Energy

Work.

Introduction –A force is a result of an interaction between objects that can change the state of motion of an object. –Work is the result of the force applied to an object and the distance the object moves as a result of the force.

This is Glen Canyon Dam on the Colorado River between Utah and Arizona. The dam is among the tallest in America, so you can imagine the tremendous pressure on the water as it moves to generators at the bottom of the dam.

–The work done on an object is the magnitude of the applied force multiplied by the parallel distance through which the force acts. Work = force X distance W = Fd –Something must move when work is done. –The movement must be in the same direction as the applied force. –A force is a vector that can be resolved into the component force that acts in the same direction as the movement.

Units of Work. –The units of force is the Newton –The distance moved is in meters. –So the units of work in the metric system is W = Fd W = (Newton)(Meter) W = N  m

The force on the book moves it through the vertical distance from the second shelf to the fifth shelf, and the work is done, W = Fd.

It is true that a force is exerted simply to hold a book, but the book does not move through a distance. Therefore the distance moved is 0, and the work accomplished is also 0.

If the direction of movement (the distance moved) is perpendicular to the direction of the force, no work is done. This person is doing no work by carrying a book across a room.

–The unit of work in the metric system is the N  m. The N  m is called a joule The units for the Newton are kg  m/s 2 The units for distance is meter, therefore. The units for the joule are kg  m 2 /s 2 The unit of work in the English system is the ft  lb

A force at some angle (not 90 O ) to the direction of movement can be resolved into the horizontal component to calculate the work done.

Work is done against gravity when lifting an object. Work is measured in joules or foot-pounds.

–Example: How much work is needed to lift a 1,000.0 kg boulder to 1,000.0 m above sea level. Since weight is really a force we can first calculate the weight (force) necessary to lift the boulder. w = mg (1,000 kg)(9.8m/s 2 ) (1,000 X 9.8)(kg X m/s 2 ) 9,800 kg  m/s2 9,800 N

Work = Force X distance W = Fd (9,800N)(1,000m) 9,800,000 N  m X 10 6 J

Power. –Power is the rate at which work is done. –Power is defined as work per unit of time. Power = work/time P = W/t –Horsepower is defined as a power rating of 550 ft  lb To convert from ft  lb to horsepower, divide by 550 ft  lb/s/hp

(A) The work accomplished in climbing a stairway is the person's weight times the vertical distance. (B) The power level is the work accomplished per unit of time.

(A) A horsepower is defined as a power rating of 550 ft?lb/s. (B) A watt is defined as a newton-meter per second, or joule per second.

If moving a book from the floor to a high shelf requires 10J of work then the book will do 10 J of work on an object of the same mass when the book falls from the shelf.

–In the metric system power is measured in J/s. This is called a watt (W). Since a J is kg  m 2 /s 2, it follows that a watt is actually kg  m 2 /s 2 /s, or kg  m 2 /s 3. Metric prefixes can be used to show very large measures of power, such as M, G, etc… 746 W = 1 hp

Motion, Position, and Energy.

Energy is the ability to do work Potential Energy. –Potential energy is the energy that an object has due to its position –Most potential energy is actually gravitational potential energy, since it is due to the gravitational attraction of the Earth for an object.

–For the metric unit of mass, weight is the product of the mass of an object times g, the acceleration due to gravity. –Potential energy = weight X height PE = mgh

The zero referenced level for potential energy is chosen for convenience. Here the reference position chosen is the third floor, so the book will have a negative potential energy at ground level.

–Example What is the potential energy of a lb boulder ft above sea level, perched above the city of Denver (elevation ft) PE = mgh = wh = (2,000.0 lb)( ft ft) = (2,000 lb) (6916ft) = ft  lb = X 10 7

Kinetic Energy. –Kinetic energy is the energy that an object contains due to its motion. –Kinetic energy can be measured: In terms of the work done to put the object in motion. In terms of the work the moving object will do in coming to rest (transfer of energy to another object). –Kinetic energy is proportional to the mass of a moving object, but the velocity of the object has a greater influence.

(A) Work is done on the bowling ball as a force (FB) moves it through a distance. (B) This gives the ball a kinetic energy equal to the amount of work done on it. (C) The ball does work on the pins and has enough remaining energy to crash into the wall behind the pins.

–Kinetic energy is proportional to the square of the velocity. The kinetic energy of an object is kinetic energy = 1/2 (mass) (velocity) 2 KE = 1/2mv 2 The unit of mass is the kg and the unit of velocity is m/s.

Therefor, the unit of kinetic energy is: KE = (kg)(m/s) 2 = (kg)(m 2 /s 2 ) = kg  m 2 /s 2 Which is the same as –(kg  m/s 2 )(m) –or –Nm–Nm –Or –Joule (J)

–Example A 3500 kg automobile is moving down the interstate with a velocity of 83 km/hr, what is the kinetic energy of the automobile? 83 km/hr X 1 hr/60min X 1 min/60s X 1000 m/km = m/s Use KE = 1/2mv 2 KE = 1/2( kg)(23.056m/s) 2 KE = 1/2 ( kg)( m 2 /s 2 ) KE = KE = 9.3 X 10 5 kg  m 2 /s 2 KE = 9.3 X 10 5 J

Energy Flow.

Work and Energy. –Energy is used to do work on an object, exerting a force through a distance. –This force is usually against something and there are five main groups of resistance. Work against inertia. –Since inertia is an objects resistance to change of motion, it naturally follows that this would resist forces acting upon it.

