential energy
Energy comes in different forms
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Radiation
Mechanical energy
Chemical energy
Atomic energy
Electromagnetic energy
Electrical energy
Heat energy
Sir Isaac Newton
1642 - 1727
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Energy = Force x distance
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Force = Acceleration x mass
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Acceleration = Speed / time
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Speed = Distance / time
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Speed = distance / time
Ways of expressing
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Miles / hour
Km / hour
Feet / second
Meters / second
Other relationships
◦ Distance = Speed x time
◦ Time = Distance / speed
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Velocity is a vector: implies speed and
direction
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1 ft/s = 0.305 m/s
1 mi/h = 0.447 m/s
1 km/hr = 0.28 m/s
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1. A car drives 72 miles in 120 minutes.
What is its velocity in miles per hour?
2. A person runs at 6 miles per hour. How
far can that person run in 10 minutes?
◦ Expressed in miles:
◦ Expressed in feet:
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3. How long does it take for that person to
run 528 feet?
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A car is traveling 60 miles per hour. How
many feet can it travel in one second?
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Acceleration = Change in velocity / time
◦ Expressed as distance / time X time
◦ Or distance / time2
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Occurs when an object is speeding up or
slowing down
Units include
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Miles / hour2
Km / hour2
Feet / second2
Meters / second2
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1 ft/s2 = 0.305 m/s2
1 m/s2 = 3.28 ft/s2
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A Kia Rio can accelerate to 30 km / hour in 6
seconds. What is its acceleration?
◦ Express in terms of m / second2
 (see Example 2.2 on p. 40)
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Velocity = Acceleration X Time
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Problem:
◦ Return to the Kia
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What
After
After
After
After
is velocity after 1 second?
3 seconds?
6 seconds?
9 seconds?
12 seconds?
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Gravity has an acceleration (Agrav)
◦ Metric: 9.8 m/s2
◦ English: 32 ft/s2
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X = (1/2) x A x T2
(see p. 62 of text for derivation)
Problem: Imagine you drop a stone from a cliff, and
it takes three seconds to hit the water below.
How high was the cliff above the water?
How fast was the stone moving when it hit the
water?
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Momentum = mass x velocity
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Force = mass x acceleration
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Common unit of measure for force:
◦ Newton (N = kg x m / s²)
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Other relationships
◦ Mass = Force / acceleration (kg=F/a)
◦ Acceleration = Force / mass (A=F/kg)
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A rock having a mass of 2 kg falls into the
water from a cliff. What is the force that it
exerts?
◦ Does that force vary if the cliff is 50’ high, as
opposed to being 100’ high?
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Mass is a property of a body (measure of
inertia).
◦ Irrespective of its position relative to gravity.
◦ Often expressed as Kg.
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Weight depends on gravity. An object will
weigh more on earth than on moon because
gravitational force greater on earth.
◦ Weight often considered to be unit of force,
expressed as Kg x Agrav
 Where Kg is mass and Agrav is acceleration due to
gravity.
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1. A body will continue to remain at rest or in
motion with a constant velocity unless it is
acted upon by an outside force.
2. The acceleration of an object is directly
proportional to the net force acting on it, and is
inversely proportional to its mass (A = F/Kg).
3. For every action force, there is an equal and
opposite reaction force.
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Energy = Force x Distance
◦ Joule (J) = Newton x meter
 Energy of an apple 1 m from the floor
◦ Some additional measures of energy
 Foot pound = 1.4 J
 1 calorie = 4.187 J
 1 BTU = 1054 J
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Potential energy
◦ Stored energy, able to do work if released. Examples
include:
 Objects placed at an elevation
 Water behind dam
 Release energy if they fall
 Objects placed at mechanical tension
 Wound up spring
 Release energy if tension is relieved
 Chemical bond energy
 Organic molecules
 Energy released if combusted
◦ Potential energy due to elevation
 PEG = weight x height = Kg x Agrav x h
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Kinetic energy
◦ Energy of motion
Examples include:
 Moving water
 Moving catapult
◦ Can be expressed mathematically as
 1/2 Kg x v2
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Rate at which energy is produced, used, or
transferred.
◦ Expressed as energy per time
◦ Common units include
 Watt (J / s)
 Ft-lb / sec
 Horsepower
 1 hp = 550 ft-lbs / sec
 1 hp = 746 Watts
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Question: A kilowatt hour is a measure of:
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Power
Energy
Force
Acceleration
None of the above
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Power = energy / time
Energy = power x time
www.belmont.k12.ca.us
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W = D (KE + PE)
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Both have two meanings
◦ Conversion
 Translating between different units of measure
 Joule <-> Calorie <-> BTU
 Changing from one form to another
 Chemical energy -> Thermal energy
◦ Conservation
 First law of thermodynamics
 Energy cannot be created or destroyed, only
converted
 Reduce wasteful energy consumption
 Switch from incandescent to light-emitting diode
(LED)
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1
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1
1
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1
kilowatt hour = 3.60 x 106 J
barrel oil equivalent = 6.119 x 109 J
ton wood equivalent = 9.83 x 109 J
ton coal equivalent = 29.31 x 109 J
ton oil equivalent = 41.87 x 109 J
quad (PBtu) = 1.055 x 1018 J
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First law: Energy cannot be created nor
destroyed, can only be converted
(conservation of energy)
◦ In an isolated system, total energy will always
remain constant
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Second law: No energy conversion is
perfect; always get some loss as heat.
◦ Gives direction to a reaction
◦ Get increase in disorder (entropy).
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In system involving movement, always get
loss as friction
Thus perpetual motion machines are
impossible (yet people still try to invent them)
Waste heat given off to environment
◦ Ultimately go off to space
Efficiency =
energy (work) output
X 100
total energy input
• Efficiencies can vary from 5% - 95%
• In multistep processes, efficiency is the product of
efficiency of each step.
• Comparative assessments of energy processes / devices
typically take great pains to accurately measure
efficiency
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Refer to Table 3.1 on p. 78 of text