7.1-7.3 Energy forms and transformations

What is energy?

• We use the word all the time – but very few people have a strong understanding what it is

• It is a difficult thing to explain – unless you use physics to do so

Why ask?

• Think very carefully about your own definition or understanding about energy

• How would you explain energy to a young child?

Improving your understanding

• It seems that we take the understanding of energy for granted – it seems like such a natural concept to us – a thing that we have always experienced, or understood

• But how much of it do we really understand?

• Here’s the problem: the textbooks don’t help too much too

The official definition• Work:• The energy transferred to an object when a force

acting on the object moves through a displacement (Note: the force must be the one responsible for the movement according to Newton’s second law)

• Energy:• The ability to do work• Though these definitions seem cyclical, it

should be apparent that force and energy are linked together – where one exists, so does the other

So how do we understand energy?

• This is the first time that you will be dealing with energy mathematically and conceptually

• The best way to study energy – is to develop a way to “see” it – but this is not always easy

Because you can’t always see it

• The challenging thing about understanding energy is “seeing” it

• You can now start to understand and “see” forces as a push or pull

• But there are many types of energy you can’t see – but somehow, we make them do work for us

Types of energy

• Heat energy – transferred thermal energy between substances

• Kinetic energy – energy found in moving objects

• Gravitational potential energy – energy stored in objects raised above the ground

Heat, as an example

• Heat is a good example of energy that does work – even though we don’t think of it as that

• It isn’t something that you can always see – but we understand it because it is something we can feel

• But you can’t always do so - when there is very little heat - you don’t notice that it’s there

Hot or cold?• For example, when things are hot – they are

hot because there is a lot of heat• But when things are cold – they are cold

because there is little heat• There is no such thing as “cold” – cold is a

description of when heat is missing• All types of energy are like this• It can be described as a thing that can move

and change from one form or another

Heat can transform into other forms of energy

• So how do we turn one form of energy, like heat, into another form, like electricity?

• Almost all of the ways we generate electricity has to rely on producing heat energy

Nuclear energy

• Nuclear energy is released when the nucleus of an atom is split open

• This type of nuclear reaction is called FISSION (splitting)

• This releases a large amount of energy in the form of heat

Energy can change • Nuclear energy doesn’t just turn into electrical

energy – it is turned into a few different types first

• This is another important characteristic of energy: it can be transformed into another form BUT: it cannot suddenly be created from nothing

• If you look carefully into everything that uses energy or creates it, you can always find out where it originally came from

So, when creating electricity from nuclear energy:

Nuclear energy – created heat

Heat energy – used to boil water to create steam

Kinetic energy – created when steam caused turbines to turn

Electrical energy – created when turning turbines moved magnet that

created an electric current

Does this give you a better understanding?

• It is important to develop your own understanding of energy in order to better understand its characteristics and how it governs interactions in the physical world

• Like force, it can be a very abstract concept and system of thinking – but it is governed by only a few simple rules and simple equations

Work – the link between force and energy

• Let’s begin by understanding that energy is a “thing” that can flow from one place to another

• This characteristic is best seen in heat transfer – but it can be applied when dealing with any type of energy

• If energy is a “thing” that can move from one place or object to another – how does it do it?

Now it’s here – not it’s not

• I want you to think of this scenario – a rolling chair or cart sitting in the middle of the room

• Someone comes by and pushes it – what is the sequence of events?

Some simple points to point out…

• The chair was at rest to start

• The chair moved forward

• The chair came to a stop

• From a dynamics point of view – we know how to describe these events

• But how do we describe this in regard to energy?

Energy• How did the energy flow?

• Energy possessed by the person

• Was transferred to the chair because the person did work on the chair

• The chair gained kinetic energy and moved

• Friction did work on the chair as it moved removing energy

• The chair lost kinetic energy and came to a stop

One way to picture energy

• A good way to become good at analyzing energy is developing a way to “see” it

• Being able to understand how an object comes to possess it – basically, how it flows from one object to another, will help you a great deal with the problems

Positive and Negative work

• Energy is a SCALAR value – although it is calculated by using force, a vector, and displacement, which is a vector

• Work can be positive or negative – but this time, the positives and negatives don’t tell you about direction

• Instead, if work is negative or positive, it tells you whether or not an object has GAINED or LOST energy

When does this happen?

• In the case of the person and the chair in the last example, the person gave energy to the chair so the energy that the chair had is POSITIVE - it gained energy

• This is because the force acted in the same direction as the displacement

• The purpose of the push from the person was to cause that displacement forward

When is it negative?

• Energy is lost when the force acting on the object is trying to STOP the displacement

• When this happens, the force is in the OPPOSITE DIRECTION to the displacement

• What is happening is that the force is taking energy away from the object in an attempt to stop it

Friction is a good way to think about it

• Friction is a good example of this – it is a force that always acts in the opposite direction of a moving object

• We understand that friction causes objects to slow down because it applies a force in the opposite direction to cause it to stop

• But another way to look at it is because friction takes kinetic energy away from objects

Think back to the chair

• If we are only looking at the chair itself at the point that it is released – to the point that it slows down, we say that it has NEGATIVE work done to it

What about pushing a wall?

• If the object you are pushing doesn’t experience a displacement – it means that you have done no work on it

• Since no energy was transferred – ie. The object didn’t gain kinetic energy and didn’t move, the work is said to be zero

• Think about pushing on a wall: even though YOU are using energy and applying a force, that’s not getting to the wall since it doesn’t move

Power

• Power refers to rate of energy use – how much energy is used per second