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Energy: Transformation and Transfer PSc.3.1 OBJECTIVE: Understand types of energy, conservation and energy transfer. Energy THERMAL The ability to cause change. internal motion of particles MECHANICAL NUCLEAR ENERGY motion of objects changes in the nucleus ELECTRICAL CHEMICAL bonding of atoms joules (J) motion of electric charges Objectives PSc.3.1.1 • Explain thermal energy and its transfer. Temperature Temperature is a measure of the average kinetic energy of the particles in a sample of matter. Thermal Energy Thermal Energy is the total energy of the particles in a material. It is the sum of the kinetic energy (due to the movement of particles) and the potential energy (due to the forces within or between particles due to position). Thermal Energy Thermal Energy depends on the temperature, mass, and type of substance. Thermal Energy Thermal Energy • As temperature increases, so does thermal energy (because the kinetic energy of the particles increased). • Even if the temperature doesn’t change, the thermal energy in a more massive substance is higher (because it is a total measure of energy). Thermal Energy Which beaker of water has more thermal energy? • B - same temperature, more mass 80ºC A 80ºC B 200 mL 400 mL Heat Heat involves a transfer of energy (a flow of thermal energy) between 2 objects due to a temperature difference. Cup gets cooler while hand gets warmer Ice gets warmer while hand gets cooler Heat flows from “hot to cold.” Heat Transfer Why does A feel hot and B feel cold? • Heat flows from A to your hand = hot. • Heat flows from your hand to B = cold. 80ºC A 10ºC B Law of Conservation of Energy When the warmer object loses heat, its temperature decreases and q is negative. When the cooler object absorbs heat, its temperature rises and q is positive. Specific Heat Some things heat up or cool down faster than others. Land heats up and cools down faster than water. Specific Heat The specific heat of any substance is the amount of heat required to raise the temperature of one gram of that substance by one degree Celsius. C water = 4184 J / kg °C C sand = 664 J / kg ° C This is why land heats up quickly during the day and cools quickly at night and why water takes longer. Specific Heat Because different substances have different compositions, each substance has its own specific heat. Specific Heat Values (J/(kg·K)) Water 4184 Alcohol 2450 Aluminum 920 Carbon (graphite) 710 Sand 664 Iron 450 Copper 380 Silver 235 Why does water have such a high specific heat? water metal Water molecules form strong bonds with each other; therefore it takes more heat energy to break them. Metals have weak bonds and do not need as much energy to break them. Heat Transfer Which sample will take longer to heat to 100°C? Specific Heat Values (J/(kg·K)) 50 g Al 50 g Cu Aluminum Copper 920 380 • Al - It has a higher specific heat. • Al will also take longer to cool down. Specific Heat q = m Cp ΔT q = heat (J) Cp = specific heat (J/(g.°C) m = mass (g) ΔT = change in temperature = Tf – Ti (°C) Exothermic and Endothermic Exothermic: Heat flows out of the system (to the surroundings). The value of ‘q’ is negative. Endothermic: Heat flows into the system (from the surroundings). The value of ‘q’ is positive. Example The temperature of a sample of iron with a mass of 10.0 g changed from 50.4°C to 25.0°C with the release of 114 J heat. What is the specific heat of iron? -114 q = 10.0 m Ciron (25.0 ∆T – 50.4) Ciron = 0.449 J/g°C Problem #1 A piece of metal absorbs 256 J of heat when its temperature increases by 182°C. If the specific heat of the metal is 0.301 J/g°C, determine the mass of the metal. m = 4.67 g Problem #2 If 335 g water at 65.5°C loses 9750 J of heat, what is the final temperature of the water? (Cp = 4.18 J/g*C) Tf = 58.5 °C Measuring Heat Heat changes that occur during chemical and physical processes can be measured accurately and precisely using a calorimeter. A calorimeter is an insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process. A coffee-cup calorimeter made of two Styrofoam cups. Phase Changes Melting Solid Vaporization Liquid Freezing Gas Condensation Sublimation Melting Vaporization Solid Liquid Freezing Gas Condensation Deposition Heating Curve for Water 120 boiling Water and 100 Steam Steam 80 Temperature is constant Water during a phase change! 