Transcript Chapter 7
Chapter Seven: Energy 7.1 Energy and Systems 7.2 Conservation of Energy 7.3 Energy Transformations 7.1 What is energy? Energy measures the ability for things to change themselves or to cause change in other things. Some examples are changes in temperature, speed, position, pressure, or any other physical variable. 7.1 Units of energy Pushing a 1-kilogram object with a force of one newton for a distance of one meter uses one joule of energy. A joule (J) is the S.I. unit of measurement for energy. 7.1 Joules One joule is a pretty small amount of energy. An ordinary 100 watt electric light bulb uses 100 joules of energy every second! 7.1 Some forms of energy Mechanical energy is the energy possessed by an object due to its motion or its position. Potential energy and kinetic energy are both forms of mechanical energy. Mechanical Energy Energy associated with motion: 6 7.1 Some forms of energy Chemical energy is a form of energy stored in molecules. Example: Batteries are storage devices for chemical energy. Chemical Energy Energy released or stored when atoms are bonded together. Bonds can be broken to release this energy. The chemical bonds in a matchstick store energy that is transformed into thermal energy when the match is struck. 8 7.1 Some forms of energy Electrical energy comes from electric charge. Caused by the flow of electrons Electric Energy Example: Power lines: Electric energy is easily transported through power lines and converted into other forms of energy 7.1 More forms of energy Nuclear energy is a form of energy stored in the nuclei of atoms. In the Sun, nuclear energy is transformed to heat that eventually escapes the sun as radiant energy. 7.1 More forms of energy Radiant energy is energy that is carried by electromagnetic waves. Light is one form of radiant energy. 7.1 More forms of energy The electromagnetic spectrum includes visible light, infrared radiation (heat), and ultraviolet light. Light energy and heat energy are included in the electromagnetic spectrum. QUIZ TIME! What type of energy cooks food in a microwave oven? ELECTROMAGNETIC ENERGY What type of energy is the spinning plate inside of a microwave oven? MECHANICAL ENERGY QUIZ TIME! Electrical energy is transported to your house through power lines. When you plug an electric fan to a power outlet, electrical energy is transform into what type of energy? MECHANICAL ENERGY QUIZ TIME! What energy transformation occurs when an electric lamp is turned on? ELECTRICAL ENERGY ELECTROMAGNETIC ENERGY What type of energy is shown below? Chemical Energy What type of energy is shown below? Chemical Energy (yummy) What type of energy is shown below? Thermal Energy 7.1 Sources of energy Without the Sun’s energy, Earth would be a cold icy place with a temperature of -273 C. As well as warming the planet, the Sun’s energy drives the entire food chain. 7.1 Sources of energy All objects with mass feel forces in the presence of Earth’s gravity. These forces are a source of energy for objects or moving matter such as falling rocks and falling water. 7.1 Energy and work In physics, the word work has a very specific meaning. A force is said to do work when it acts on an object & there is a displacement in the direction of the force. Work is the transfer of energy that results from applying a force over a distance. Work and Potential Energy The work done on the ball gives the ball gravitational potential energy. Gravitational potential energy = mgh 24 Potential Energy Energy stored in an object due to its position. Examples: 25 Gravitational potential energy Depends on the weight of the object and its height above some reference point. Formula: 26 Gravitational Potential Energy Example: Both blocks acquire the same gravitational potential energy, mgh. The same work is done on each block. What matters is the final elevation, not the path followed. 27 Elastic Potential Energy The potential energy due to the physical distortion (stretched or compressed) of an object. Depends on: Type of material Amount of distortion 28 Elastic Potential Energy Example: Work is done on the bow. The work done is stored in the bow and string as elastic potential energy. After release, the arrow is said to have kinetic energy, 1/2 mv2. Energy is measured in the same units (joules) as work. 29 Potential Energy Systems or objects with potential energy are able to exert forces (exchange energy) as they change. Potential energy is energy due to position, which can change. Potential Energy mass of object (g) PE (joules) height object raised (m) EP = mgh gravity (9.8 m/sec2) Kinetic energy Energy of motion is called kinetic energy. A moving cart has kinetic energy because it can hit another object (like clay) and cause change. Kinetic Energy Energy of motion All moving objects possess kinetic energy Can change from one type to another 33 Factors Which Affect Kinetic Energy: Mass & Velocity KE = ½ 2 mv This makes the velocity much more important than the mass. Doubling the velocity will quadruple the kinetic energy. 