Energy

Law of the Conservation of Energy

The amount of energy in the universe is NOT Infinite

Rather, there is only a finite amount of energy in the universe

Energy cannot be created

Energy cannot be destroyed

Rather, it just changes forms

Forever changing


But what is Energy?

It is surprisingly hard to define - but I see it as being, the ability to do stuff

Without energy the universe would be like a dark, frozen photograph. Everything frozen, nothing moving, no life, no heat, and no anything, as matter is condensed energy.

Energy, moreover the transfers and transformations of energy allow everything to happen.........everything! Light, a thought, an emotion, the moon orbiting the earth, the light and heat of the sun..... everything!

This is the Law of the Conservation of energy: Energy can not be created or destroyed, it just changes forms

So, if we go through a brief history of the energy you are using to read this, it will look like:

Big bang, nuclear fusion, light energy , photosynthesis, chemical potential energy in grass, chemical potential energy in sheep, chemical potential energy in you, kinetic energy in the muscles of your eyes, heat energy

Energy keeps changing forms, or Transforming

Energy can transform from and to any of the types showing in the images below

Energy can be stored Potential Energy, such as Chemical PE, Nuclear PE, Gravitational PE or Elastic PE

Energy can be moving, like Light Energy, Thermal, Sound, Electrical or Kinetic

Joule

James Joule spent time working on energy.

The unit used for quantifying energy is named after him, the Joule


1 Joule is the quantity of energy transformed to accelerate a 1kg mass at a rate of 1ms-2

Likewise,

1 Joule is the quantity of work done when 1 Newton of force moves an object by 1 meter

Likewise,

1 Joule is roughly the energy that a golf ball has when it lands on your bare foot when dropped from 2 meters

Likewise,

1 Joule is the amount of energy used by a 1 Watt led in 1 second


1 Joule = 1kg moved at 1ms-2

and

1 J = 1N.m


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Pages 57 & 58

Gravitational Potential Energy

My dog Zed likes to jump from the table

However, if I stand on the table, pick him up and throw him, will there be a difference?

The higher you are when you jump the greater the impact. You know this when you are up high and look over the edge. Part of your brain goes, that would be bad.

So can we quantify the energy of something up high?

Yes

The higher something is, the more energy it has. It has more energy, because it transformed more energy getting up there.

I climbed Mt Tongariro, I had to put alot of energy in to get to the summit.

The earth itself had to transform alot of energy to create Mr Tongariro.

The Helicopter that takes Fabio Wibmer to the top of his Mountain Bike ride transformed alot of chemical potential energy into kinetic energy and then into gravitational potential energy.

To lift my dog up, I had to transform chemical potential energy into kinetic energy and this energy into gravitational potential energy.

But how much did I transform? How much energy does Zed have when he is 2 meters above the ground? How much Gravitational Potential Energy does he have?

We can calculate that by taking 3 factors

Mass - how massive is the object (kg)

Gravity - with how gravity is on that planet (ms-2 )

Height - how far above the other surface is something (m)

As we can see from this, how much mass something has matters. If I was lifting my big dog Luka up, then, as he is more massive, he would have more gravitational potential energy

Gravity matters as well, if we are on the moon, then jumping from 2 or 3 meters would be fun as there would be little ground shock

On Earth gravity has a value of 9.81ms-2

However, you can round gravity to 10ms-2

Height relative to a surface matters. Notice that it is not height relative to the ground. If I lift up Zed and I drop him on the table, that would be very different to dropping him onto the ground. He his closer to the table, so the height relative to the table is less, thus he has less fall time, his velocity will be less and thus so too will be his impact. So, when calculating height it is relative to a surface. If you are in the Skytower and jump and land on the same floor, because you are still inside, then you wont suddenly splatter on the floor. Even though, relative to the earth outside, your GPE is huge.

Mass, Gravity and Height.... Times these together and you get the objects Gravitational Potential Energy

Gravitational Potential Energy = mass * gravity * height


Ep = m g h

Lets take Zed and me. Zed's mass is 20kg, mine is 75kg. The table is 1 meter high

How much GPE do we both have whilst we are both standing on the table?

  1. Zed's GPE

GPE = m g h

GPE = 20kg x 9.81ms-2 x 1m

Zeds GPE = 196.2 Joules


  1. Mr Cowley's GPE

GPE = m g h

GPE = 75kg x 9.81ms-2 x 1m

GPE = 735.75 Joules

Then, what is Zed's GPE when I lift him up so that he is 2 meters above the ground?