60 40 20 melting 0 Ice Water and Ice -20 0 40 120 220 760 800 Energy and Phase Change Latent heat is the energy released or absorbed by a substance in order for a phase change to occur. Latent heat relates to potential energy, NOT the average kinetic energy of the particles because the temperature remains the same. Energy and Phase Change If heat is used to change state, then that energy is used for that purpose and the substance does not get any hotter. It gives the particles in the substance more ‘freedom’ rather than increasing their kinetic energy. It is increasing their potential energy. Heat Transfer Method Conduction Convection Radiation Notes Heat Transfer Thermal energy flows from higher temperature to lower temperature. This process is called heat transfer. There are three ways heat flows: • heat conduction, • convection, and • thermal radiation. Heat Transfer Heat conduction is the transfer of heat by the direct contact of particles of matter. Heat Transfer Conduction occurs between two materials at different temperatures when they are touching each other. Heat Transfer Thermal equilibrium occurs when two bodies have the same temperature. No heat flows in thermal equilibrium because the temperature is the same in the two materials. Thermal Conductors and Insulators Materials that conduct heat easily are called thermal conductors and those that conduct heat poorly are called thermal insulators. Convection Convection is the transfer of heat through the motion of matter such as air and water. Convection The hot water at the bottom of the pot rises to the top and replaces the cold water. Convection Convection is mainly what distributes heat throughout a room. Thermal Radiation Heat from the Sun is transferred to Earth by thermal radiation. All the energy the Earth receives from the Sun comes from thermal radiation. Thermal Radiation The higher the temperature of an object, the more thermal radiation it emits. Thermal Radiation Thermal radiation is also absorbed by objects. The amount of thermal radiation absorbed depends on the surface of a material. Thermal Radiation Dark surfaces absorb most of the thermal radiation they receive. Silver or mirrored surfaces reflect thermal radiation. Objectives PSc.3.1.2 • Explain the law of conservation of energy in a mechanical system in terms of kinetic energy, potential energy and heat. The Nature of Energy Energy is the ability to do work or produce heat. It exists in two basic forms, potential energy and kinetic energy. The Nature of Energy Potential energy is energy due to the composition or position of an object. Gravitational potential energy is energy stored by objects due to their position above the Earth’s surface. Potential Energy ♦ Which boulder has greater gravitational potential energy? It has a greater height from the ground. The Nature of Energy Kinetic energy is energy of motion. The kinetic energy of a moving object depends on the object’s mass and its velocity. Kinetic Energy ♦ Which has the most kinetic energy? 80 km/h truck Which has the least kinetic energy? 80 km/h 50 km/h 50 km/h motorcycle 80 km/h Law of Conservation of Energy The law of conservation of energy states that in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed. The Nature of Energy The potential energy of dammed water is converted to kinetic energy as the dam gates are opened and the water flows out. Law of Conservation of Energy To better understand the conservation of energy, suppose you have money in two accounts at a bank and you transfer funds from one account to the other. Law of Conservation of Energy Although the amount of money in each account has changed, the total amount of your money in the bank remains the same. Law of Conservation of Energy • Mechanical energy is the total amount of potential and kinetic energy in a system and can be expressed by this equation. mechanical energy = potential energy + kinetic energy Conservation of Energy Potential Energy Kinetic Energy Conservation of Energy Potential Energy Kinetic Energy Conservation of Energy Potential Energy Kinetic Energy Is Energy Always Conserved? ♦ While coasting along a flat road on a bicycle, you know that you will eventually stop if you don’t pedal. ♦ If energy is conserved, why wouldn’t your kinetic energy stay constant so that you would coast forever? The Effect of Friction ♦ You know from experience that if you don’t continue to pump a swing or be pushed by somebody else, your arcs will become lower and you eventually will stop swinging. The Effect of Friction ♦ In other words, the mechanical (kinetic and potential) energy of the swing seems to decrease, as if the energy were being destroyed. Is this a violation of the law of conservation of energy? The Effect of Friction ♦ With every movement, the swing’s ropes or chains rub on their hooks and air pushes on the rider. ♦ Friction and air resistance cause some of the mechanical energy of the swing to change to thermal energy. The Effect of Friction ♦ With every pass of the swing, the temperature of the hooks and the air increases a little, so the mechanical energy of the swing is not destroyed. ♦ Rather, it is transformed