Types of EnergyChapter 11

System – The object or objects of interest that can interact with each other and with the outside world.

Work-energy theorem – Doing work on a system, the result is a change in the system’s energy.

Review Vocabulary

What are some examples you use?

Food/drinks, athletes in sports, the companies that supply your home with electricity, gas and heat.

How is energy used in every day speech?

Work is the process that transfers energy between a system and the external world. When an agent performs work on a system, the system’s energy increases, when the system does work on the surroundings, the system’s energy decreases.

This chapter looks at mechanical energy.

How do scientist and engineers use the term energy?

Yes. It is like ice cream – there are a lot of varieties, but they are all types of energy

Can energy change forms?

Energy Bar diagram – keeps track of the energy stored or used

We will use these diagrams when we look at the conservation of energy

What diagram is used to show the transformation of energy?

What is kinetic energy? The energy that is due to the motion of a

system’s center of mass. Think of it as your moving energy.

What is potential energy?

Energy that is stored due to interactions between objects in a system. There is more than one type of potential energy. Brain POP

Energy in Roller Coasters

Types of Mechanical Energy

Let h represent the orange’s height measured from the jugglers hand. On the way up, its displacement is upward, but the gravitational force on the orange (Fg) is downward, so the work done is negative. On the way back down, the force and displacement are in the same direction so work is positive.

Describe the work done by gravity as the orange rises from the

juggler’s hand.

While the orange moves upward, gravity does negative work which slows the orange to a stop. When it goes back down, gravity does positive work, increasing the orange’s speed and kinetic energy.

At the top, the orange has all GPE. At the person’s hand, the orange has all KE. Halfway between these points there is equal GPE and KE.

Describe the work done by gravity as the orange rises from the

juggler’s hand.

Type of Potential Energy

The stored energy due to gravity.

Gravitational Potential Energy (GPE).

The position defined to be zero. If you look at the picture of the girl juggling oranges, you could use her hands as the reference level or the top height of the orange as a reference level.

What is a reference level?

The stored energy due to the object’s change in shape.

What types of objects have elastic potential energy?

Springs, rubber bands, trampolines, string on a bow and arrow.

Elastic Potential Energy (EPE)

Recognized that mass is a type of energy called rest energy (E0).

Albert Einstein

Chemical and nuclear – the chemical bonds (which store energy) are broken down. The energy is released when the structure of the atom’s nucleus changes.

Thermal Energy– the sum of kinetic and potential energy of the particles in the system. Ex: rubbing your hands together.

Electrical energy – associated with charged particles. Ex. Power plants

Radiant energy – energy transferred by electromagnetic waves.

Other forms of energy

EPE = k = spring constant x = displacement of object (how far it is

stretched) KE =

m = mass v = velocity

Mechanical Energy Equations

GPE = m = mass g = gravity (we are now using down as

positive) h = height

E0 =

m = mass c = speed of light (3.0 x 108 m/s)

Mechanical Energy Equations

Energy is measured in Joules (J)

This make sense because the change in energy is equal to work.

Unit of Energy

Conservation of Energy

Closed System – A system that does not gain or lose mass.

Law of Conservation of Energy – states that in a closed, isolated system, energy can neither be created nor destroyed; energy is conserved.

Mechanical Energy – the sum of the kinetic and potential energy of the system’s object.

Vocabulary

ME = KE + PE

Mechanical energy is the sum of kinetic and potential energy.

What is the equation that represents mechanical energy?

Yes – gravitational and elastic

Can you use both types of potential energy to find the

mechanical energy of a system?

Match the scenario with the correct energy bar diagram.

1 2 3

B C A

Can mechanical energy be conserved? Yes

What is the conservation of mechanical energy (KEi + PEi = KEf + PEf)?

The sum of the system’s kinetic energy and potential energy before an event is equal to the sum of the system’s kinetic energy and potential energy after that event.

Conservation of Mechanical Energy

Yes, the energy is just transformed into a different type (non-mechanical energy). Energy cannot just disappear.

EX: Friction and air resistance

If friction is present in a system, is energy is still conserved? Why?

Some of the original mechanical energy in the system transforms into another form of energy within the system or transmits energy outside the system.

Ex: warmer = thermal energy, sound = sound energy

How do objects in everyday life (roller coasters, bouncy ball, etc.) stop

without violating the law of conservation of energy?

1. Carefully identify the system.2. Identify the forms of energy in the system.

Identify which forms are part of the mechanical energy of the system.

3. Identify the initial and final states of the system.4. Is the system isolated?

There are no external forces acting on the system. Einitial = Efinal

If there are external forces, then final energy is the sum of the inital energy and the work done on the system. Remember, work can be negative. Einital + W = Efinal

What are the problem solving strategies for conservation of energy?

5. For an isolated system identify the types of energy in the system before and after. If the only forms of energy are potential and kinetic, mechanical energy is conserved. KEi + PEi = KEf + PEf

6. Decide on the reference level for gravitational potential energy.

7. Drawing an energy bar diagram showing the initial and final energies may help.

What are the problem solving strategies for conservation of energy?