Energy. Copyright 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Transcription

1 Energy Copyright 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings 1

2 Energy is a substance like quantity that can cause change. Makes objects move. Makes things stop. Is needed to do work. It does NOT have mass or volume. 2

3 Energy is stored in objects using one or more storage mechanisms. flows from one storage mechanism to another storage mechanism within the same object. flows from one object to another object. 3

4 Storage Mechanisms are ways in which energy is stored in objects. Storage mechanisms fall into one of three broad categories: Potential Energy Kinetic Energy, E k Thermal Energy, E th 4

5 Potential Energy is energy stored in the structural composition of an object or the position of an object. Potential energy mechanisms: Gravitational Potential Energy, E g Elastic Potential Energy, E el Chemical Energy, E ch Interaction Energy, E i 5

6 Gravitational Potential Energy is stored in an object that is positioned above another object that exerts a gravitational pull on it. Examples: a boulder positioned above the earth a roller coaster at the top of a hill water stored in a reservoir above a turbine at the bottom of a dam 6

7 Elastic Potential Energy is stored in an object that is compressed or stretched. Examples: the twisted rubber band in a toy airplane an archer s bow a stretched bungee cord 7

8 Chemical Energy is stored in the chemical bonds of particles. Examples: the chemical bonds in a peanut butter and jelly sandwich the chemical bonds in coal the chemical bonds in the anodes and cathodes of a battery 8

9 Interaction Energy is stored in the energy that keeps particles separated from each other. Example: Steam has a higher interaction energy than either water or ice because the water particles are kept farther apart when they are in the gas phase and closer together when they are in the liquid and solid phases. 9

10 Learning Check What are the four mechanisms for storing potential energy? 10

11 Learning Check What are the four mechanisms for storing potential energy? Gravitational potential energy Elastic potential energy Chemical energy Interaction energy 11

12 Kinetic Energy is energy stored in the purposeful movement of an object. Examples: a bolder falling off a cliff to the earth below a roller coaster going down its tracks water moving past a turbine blade 12

13 Thermal Energy is energy stored in the random motion of particles (aka atoms and molecules). Examples: the random motion of the hot gas particles in the chimney of a fireplace random motion of particles that make up hot water the random vibration of particles in ice (even cold objects have thermal energy!) 13

14 Internal Energy is the sum of all the energy stored in an object: gravitational potential energy + elastic potential energy + chemical energy + interaction energy = internal energy + kinetic energy + thermal energy Internal energy is represented by the symbol U U = E k + E th + E g + E el + E ch + E i 14

15 Learning Check What are the three broad categories of energy storage mechanisms? What is the name for the total energy stored in an object? 15

16 Learning Check What are the three broad categories of energy storage mechanisms? Potential energy Kinetic energy Thermal energy What is the name for the total energy stored in an object? Internal energy 16

17 Energy Transfer Mechanisms transfer energy from one type of storage mechanism to another type of storage mechanism within the same object, or from one object to another object The three transfer mechanisms are work, W radiation, R heat, Q 17

18 Work transfers energy by pushing, pulling, lifting, falling, expanding or contracting. Examples A person performs work to lift a box from the floor to the top shelf. This increases the gravitational potential energy of the box and decreases the chemical potential energy of the person. Hot gasses perform work to expand the piston in a car s engine. This decreases the chemical potential energy and thermal energy of the gas and increases the kinetic energy of the car. 18

19 Radiation transfers energy stored in one object to energy stored in another object through waves known as electromagnetic radiation. Examples When you feel the sun s warmth on your face. Radiation is transferring thermal energy stored in the sun to thermal energy stored in the particles of your skin. When you put your hand a few inches away from a light bulb. Radiation is transferring thermal energy stored in the light bulb to thermal energy stored in the particles of your skin. 19

20 Heat transfers thermal energy, interaction energy and/or chemical potential energy stored in one object to thermal energy, interaction energy and/or chemical potential energy stored in another object. Examples When you use the stove to boil a pot of water. Heat is transferring thermal energy stored in the hot burner to energy that is stored in the water as thermal energy and interaction energy. When you use a chemical ice pack to stop the swelling of a sports injury. Heat is transferring the thermal energy stored in your body to energy that is stored in the chemicals as chemical energy. 20

21 Heat can transfer energy between objects by Conduction when two objects are in direct contact with each other. Example, touching a pot of boiling water. Convection when the hotter object warms the air around it which in turn warms the colder object. Example, warming your hands above a fire. Thermal energy always flows from the hotter object to the colder object. 21