into thermal energy (heat). The Effect of Friction Remember heat, which is represented by the symbol q, is energy that is in the process of flowing from a warmer object to a cooler object. The SI unit of heat and energy is the joule (J). The Effect of Friction Conservation of Energy Mechanical Energy Thermal Energy Near the end of the run, the skier encounters the force of friction. Objectives PSc.3.1.3 • Explain work in terms of the relationship among the applied force to an object, the resulting displacement of the object, and the energy transferred to an object. Work Work is the transfer of energy through motion. It is also a force exerted through a distance. W = Fd W: F: d: work (J) force (N) distance (m) 1 J = 1 N·m Distance must be in direction of force! Work Work ♦ When you lift a stack of books, your arms apply a force upward and the books move upward. Because the force and distance are in the same direction, your arms have done work on the books. Work ♦ When you carry books while walking, you might think that your arms are doing work. ♦ However, in this case, the force exerted by your arms does no work on the books. Work Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back? GIVEN: F = 30 N d = 1.5 m W=? WORK: W = F·d W = (30 N) (1.5 m) W = 45 J W F d Work A forklift does 12300 J of work while raising a pallet with 4900 N of physics textbooks from the ground to an unknown height. Calculate the distance the books were raised. GIVEN: F = 4900 N d=? W = 12300 J W F d WORK: d=WF d = (12300 J) (4900 N) d = 2.5 m Work Two men do 235440 J of work to push a car 218 m to the nearest fuel station. Determine the force applied to the car. GIVEN: W = 2354400 J d = 218 m F=? W F d WORK: F=Wd F = (235440 J) (218 m) F = 1080 N Work A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during the lift? GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - after W=? W F d WORK: F = m·a F =(40 kg)(9.8 m/s2) = 392 N W = F·d W = (392 N)(1.4 m) W = 549 J during lift Work A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina after the lift? GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - after W=? W F d WORK: W = F·d No work after lift. “d” is not in the direction of the force. Objectives PSc.3.1.4 • Explain the relationship among work, power and simple machines both qualitatively and quantitatively. Power ♦ Power is the amount of work done in one second. It is a rate — the rate at which work is done. Power W Fd P Fv t t P = Power (Watts - W) v = average velocity (m/s) Power WW Fd P P P t FFvv t t Fd P t Power A 5 kg cart is pushed by a 30 N force against friction for a distance of 10 m in 5 seconds. Determine the power needed to move the cart. GIVEN: m = 5 kg F = 30 N d = 10 m t=5s P=? WORK: P = Fd t P = (30 N) (10 m) 5 P = 60 W Fd P t Power Jerome does 2289 J of work while running up a flight of stairs. If his power is 1730 W, how long does it take him to climb the stairs? GIVEN: W = 2289 J P = 1730 J t=? W P t WORK: t=WP t = 2289 J 1730 W t = 1.32 s Power A 245 N crate it lifted on to a ledge by a worker that exerts 325 W of power. With what speed was the crate lifted to the ledge? GIVEN: F = 245 N P = 325 W v= ? P F v WORK: v=PF v = 325 W 245 N v = 1.33 m/s Simple Machines • A machine is a device that makes doing work easier. • Machines can be simple. Simple Machines • Some, like knives, scissors, and doorknobs, are used everyday to make doing work easier. • Machines can make work easier by increasing the force that can be applied to an object. Simple Machines • A second way that machines can make work easier is by increasing the distance over which a force can be applied. • Machines can also make work easier by changing the direction of an applied force. Increasing Force • A car jack is an example of a machine that increases an applied force. • The upward force exerted by the jack is greater than the downward force you exert on the handle. Increasing Force • However, the distance you push the handle downward is greater than the distance the car is pushed upward. • The jack increases the applied force, but doesn't increase the work done. Force and Distance • The work done in lifting an object depends on the change in height of the object. Force and Distance • The same amount of work is done whether the mover pushed the container up the long ramp or lifts it straight up. • If work stays the same and the distance is increased, then less force will be needed to do the work. Changing Direction • Some machines change the direction of the force you apply. • The wedge-shaped blade of an axe is one example. The Work Done by Machines • When you use an axe to split wood, you exert a downward force as you swing the axe toward the wood. The Work Done by Machines • The blade changes the downward force into a