Work against fundamental forces. –Gravitational attraction. –Electromagnetic forces. –Nuclear forces. Work against friction –Friction is always present when two objects are in contact with each other. –Friction is always a force in the opposite direction of the applied force.

Work against shape. –Work is needed to stretch or compress an object. –This is what happens when we work against the shape of a spring. Work against any combination of inertia, fundamental forces, friction, or shape.

Examples of working against: (A) inertia, (B) gravity, (C) friction, and (D) shape.

–Some kind of energy change has taken place, which may include one of the following: Increased kinetic energy. –Work against inertia results in energy of motion for an object. Increased potential energy. –Work against fundamental forces and work against shape result in an increase in energy of position (potential energy)

Increased temperature. –Work against friction always results in an increase in temperature. Increased combination of kinetic energy, potential energy, and/or temperature.

Energy Forms. (five forms). –Mechanical energy. Usually associated with the kinetic energy of everyday objects and potential energy that results from the effect of gravity.

Mechanical energy is the energy of motion, or the energy of position, of many familiar objects. This boat has energy of motion.

–Chemical energy. Chemical energy is the form of energy associated with chemical reactions. Chemical energy is released during the process known as oxidation. Chemical energy is potential energy that is released when chemical reactions break bonds in molecules.

Chemical energy is a form of potential energy that is released during a chemical reaction. Both (A) wood and (B) coal have chemical energy that has been stored through the process of photosynthesis. The pile of wood may provide fuel for a small fireplace for several days. The pile of coal might provide fuel for a power plant for a hundred days.

–Radiant energy. Radiant energy is the form of energy that travels through space. Also called electromagnetic radiation. Visible light is one small part of the electromagnetic radiation.

Radiant energy is energy that travels through space.. (A) This demonstration solar cell array converts radiant energy from the sun to electrical energy, producing an average of 200,000 watts of electric power (after conversion). (B) Solar panels are mounted on the roof of this house.

The electromagnetic spectrum includes many forms of radiant energy. Note that visible light occupies only a tiny part of the entire spectrum.

–Electrical energy. Electrical energy is a form of energy that comes from electromagnetic interactions. Electrical energy that travels through the wires in our homes to light or houses is a familiar form of electrical energy.

–Nuclear energy. this is the form of energy generated in nuclear power plants.

The blades of a steam turbine. In a power plant, chemical or nuclear energy is used to heat water to steam, which is directed against the turbine blades. The mechanical energy of the turbine turns an electric generator. Thus a power plant converts chemical or nuclear energy to mechanical energy, which is then converted to electrical energy.

Energy Conversion. –Energy can be converted from one form to another. –For example, during a fall PE lost = KE gained –mgh = 1/2mv 2 –Solving for v f –v f =  2gh –This allows you to calculate the final velocity of a falling object after its potential energy is converted into kinetic energy.

This pendulum bob loses potential energy (PE) and gains an equal amount of kinetic energy (KE) as it falls through as distance h. The process reverses as the bob moves up the other side of its swing.

The ball trades potential energy for kinetic energy as it falls. Notice that the ball had 98 J of potential energy when dropped and has a kinetic energy of 98 J just as it hits the ground.

The energy forms and some conversion pathways.

Energy arrives from the sun, goes through a number of conversions, then radiates back into space. The total sum eventually equals the original amount that arrived.

–Example a 11.1 kg rock falls from a height of 10.1 m. What is its velocity as it hits the floor. v f =  2gh =  2(9.8m/s 2 )(10.1m) =  m 2 /s 2 = 14.1 m/s

Energy Conservation. –Any form of energy can be converted into another form. –The total amount of energy remains constant. –Law of Conservation of Energy: Energy is never created or destroyed. Energy can be converted from one form to another, but the total energy remains constant.

Energy Transfer. –Any time energy is transferred, either work or heat is involved.

Energy Sources Today.

Introduction –Petroleum is our most widely used source of energy. Petroleum provides about 40 percent of the energy used by the US. –Natural gas provides about 23 percent of our energy needs. –Coal provides about 23 percent of our energy needs. –Alternative energies (solar, wind, geothermal) provide less than 0.5 percent of the total.

–Over 99 percent of our energy needs are supplied by 4 sources: Petroleum. Coal. Hydropower. Nuclear.

Petroleum. –Petroleum is oil that comes from oil bearing rocks. –Petroleum and natural gas come from organic sediments, material that have settled out of water. –Most of the organic material comes from plankton (phyto- and zoo-) –The process of converted live organisms into petroleum and natural gas takes millions of years. –Natural gas forms under higher temperatures that petroleum.

Coal. –Coal forms from an accumulation of plant materials that collected millions of years ago. –Carbon rich decayed plant material is called peat. –Pressure, compaction, and heating are brought about by movement of the Earth's crust eventually change the water content and release the carbon in the materials, it has now begun the process toward coal formation.

–Coal is ranked according to how long it took to form and how hard it is. Lignite is the lowest ranked and is softest, took the least time to form, and burns quickest so contains the least amount of usable energy. Bituminous is the next highest raking. Anthracite is the hardest and took the longest to form and so contains the most usable energy. Softer coal also has more impurities which contribute to increased pollution levels.

Water Power. –Moving water is a source of renewable energy that has been used for thousands of years. –At present in the US we have built about all of the hydropower plants that we can as we have no usable sources of moving water left.

Nuclear Power. –Nuclear power plants use the energy that is release from the splitting of uranium atoms and plutonium atoms to produce electrical energy.

(A) The sources of energy and (B) the uses of energy during the 1990s.