Unit 3 Section 2 Notes

1. Unit 3 Section 2 Notes What is Energy?

2. Energy and Work • Energy can be defined as: the ability to do work • Most of the time we can’t see energy but it is everywhere around us. • Energy can never be created or destroyed…. It can only be STORED or TRANSFERRED • SI unit for energy is: Joules

3. Energy and Work • All energy is either Potential Energy or Kinetic Energy All Energy Potential Energy Kinetic Energy

4. Potential Energy • PE is the energy stored in an object • “Potential” means the energy has the ability to do something useful later on.

5. Examples of Potential Energy • A stretched rubber band. • Water at the top of a waterfall. • A battery. • A drawn bow and arrow.

6. Potential Energy • Elastic Potential Energy: the energy stored in any type of stretched or compressed elastic material.

7. Potential Energy • This is an example of Potential Energy: there is stored energy in the elastic bands which will convert to kinetic energy when it is released.

8. How are these pictures examples of potential energy?

9. Gravitational Potential Energy • Gravitational Potential Energy: energy stored in objects separated by a distance; results because of gravitational attraction between objects. • Depends on 2 factors: • mass • height

10. Gravitational Potential Energy • The higher an object and the more massive, the more gravitational PE it has. • Gravitational Potential Energy Equation: PE = mgh • grav. PE = mass x free fall acceleration x height

11. Gravitational Potential Energy • A flower pot with a mass of 15 kg is sitting on a window sill 10 m above the ground. How much potential energy does the flower pot contain? • PEgrav = mgh • PEgrav = (15 kg)·(10 m)·(9.8 m/s2) • PEgrav= 1500 J

12. Potential Energy

13. Potential Energy

14. Potential Energy& Chemical Reactions • Chemical Energy: energy stored within atoms and molecules that can be released when a substance reacts.

15. Potential Energy& Chemical Reactions • Chemical reactions involve PE. Why? When a chemical reaction takes place, bonds between atoms break apart and a new substance is formed, which involves changes in energy from the relative positions of atoms in the substance. • If a reaction releases energy, there is a decrease in PE. Example: when a match is struck.

17. Kinetic Energy • KE is the energy of moving objects due to their motion • Depends on 2 factors: • mass • Speed • “Kinetic” means movement

18. Kinetic Energy • Kinetic Energy Equation: KE = ½ mv2 • KE = ½ x mass x speed squared

19. Kinetic Energy • Example: How much kinetic energy does a bicycle with a mass of 14 kg traveling at a velocity of 3.0 m/s east have? • KE = ½ mv2 • KE = ½ (14 kg)·(3.0 m/s)2 • KE = ½ (14 kg)·(9.0 m2/s2) • KE = 63 J

20. Examples of Kinetic Energy

21. Kinetic Energy • Scientific proof for why car crashes are more dangerous at high speeds: KE depends on speed more than mass because speed is squared; so increase in speed = large increase in KE • SI unit for KE: Joules

22. Kinetic Energy • A water bottle is knocked off of a desk. When does it have the MOST KE? • At the top of the fall. • In the middle of the fall. • At the bottom right before it hits the ground.

23. Kinetic Energy • A water bottle is knocked off of a desk. When does it have the MOST KE? • At the top of the fall. • In the middle of the fall. • At the bottom right before it hits the ground.

24. Kinetic Energy • Atoms and molecules have KE. Why? Because atoms and molecules are constantly in motion, and KE is energy of motion. • KE increases as objects get hotter and decreases as objects get cooler.

25. Why Physics is Important! • Someone failed their high school physics, loading the cart so much that it tipped backwards. Poor Donkey!

27. Mechanical Energy • Mechanical energy: the sum of PE and KE in a system • Example: An apple falling from a tree has both PE and KE

28. Mechanical Energy

29. Nonmechanical energy • In almost every system, there are hidden forms of energy that are related to the arrangement of atoms that make up the objects in the system. For example, when you eat an apple, it gives you energy. • Nonmechanical energy: energy that lies at the level of atoms and doesn’t affect motion on a large scale. • In most cases, nonmechanical forms of energy are just special forms of either kinetic or potential energy.

30. Living Things & Energy • Where do we get the energy needed to live? • The energy comes from food. When we eat a meal, we eat plants, animals, or both. Animals also eat plants, other animals or both.

31. Living Things & Energy • Plants and algae do not need to eat because they get their energy directly from sunlight. Plants use photosynthesis to turn the energy in sunlight into chemical energy. This energy is stored in sugars and other organic molecules that make up cells in living tissue. Thus, when you eat a meal, you are really eating stored (potential) energy.

32. Nuclear Reactions • How does the sun get its energy? From nuclear fusion; (fusion is the process in which light nuclei join to form heavy nuclei with a large energy release).

33. Nuclear Reactions • Nuclear power plants use nuclear fission to release energy. In fission, a single large nucleus is split into two or more smaller nuclei with a large release of energy.

34. Electricity • Electricity: a form of energy that results from the flow of charged particles in an electric field. • This is how the lights and appliances in our homes get their energy. Lightning, a form of electrical energy, results from moving electrons between the ground and a cloud.

35. Light Energy • Consider a bright sunny day at the beach. Where is it hotter: in the sand or under the umbrella?

36. Light Energy • The reason its hotter in the sand is that light carries energy. • Light energy travels from the sun to Earth across empty space in the form of electromagnetic waves. Electromagnetic waves are made of electric and magnetic fields, so light is another example of energy stored in a field.