34 Kinetic Energy KE (joules) mass of object (kg) EK = ½ mv2 velocity (m/sec) Solving Problems A 2 kg rock is at the edge of a cliff 20 meters above a lake. It becomes loose and falls toward the water below. Calculate its potential and kinetic energy when it is at the top and when it is halfway down. Its speed is 14 m/s at the halfway point. Solving Problems 1. Looking for: …initial EK, EP and EK, EP half way down. 2. Given: mass = 2.0 kg; h = 20 m v = 14 m/s (half way) 3. Relationships: EP =mgh EK = ½ mv2 Assume rock starts from rest. Solving Problems 4. Solution Draw a free body diagram. m = 20 kg EP = (2 kg)(9.8 N/kg)(20 m) = 392 J at top EP = (2 kg)(9.8 N/kg)(10 m) h = 20 m EP = mgh EK = 0 J = 196 J half way EK = 0 J, rock is at rest EK = (1/2)(2 kg)(14 m/s)2 = 196 J half way h = 10 m EP = mgh EK = ½ mv2 Chapter Seven: Energy 7.1 Energy and Systems 7.2 Conservation of Energy 7.3 Energy Transformations 7.2 Conservation of Energy Systems change as energy flows and changes from one part of the system to another. Each change transfers energy or transforms energy from one form to another. 7.2 Energy flow How can we predict how energy will flow? One thing we can always be sure of is that systems tend to move from higher to lower energy. 7.2 Sources of energy The chemical potential energy stored in the food you eat is converted into simple sugars that are burned as your muscles work against gravity as you climb the hill. Bell Work! Energy Transformation 1. What is the formula for Gravitational potential energy? 2. What is the formula for Kinetic energy? 3. How does the Pe at the top of a hill compare to the Ke at the bottom? Energy Transformation The work done in lifting the mass gave the mass gravitational potential energy. Potential energy then becomes kinetic energy. Kinetic energy then does work to push stake into ground. 44 7.2 Units of energy Some units of energy that are more appropriate for everyday use are the kilowatt hour (kWh), food Calorie, and British thermal unit. o F Chapter Seven: Energy 7.1 Energy and Systems 7.2 Conservation of Energy 7.3 Energy Transformations 7.3 Conservation of Energy The idea that energy tranforms from one form into another without a change in the total amount is called the law of conservation of energy. The law of energy conservation says the total energy before the change equals the total energy after it. 7.3 Conservation of Energy When you throw a ball in the air, the energy transforms from kinetic to potential and then back to kinetic. Energy Conservation Total energy is the sum of both types of energy. 50 Energy Conversion in Pendulums 51 Energy conversions All forms of energy can change from one form to another. Examples: 52 Solving Problems A 2 kg car moving with a speed of 2 m/sec starts up a hill. How high does the car roll before it stops? mgh = ½ 2 mv A 2 kg car moving with a speed of 2 m/sec starts up a hill. How high does the car roll before it stops? E = ½ 2kg (2 m/s)2 E=4J K E = 2kg (9.8 m/s2) X p K mgh = ½ 2 mv Solving Problems 1. Looking for: …height of hill 2. Given … mass = 2 kg, v = 2 m/s 3. Relationships: Energy transformed from EK to EP EK = ½ mv2 EP =mgh Solving Problems 1. Solution Find beginning EK EK = ½ (2 kg) (2 m/s)2 = 4 Joules Assume energy before = energy after EK = EP EP =mgh 4 J = mgh h = (4 Nm)/(2 kg)(9.8 N/kg) = .2 m 7.3 Conservation of Energy Many people are concerned about “running out” of energy. What they worry about is running out of certain forms of energy that are easy to use, such as fossil fuels like oil and gas. 7.3 Conservation of Energy It took millions of years to accumulate these fuels because they are derived from decaying, ancient plants that obtained their energy from the Sun when they were alive. Because it took a long time for these plants to grow, decay, and become oil and gas, fossil fuels are a limited resource. 7.3 Conservation of Energy Regular (incandescent) light bulbs convert only 10% of electrical energy to light. That means 90% of the energy is released as wasted heat. 7.3 Conservation of Energy Other forms of energy, such as thermal energy, flowing water, wind, and solar energy are not as limited. Conservation of Energy Energy can be neither created nor destroyed. 62 EOC Pages 184-186 Concepts 5-6, 10-12, Problems 1-5, 7-8, Applying your Knowledge # 4 1.9 cm = 0.019m 62 g = 0.062 kg Speed = 0.01 m 0.01 m