y Referencepoint ENERGY Part of our everyday lives: • Energetic people • Food that is “full of energy” • High cost of electric energy • Risks of nuclear energy
ENERGY Energy: The ability to do work Energy can neither be created nor consumed or destroyed When anything happens in the physical world, energy is somehow involved. Our quality of life is related to energy use
Energy • Energy can be transformed or converted from one form to another • Energy can be released • Energy can be transferred or given from one object to another
Forms of Energy • Chemical Energy • Chemical bonds, atomic or molecular structure • Biological Energy • Cells, proteins, carbohydrates • Mechanical Energy • Associated with motion and position • Heat Energy • Pressure and volume change
Work • Work is the useful result of energy • Work has several forms: • Mechanical work • Work = force times distance • Electrical work • Work = voltage times current times time • Fluid work • Work = pressure times volume
Mechanical 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”.
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. 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 Cos θ
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:
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.
Power • The RATE of Doing Work… • How fast work is done • Related to how long it takes to do work • The more powerful something is, the faster it can do work
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 more common
Types of Energy • Kinetic – Energy of Motion • Potential – Energy of Position • Chemical Energy • Food converted to energy in our bodies • Gasoline used in cars and trucks • Heat Energy • Heat from burning coal to make steam to drive turbines • Electric Energy • Electricity turns motors in homes and factories • Radiant Energy • Energy from the sun
Kinetic Energy Kinetic energy = the energy a body possesses due to its motion Translational Rotational velocity, v (ms-1) Kinetic energy = ½ mv2 mass, m (kg)
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 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)
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
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 • Stored Energy Elastic Potential Energy Example: a spring 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) Chemical Potential Energy Batteries, food, gasoline
h mg Potential Energy • Gravitational Potential energy = the energy a body possesses because of its position (relative to the ground) Gravitational Potential = mgh Energy
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)
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
Potential Energy is Relative • Amount of potential energy is a function of the relative height of the objects • Gravitational PE is relative
Elastic Potential Energy • Another important form of potential 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.
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.
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
Total Energy • Total Energy Is Constant • Total Energy = KE + PE • The total energy is usually given or calculated when one value is zero • Dropped from a height h, v is zero • Tossed up in the air with a velocity, h is zero + mgh 1 mv2 2
A C D E B B C A 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 All potential energy (stops for an instant) P.E. = 10 J K.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
m = 4kg 5m v 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
Example What is the speed of the rollercoaster at P, Q and R? Q 16.2 m (h1) R 11.2 m (h2) P 9.0 m (h3) At P: P.E. = 0 J K.E. = maximum K.E. = P.E. at the start ½ mv2 = mgh1 v = 18 ms-1 At R: P.E. = mgh3 K.E. = loss in P.E. ½ mv2 = mgh1 – mgh3 = mg(h1-h3) v = 12 ms-1 At Q: P.E. = mgh2 K.E. = loss in P.E. ½ mv2 = mgh1 – mgh2 = mg(h1-h2) v = 10 ms-1
Work – Energy Theorem • Work added to an object increases the kinetic energy = Work + 1 1 1 1 m(v1)2 m(v2)2 m(v2)2 m(v1)2 or 2 2 2 2 Work =
Energy Loss • Even though energy is conserved, some energy is lost • Friction is wasted energy • Energy is converted to heat
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) • 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 • 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
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 • Variation due to climate and latitude • $70/watt in 1960, $3/watt today • Economics still limit widespread application
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 • 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 • 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 • Geothermal Energy • Nuclear Energy • Tidal Energy
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