horizontal force that splits the wood apart. The Work Done by Machines • When you use a machine such as a crowbar, you are trying to move something that resists being moved. The Work Done by Machines • If you use a crowbar to pry the lid off a crate, you are working against the friction between the nails in the lid and the crate. The Work Done by Machines • You also could use a crowbar to move a large rock. • In this case, you would be working against gravity—the weight of the rock. Input and Output Forces • Two forces are involved when a machine is used to do work. • The force that is applied to the machine is called the input force. • The input force is also referred to as the effort force. • FE stands for the effort force. Input and Output Forces • The force applied by the machine is called the output force. • The output force is also called the resistance force, symbolized by FR. Work • Two kinds of work need to be considered when you use a machine—the work done by you on the machine and the work done by the machine. • The work done by you on a machine is called the input work and is symbolized by Win. Work • The work done by the machine is called the output work and is abbreviated Wout. Conserving Energy • When you do work on the machine, you transfer energy to the machine. • When the machine does work on an object, energy is transferred from the machine to the object. Conserving Energy • The amount of energy the machine transfers to the object cannot be greater than the amount of energy you transfer to the machine. • A machine cannot create energy, so Wout is never greater than Win. Conserving Energy • When a machine is used, some of the energy transferred changes to heat due to friction. • The energy that changes to heat cannot be used to do work, so Wout is always smaller than Win. Ideal Machines • Suppose a perfect machine could be built in which there was no friction. • None of the input work or output work would be converted to heat. Ideal Machines • For such an ideal machine, the input work equals the output work. Ideal Machines • Suppose the ideal machine increases the force applied to it. • This means that the output force is greater than the input force. • Recall that work is equal to force times distance. Ideal Machines • If the output force is greater than input force, then Win and Wout can be equal only if the input force is applied over a greater distance than the output force is exerted over. Actual Mechanical Advantage • The ratio of the resistance force (output force) to the effort force (input force) is the actual mechanical advantage (AMA) of a machine. Actual Mechanical Advantage • The actual mechanical advantage of a machine can be calculated from the following equation. FR AMA FE • FR = resistance force • FE = effort force Mechanical Advantage • Window blinds are a machine that changes the direction of an input force. • A downward pull on the cord is changed to an upward force on the blinds. Mechanical Advantage • The input and output forces are equal, so the MA is 1. Ideal Mechanical Advantage • The mechanical advantage of a machine without friction is called the ideal mechanical advantage, or IMA. • The IMA can be calculated by dividing the effort (input) distance by the resistance (output) distance. Ideal Mechanical Advantage dE IMA dR • dR = resistance distance • dE = effort distance Efficiency • Efficiency is a measure of how much of the work put into a machine is changed into useful output work by the machine. • A machine with high efficiency produces less heat from friction so more of the input work is changed to useful output work. Calculating Efficiency • To calculate the efficiency of a machine, the output work is divided by the input work. • Efficiency is usually expressed as a percentage by this equation: Wout efficiency x100 Win Calculating Efficiency • In an ideal machine there is no friction and the output work equals the input work. So the efficiency of an ideal machine is 100 percent. • The efficiency of a real machine is always less than 100 percent. Increasing Efficiency • Machines can be made more efficient by reducing friction. This usually is done by adding a lubricant, such as oil or grease, to surfaces that rub together. Increasing Efficiency • A lubricant fills in the gaps between the surfaces, enabling the surfaces to slide past each other more easily. Machines and Work – Problem #1 You are using a lever to lift the edge of a crate in order to slide a roller under it. The crate weighs 5250 N. You are able to exert a force of 400 N and move the handle of the lever 1.20 meters. The crate is lifted a distance of 0.0800 meters. (a) What is the input work done? (a) Win = 480 J (b) What is the output work done? (b) Wout = 420 J Machines and Work – Problem #1 You are using a lever to lift the edge of a crate in order to slide a roller under it. The crate weighs 5250 N. You are able to exert a force of 400 N and move the handle of the lever 1.20 meters. The crates is lifted a distance of 0.0800 meters. (c) What is the efficiency of the lever? (c) eff = 87.5% Machines and Work – Problem #1 You are using a lever to lift the edge of a crate in order to slide a roller under it. The crate weighs 5250 N. You are able to exert a force of 400 N and move the handle of the lever 1.20 meters. The crate is lifted a distance of 0.0800 meters. (d) What is the actual mechanical advantage of the lever? (d) AMA = 13.1 (e) What is the ideal mechanical advantage? (e) IMA = 15 Machines and Work – Problem #2 A ramp is used to raise barrels that weigh 824 N up onto a 1.50 meter high loading dock. The ramp is 4.00 m long. The ramp’s actual mechanical advantage is 2.20. (a) How much effort must be exerted to roll the barrels up the ramp? (a) FE = 375 N (b) What is the input work? (b) Win = 1500 J Machines and Work – Problem #2 A ramp is used to raise barrels that weigh 824 N up onto a 1.50 meter high loading dock. The ramp is 4.00 m long. The ramp’s actual mechanical advantage is 2.20. (c) What is the output work? (c) Wout = 1236 J (d) What is the efficiency of the lever? (d) eff = 82.4% Machines and Work – Problem #2 A ramp is used to raise barrels that weigh 824 N up onto a 1.50 meter high loading dock. The ramp is 4.00 m long. The ramp’s actual mechanical advantage is 2.20. (e) What is the lever’s ideal mechanical advantage? (e) IMA = 2.67 Machines and Work – Problem #3 A pulley’s actual mechanical advantage is 8.00 and you are using it to raise containers weighing 12,400 N to a height of 14.0 m. You need to pull out 122 m of cable to lift the crates. (a) What amount of effort is required to lift the containers with this pulley? (a) FE = 1550 N (b) What is the input work done? (b) Win = 189100 J Machines and Work – Problem #3 A pulley’s actual mechanical advantage is 8.00 and you are using it to raise containers weighing 12,400 N to a height of 14.0 m. You need to pull out 122 m of cable to lift the crates. (c) What is the output work done? (c) Wout = 173600 J (d) What is the pulley’s efficiency? (d) eff = 91.8% Types of Simple Machines • A simple machine is a machine that does work with only one movement of the machine. • There are six types of simple machines: lever, pulley, wheel and axle, inclined plane, screw and wedge. Types of Simple Machines Levers • A lever is a bar that is free to pivot or turn around a fixed point. • The fixed point the lever pivots on is called the fulcrum. Levers • The input arm of the lever is the distance from the fulcrum to the point where the input force is applied. • The output arm is the distance from the fulcrum to the point where the output force is exerted by the lever. Levers • If the output arm is shorter than the input arm, then the output force is greater than the input force. Ideal Mechanical Advantage of a Lever • The IMA of a lever can be calculated from this equation: Pulleys • A pulley is a grooved wheel with a rope, chain, or cable running along the groove. • A fixed pulley is a modified first-class lever. • The axle of the pulley acts as the fulcrum. Fixed Pulleys • A fixed pulley is attached to something that doesn't move, such as a ceiling or wall. • Because a fixed pulley changes only the direction of force, the IMA is 1. Wheel and Axle • A wheel and axle is a simple machine consisting of a shaft or axle attached to the center of a larger wheel, so that the wheel and axle rotate together. Wheel and Axle • A wheel and axle is a also a variation of the lever. Mechanical Advantage of the Wheel and Axle • The output force is exerted at the rim of the axle. • So the length of the output arm is the radius of the axle. Mechanical Advantage of the Wheel and Axle • The IMA of a wheel and axle is given by this equation: Inclined Planes • A sloping surface, such as a ramp that reduces the amount of force required to do work, is an inclined plane. Mechanical Advantage of an Inclined Plane • By pushing a box up an inclined plane, the input force is exerted over a longer distance compared to lifting the box straight up. Mechanical Advantage of an Inclined Plane • The IMA of an inclined plane can be calculated from this equation. The Screw • A screw is an inclined plane wrapped in a spiral around a cylindrical post. • You apply the input force by turning the screw. • The output force is exerted along the threads of the screw. The Wedge • The wedge is also a simple machine where the inclined plane moves through an object or material. • A wedge is an inclined plane with one or two sloping sides. It changes the direction of the input force.