ENERGY KEY IDEAS

〉What is the relationship between energy and work?

〉Why is potential energy called energy of position?

〉What factors does kinetic energy depend on?

〉What is nonmechanical energy?

Whenever work is done, energy is transformed or is transferred from one system to another system.

Energy: the capacity to do work

Energy is measured in joules (J).

Energy Potential Energy Kinetic Energy Other Forms of

Energy Mechanical Chemical Nuclear Electrical Light

ENERGY OVERVIEW

〉Potential energy (PE) is sometimes called energy of position because it results from the relative positions of objects in a system.

Potential Energy: the energy that an object has because of the position, shape, or condition of the object

FORMS OF ENERGY

〉Kinetic energy depends on both the mass and the speed of an object.

Kinetic Energy: the energy of an object due to the object’s motion

KE = ½ mass speed squared, or KE= ½mv2

FORMS OF ENERGY

Mechanical Energy: the amount of work an object can do because of the object’s kinetic and potential energies

In most cases, nonmechanical forms of energy are just special forms of either kinetic or potential energy.

FORMS OF ENERGY

Chemical reactions involve potential energy. The amount of chemical

energy associated with a substance depends in part on the relative positions of the atoms it contains.

Living things get energy from the sun. Plants use photosynthesis

to turn the energy in sunlight into chemical energy.

The sun gets energy from nuclear reactions. The sun is fueled by nuclear

fusion reactions in its core.

FORMS OF ENERGY

Energy can be stored in fields.

Electrical energy results from the location of charged particles in an electric field.

When electrons move from an area of higher electric potential to an area of lower electric potential, they gain energy.

FORMS OF ENERGY

Light can carry energy across empty space.

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 energy is another example of energy stored in a field.

FORMS OF ENERGY

You should already have learned that energy is always conserved. Instead of being created or destroyed, energy just changes from one form to another. For example, sunlight is the ultimate source of energy on Earth. Look at the illustration below, and identify the types of energy involved.

1. How does sunlight provide the energy the gir l needs to swing the bat? (Hint: What do you need to have energy?)

2. When the gir l hits the bal l , she exerts a force on it. Does she do work on the bal l in the scientifi c sense of the term? Explain your answer.

3. After the gir l hits the bal l , the bal l moves very fast and has energy. When the ball hits the fi elder’s glove, it stops moving. Given that energy can never be destroyed but merely changes form, what happens to the energy the bal l once had? (Hint: I f you are the fi elder, what do you hear and feel as you catch the bal l?)

PRACTICE

1. When does the sled have the most potential energy? When does it have the least potential energy?

2. Where does the sled have the most kinetic energy and the least kinetic energy?

3. What happens to the relative amounts of potential and kinetic energy as the sled sl ides down the hil l? What happens to the total energy?

4. After the sled reaches the bottom of the hi l l , i t coasts across level ground and eventually stops. What happened to the energy the sled had?

DISCUSS WITH YOUR PARTNER

〉How does energy change?

〉What is the law of conservation of energy?

〉How much of the work done by a machine is actually useful work?

CONSERVATION OF ENERGYKEY IDEAS

Energy cannot be created or destroyed. In other words, the total amount of energy in the universe never changes, although energy may change from one form to another.

Energy does not appear or disappear.

Whenever the total energy in a system increases, it must be due to energy that enters the system from an external source.

LAW OF CONSERVATION OF ENERGY

Systems may be open, closed, or isolated.

• open system: energy and matter are exchanged with the surroundings

• closed system: energy but not matter is exchanged

• isolated system: neither energy nor matter is exchanged

Most real-world systems are open.

ENERGY IN SYSTEMS

〉How much of the work done by a machine is actually useful work?

〉Only a portion of the work done by any machine is useful work— that is, work that the machine is designed or intended to do.

Not all of the work done by a machine is useful work.

because of friction, work output < work input

Effi ciency is the ratio of useful work out to work in.

EFFICIENCY OF MACHINES

State the Law of Conservation of Energy in your own words. Give an example of a situation you have encountered in which this law was demonstrated.

PRACTICE