Integrated Science Unit 2, Chapter 5

Unit Two: Work and Energy Chapter 5 Work, Energy and Power 5.1 Work 5.2 Energy Conservation 5.3 Energy Transformations

Chapter 5 Learning Goals Calculate the amount of work done by a simple machine. Use units of joules to measure the amount of work done. Analyze the effects of changing force or distance in a simple machine. Calculate the efficiency of a machine. Calculate power in machines. Discuss perpetual motion machines.

Chapter 5 Vocabulary Terms chemical energy electrical energy energy transformations heat efficiency horsepower joule energy kinetic energy law of conservation of energy nuclear energy potential energy watt power work radiant energy radiation solar power

5.1 The Meaning of Work The word work is used in many different ways. You work on science problems. You go to work. Your toaster doesn’t work. Taking out the trash is too much work. If you push a box with a force of one newton for a distance of one meter, you have done exactly one joule of work.

5.1 Work Force (N) Work (joules) W = F x d Distance (m)

5.1 Efficiency In a very efficient machine, all (or most) of the work input becomes work output. An engineer would say the machine was 100 percent efficient, because all the input work became output work and none was lost. You calculate efficiency by dividing the work output by the work input.

5.1 Power The rate at which work is done is called power. You can determine the power of a machine by dividing the amount of work done by the time it takes in seconds. The unit of power is called the watt. One watt is equal to one joule of work done in one second. Another unit of power commonly used is the horsepower. One horsepower is equal to 746 watts.

5.1 Power Work (joules) Power (watts) P = W t Time (sec)

5.1 Work What happens when you multiply forces in machines? Key Question: What happens when you multiply forces in machines?

5.1 Mini Quiz How is work defined in science? What are the units of work? What is meant by an efficient machine? What is power? What are the units of power?

5.2 Energy Energy is the ability to do work. Any object that has energy has the ability to create force. Energy is measured in the same units as work. A joule is a unit of force that acts over a distance.

5.2 Potential Energy The first type of energy we will explore is called potential energy. Potential energy comes from the position of an object relative to the Earth. Objects that have potential energy don’t use their energy until they move.

5.2 Potential Energy Ep = mgh Mass (kg) Potential Energy (joules) Height (m) Acceleration of gravity (m/sec2)

5.2 Kinetic Energy Objects also store energy in motion. Energy of motion is called kinetic energy. Applying a force means you do some work, which is stored as energy. Kinetic energy depends on two things: mass and speed.

5.2 Kinetic Energy Ek = 1 mv2 2 Mass (kg) Kinetic Energy Speed (m/sec) (joules) Ek = 1 mv2 2 Speed (m/sec)

5.2 Energy Conservation Nature never creates or destroys energy; energy only gets converted from one form to another. This concept is called the law of conservation of energy. At any moment in its flight, the ball has exactly the same energy it had at the start. The energy is divided between potential and kinetic, but the total is unchanged.

5.2 Energy Conservation What is energy and how does it behave? Key Question: What is energy and how does it behave?

5.2 Mini Quiz What is energy? What are the units of energy? What is potential energy? What is kinetic energy? What is the law of conservation of energy?

5.3 Energy Transformations Energy transformations occur between different types of energy. radiant energy electrical energy chemical energy nuclear energy

5.3 Energy Transformations On a trip up a hill, work is transferred to potential. Potential is transferred to kinetic on the way down the hill. The total energy is conserved.

Energy Transformations in a Power Plant Power plants convert chemical energy into electrical energy. chemical energy heat energy mechanical energy electrical energy

5.2 Other Forms of Energy Radiant energy is also known as electromagnetic energy. Electrical energy we use in our daily lives is actually derived from other sources of energy. Chemical energy is the type of energy stored in molecules. Nuclear energy comes from splitting an atom, or fusing two atoms together. Heat is a form of thermal energy.

5.3 Energy Transformations Key Question: Where did the energy go?

5.3 Mini Quiz What is mechanical energy? How does potential energy get used up? What is radiant energy? What is nuclear energy? What is thermal energy?