22 Learning Check What are the 3 transfer mechanisms that transfer energy between objects? What transfer mechanism transfers energy by pulling, pushing, lifting, falling, expanding or contracting? What transfer mechanism transfers energy through waves of energy? What transfer mechanism transfers energy by convection and conduction? 22

23 Learning Check What are the 3 transfer mechanisms that transfer energy between objects? work, radiation & heat What transfer mechanism transfers energy by pulling, pushing, lifting, falling, expanding or contracting? work What transfer mechanism transfers energy through waves of energy? radiation What transfer mechanism transfers energy by convection and conduction? heat 23

24 The Law of Conservation of Energy Energy can neither be created nor destroyed during physical and chemical changes. Nuclear changes are the exception to this law and the law of conservation of matter. During nuclear changes mass is changed into energy. 24

25 The Law of Conservation of Energy Since energy can neither be created nor destroyed, Amt of energy Amt of energy lost from = is gained by one object another object(s) Wood loses heat Surroundings gain energy stored transfers energy stored as chemical energy energy as thermal energy 25

26 The Law of Conservation of Energy changes in the equal the sum of kinetic energy work thermal energy + gravitational PE = radiation elastic PE + chemical energy heat interactive energy of an object added or removed from the object 26

27 The Law of Conservation of Energy The conservation of energy equation ΔU = ΔE k + ΔE th + ΔE g + ΔE el + ΔE ch + ΔE i = W + Q + R Sign Convention Negative numbers = Energy transferred out Positive numbers = Energy transferred in 27

28 Systems When investigating energy changes it is essential to clearly define the system you are investigating. A system is simply the environment you are studying. It could be a single object or a group of objects. In this picture the system could be the beaker of water and the Bunsen burner or it could be just the beaker of water. 28

29 Some Simplifications for Chemistry In chemistry we will focus on chemical energy interaction energy thermal energy heat Storage mechanisms Transfer mechanism So The change in a system s thermal, interactive and chemical energies is equal to the energy heat transfers in or out of the system. ΔU = ΔE th + ΔE ch + ΔE i = Q 29

30 Learning Check What does the Law of Conservation of Energy mean? Why is it important to define the system when investigating or analyzing energy? 30

31 Learning Check What does the Law of Conservation of Energy mean? Energy is neither created nor destroyed so the amount of energy lost from one object is gained by another object(s). Why is it important to define the system when investigating or analyzing energy? We must define the system so we can accurately track energy changes into and out of the system. 31

32 Learning Check What storage mechanisms do we focus on in chemistry? How are a system s total stored energy and the energy transferred into and out of the system related? 32

33 Learning Check What storage mechanisms do we focus on in chemistry? thermal energy, chemical energy and interactive energy How are a system s total stored energy and the energy transferred into and out of the system related? The change in a system s total stored energy is equal to the energy transferred into or out of the system. 33

34 Tracking Energy Changes We will use column graphs and arrows to track the flow of energy between objects. Here are the key concepts to follow: 34 Identify the storage mechanism(s) in each object that gain and lose energy during the change. Identify the transfer mechanism(s) that transfer the energy between the objects. Determine which object gains energy and which one looses energy. Use column graphs with labels to show the units of energy that are lost or gained by each storage mechanism. Use arrows with arrows to show the units of energy being transferred. Let s look at the example on the next slide.

35 Tracking Energy Changes Lets study how energy flows when an ice cube is put in a glass of room temperature water. Energy flows from hot to cold, so energy stored as E th is transferred via the heat transfer mechanism, Q, to the ice cube. The ice cube stores this energy as E th because its temperature increases and as E i because it changes from a solid to a liquid. E i does not change for the water because it started as a liquid and remains a liquid. Energy is conserved because the energy the warm water lost is equal to the energy the ice cube water gained. 35

36 Tracking Energy Changes Lets study how energy flows when a student lifts their backpack from the floor to the desk. The student uses his or her muscles to lift the backpack, so they are using the energy stored in their muscles. Therefore, the energy in the E ch mechanism decreases. The energy flows from the student to the box via the Work transfer mechanism. The position of the box is higher when it is sitting on the desk, so the energy coming into the box is stored in the E g mechanism. Energy is conserved because the amount of energy the person lost is equal to the amount of energy the box gained. 36