ENERGY : ENERGY Part of our everyday lives:
Energetic people
Food that is “full of energy”
High cost of electric energy
Risks of nuclear energy Log on to : www.mathscienceguru.com
ENERGY : ENERGY Energy:
the amount of work a physical system is capable of performing.
Energy can neither be created nor consumed or destroyed
When anything happens in the physical world, energy is somehow involved. Log on to : www.mathscienceguru.com
Work : Work Definition:
A measure of the change a force produces:
“The work done by a force acting on an object is equal to the magnitude of the force multiplied by the distance through which the force acts”. Log on to : www.mathscienceguru.com
Work : Work Work is done…
…by a force when the object it acts on moves
NO work is done by pushing against a stationary wall.
Work IS done throwing a ball because the ball MOVES while being pushed during the throw. Log on to : www.mathscienceguru.com
Work : Work Equation for work:
In words:
The direction of the force (F) is assumed to be the same as the direction of the distance (d)
A force perpendicular to the direction of motion of an object cannot do work on the object Log on to : www.mathscienceguru.com
The Joule : The Joule joule (J)
The SI unit of energy
Amount of work done by a force of one newton when it acts through a distance of one meter:
Example:
Push a box 8 m across the floor with a force of 100 N performs 800 J of work: Log on to : www.mathscienceguru.com
Direction of Force : Direction of Force When a force and the distance through which it acts are parallel, the work done is equal to the product of F and d
If the forces are NOT parallel, work done is equal to the product of d and the projection of F in the direction of d. Log on to : www.mathscienceguru.com
Power : Power The RATE of Doing Work…
Rate is the amount of work done in a specified period of time
The more powerful something is, the faster it can do work Log on to : www.mathscienceguru.com
Units of Power : Units of Power Standard (SI) unit of power is the watt
Example:
500W motor can perform 500J of work in 1 s
… or 250J of work in 0.5 s
… or 5000J of work in 10 s
Watts are very small units
Kilowatts are used most commonly Log on to : www.mathscienceguru.com
Types of Energy : Types of Energy Kinetic – Energy of Motion
Potential – Energy of Position
Chemical Energy
Food converted to energy in our bodies
Heat Energy
Heat from burning oil to make steam to drive turbines
Electric Energy
Electricity turns motors in homes and factories
Radiant Energy
Energy from the sun Log on to : www.mathscienceguru.com Log on to : www.mathscienceguru.com
Kinetic Energy : velocity, v
(ms-1) mass, m (kg) Translational Rotational Kinetic energy
= ½ mv2 Kinetic Energy Kinetic energy = the energy a body possesses due to its motion
Kinetic Energy : Kinetic Energy Moving objects can exert forces on other moving or stationary objects
Kinetic energy depends on the mass and speed of a moving object Note that v2 factor means that KE increases VERY rapidly with increasing speed
Kinetic Energy : Kinetic Energy Example
Kinetic energy of a 1000kg car moving at 10 m/s is 50kJ
( 50kJ of work must be done to start the car from a stop, or stop it when it is moving) Log on to : www.mathscienceguru.com
Force on a Nail : Force on a Nail When a hammer strikes a nail, the hammer’s kinetic energy is converted into work, which pushes the nail into the wood Log on to : www.mathscienceguru.com
Force on a Nail : Force on a Nail Example:
Using a hammer with a 600g head moving at 4 m/s to drive a 5mm nail into a piece of wood, what is the force exerted on the nail on impact?
Potential Energy : Potential Energy Energy stored in the set-up of a mechanical system
Elastic potential Energy
Gravitational Potential Energy
When a stone is dropped, it falls towards the ground, until it hits the ground
(if the ground is soft, the stone will make a small depression in the ground) Log on to : www.mathscienceguru.com
Potential Energy : Gravitational Potential energy = the energy a body possesses because of its position relative to the ground Gravitational
Potential = mgh
Energy Potential Energy Log on to : www.mathscienceguru.com
Potential Energy Example : Potential Energy Example Potential energy of a car pushed off a 45m cliff
Compare with amount of KE done by a car moving at 30m/s (108 km/hr) Log on to : www.mathscienceguru.com
Examples of Potential Energy : Examples of Potential Energy Examples are almost everywhere
Book on the table
Skier on the top of a slope
Water at the top of a waterfall
Car at the top of a hill
A stretched spring
A nail near a magnet Log on to : www.mathscienceguru.com
Potential Energy is Relative : Potential Energy is Relative Amount of potential energy is a function of the relative height of the objects
Gravitational PE is relative Log on to : www.mathscienceguru.com
Conservation of Energy : Conservation of Energy The Law of Conservation of Energy:
Energy cannot be created or destroyed, although it can be changed from one form to another.