  1. Zed lifted by Mr Cowley

GPE = m g h

GPE = 20kg x 9.81ms-2 x 2m

Zeds GPE = 392.4 Joules

Note, GPE is often written as Energy potential, or Ep = mgh

This is because it is a very common calculation, so common that by dropping the G everyone still knows that the calculation is for GPE. And if you forget that Ep = GPE, thats ok because the g is in the other side of the formula :-)

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Page 59

Kinetic Energy

Kinetic energy is like X-Box Kinect - its all about movement

Kinetic energy is the energy of movement

2 things matter

How big you are and how fast you are going

The bigger you are the harder it is to stop you

In other words, the greater the mass, the greater the Kinetic Energy

Also, the Faster you are going, or the higher your Velocity the harder it is to stop you

In other words, the greater the velocity the greater the Kinetic Energy

But what matters more, velocity or size?

Well its relative, if you are hit by a bullet you can die, but if you are hit by a train you can also die. Lets have a look at two things that are similar in size, a bullet and a rock, explained by Adam Savage from Mythbusters

Kinetic Energy, or Energy Kinetic is the same thing

It is written as EK, Ek, Ek Ek, or even KE

With this we take the two things that matter; the mass of the object and how fast it is moving

Mass is not nearly as important as how fast something is moving, that is why a bullet is so deadly

Because velocity is so very important, we will square it: v2

Because mass is not nearly as important we will half it: ½m

This makes our equation;

Kinetic Energy Equals = half mass times velocity squared

Kinetic Energy is in Joules

Mass is in Kilograms

Velocity is in meters per second


We can write this as Ek = ½ mass * velocity2

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Page 60, 61

Gravitational Potential Energy into Kinetic Energy

SciPad

Page 62, 63, 64, 65, 66, 67

When something falls, it speeds up

If a ball is at the top of a cliff and is then pushed off, at the top of the cliff it has maximum GPE, as it falls it gets closer to the ground, so it has less GPE, but it gets faster so it gets more EK. In the moment before it hits the ground it is at its maximum velocity, so almost all of the energy is converted into Ek and there is almost no GPE is left

Ep = m*g*h

Ek =½ *m* v2

Energy is Energy, so Ep is equal to Ek

Ep = Ek

This means that the energy that an object has before it falls from a height, will be the same as the amount of energy that object has when it hits the ground

m*g*h = ½ *m* v2

If we cancel 'm' as its on both side of the equation, then

g*h = ½ v2

Then we can rearrange to use Ep information to calculate the expected velocity if an object falls

2*g*h = v

This will give a theoretical velocity, the actual velocity will be less due to air resistance. Air resistance causes some of the energy to transform into heat energy

Remember, some energy can be 'lost' or transformed into Heat Energy

  • Carry out an experiment to investigate the relationship between kinetic energy gain and gravitational energy loss.

Work

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Page 68, 69

When you mow the lawns, you do work

The longer the grass is the more work you have to do

The muddier the ground, the more force you have to push with to move the lawnmower, and the more work you do

The bigger the lawn, the more work you do

These are the two big things with work. How much force you have to apply, and how far you have to go

So work is Force applied across a distance

Work = Force times Distance

Work is measured in Joules

Force in Newtons

Distance in Meters

W = F d

Power

Power is how quickly you can do work

The more powerful you are, the faster you can do the work

How long would it take to mow your lawn with one of the lawnmowers in the clip to the right?

To be powerful, you have to be able to do the work faster. If the Rock, Dwaine Johnson and I were to lift 4 20kg weight plates across the room, I would carry each individually, and he would carry them all at once. So it would take me four times longer. So, the Rock is at least 4 times more powerful than I am

In the 'Hammock Run" clip, I am doing the same as I was in the "Hammock Walk" clip, the same amount of work, but in far shorter time. So I am more powerful in the Hammock Run than I was in the Walk

So, Work and Time are the important factors when it comes to the Power

You are more Powerful if you can do the Work in less time

Power is thus Work divided by time

Power = Work / Time

P = W/t


Power is measured in what? Watts

Work is measured in Joules

Time is in Seconds (as it always is in Science, Science Seconds)


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Page 70,71, 72, 73