This principle has the widest application to all science
Applies equally to distant stars and biological processes in living cells. Log on to : www.mathscienceguru.com
Conservation Principles : Conservation Principles Conservation of energy is significant because, if the laws of nature…
…are the same at all times (past, present, and future), then energy must be conserved. Log on to : www.mathscienceguru.com
Energy Transformations : Energy Transformations Most mechanical processes involve conversions between KE, PE, and work
A car rolling down a hill into a valley
PE at the top of the hill is converted into KE as the car rolls down the hill
KE is converted to PE as the car rolls up the other side
Total amount of energy (KE+PE) remains constant Log on to : www.mathscienceguru.com
Slide 24 : A B C D E A B C All potential energy
(stops for an instant)
P.E. = 10 J
K.E. = 0 J All potential energy
(stops for an instant)
P.E. = 10 J
K.E. = 0 J All kinetic energy
(greatest speed)
K.E. = 10 J
P.E. = 0 J D E Potential energy &
Kinetic energy
P.E. = 5 J; K.E. = 5 J Potential energy &
Kinetic energy
P.E. = 4 J; K.E. = 6 J
Energy Transformations : Energy Transformations Log on to : www.mathscienceguru.com
Slide 26 : Example A parcel of mass 4 kg slides down a smooth curved ramp. What is the speed of the parcel when it reaches the bottom. Top of ramp: all potential energy
P.E. = mgh = 4 kg ? 10 ms-2 ? 5 m = 200 J Bottom of ramp: all kinetic energy
(all P.E. has changed to K.E.)
K.E. = ½ mv2 = 200 J
½ ? 4 kg ? v2 = 200 J
v2 = 100 ? v = 10 ms-1
Slide 27 : Example 16.2 m
(h1) 11.2 m
(h2) 9.0 m (h3) P Q R What is the speed of the rollercoaster at P, Q and R? At P: P.E. = 0 J
K.E. = maximum
K.E. = P.E. at the start
½ mv2 = mgh1
v = 18 ms-1 At Q: P.E. = mgh2
K.E. = loss in P.E.
½ mv2 = mgh1 – mgh2
= mg(h1-h2)
v = 10 ms-1 At R: P.E. = mgh3
K.E. = loss in P.E.
½ mv2 = mgh1 – mgh3
= mg(h1-h3)
v = 12 ms-1
Slide 28 :
Slide 29 :
Rest Energy : Rest Energy Matter is a form of energy
Most important conclusion of special relativity theory is that matter and energy are closely related
Matter ? Energy and Energy ? Matter
Rest Energy
The energy equivalent of an objects mass
Albert Einstein (1879-1955) : Albert Einstein (1879-1955) Left school at 16 to work in the Swiss patent office (his math teacher called him a “lazy” dog)
Developed 3 papers that would revolutionize physics and modern civilization:
Wave and particle theory of light
Brownian motion of particles
Introduction of the theory of relativity
In 1919, his predictions on gravitational effects on light were proven…became a world celebrity
Left Germany in 1933 and spent rest of his life at Princeton University
Searched for a “unified field theory” that would relate gravitation and electromagnetism.
Energy and Civilization : Energy and Civilization The rise of modern civilization
Impossible without vast resources of energy
Development of ways to convert energy forms
Most convenient fuels are limited
Oil, natural gas, and coal
Other sources of energy have various problems
Population increasing, as is demand for energy
Energy Demand and Type : Energy Demand and Type
The Energy Problem : The Energy Problem Limited Supply, Unlimited Demand
The sun – source of most of our energy
Food, wood, plants
Water power – The hydrological cycle
Wind power – Temperature changes
Fossil Fuels
Nuclear and hydrothermal power
Not related to the sun
Solar Cells : Solar Cells Variation due to climate and latitude
$70/watt in 1960, $3/watt today
Economics still limit widespread application
Fossil Fuels : Fossil Fuels Limited Supply
Most large deposits of oil and gas found
Remaining reserves = 100 years??
No new deposits being formed
Problems with coal
Mining needed to extract from earth
Air pollution – dangerous to health
All Fossil Fuels
Adds CO2 to atmosphere – greenhouse effect
Hydroelectric Power : Hydroelectric Power Kinetic energy of falling water converted into electricity using turbines
New hydro projects unlikely due to environmental and land-use constraints
Two-sided arguments
Environmental concerns
Development concerns
Wind Energy : Wind Energy Advantages
Non-polluting
Don’t contribute to global warming
Renewable resource
Disadvantages
Only work where winds are powerful and reliable
Take up a lot of space
Noisy, some environmental concerns
Other Energy Sources : Other Energy Sources Geothermal Energy
Nuclear Energy
Tidal Energy
Future Energy Supplies : Future Energy Supplies Fusion Energy
Technology may be many years into the future
Most alternate energy sources are very expensive
Cost of fossil fuels is still the lowest and easiest to distribute