Unit 7 Energy

Unit 7 Energy

This is a difficult concept since we have all experienced and heard about this for a long time but we don’t have a firm grasp of what energy really is

Have students give their ideas of what energy and work are.

What is energy                                                                  What is work

Ability to do work                                                            transfer of energy

Power

Ability to produce change

Fossil fuels

Nuclear

Red bull

5 hours

Light

Heat

We did a little aside to cover the idea of gravitational field to be used later on in this unit I assume

I’m wondering how to incorporate the field concept into the modeling because I think it is a part of NGSS.

On Earth

Gravitational Field  10 N/kg  is the gravitational field strength  sometimes represented by 10N/kg = g

On the moon (trusting people who know stuff) g=1.6N/kg, Mars 4 N/kg

Someone also asked the following question and so we took some time on this as well.

Why do objects of different masses fall with equal acceleration?  The earth pulls on more massive objects with more force and more massive objects resist changes in motion more..

Energy resumption  We already have an idea of force and energy before “this isn’t your first day as a human”

Examples of energy

 Electrical

Mechanical

Chemical

Nuclear

Wind

Solar

Green

Geothermal

Food

Dark

Kinetic

Potential

Electromagnetic

Elastic

Tidal

Thermal

Light

Heat

1^st rule of energy—All energy is stored energy! You must name where/how it is stored We will start here  (Give this as a place to start since we really don’t know anything)

Go to the list and try to fill it in with the where/how

 Energy is stored in the motion of the car and then in the stretchy string then back in the motion of the car

(I can really see where students would be apprehensive to answer in fear of being wrong)

We roll out some analogies.  Currency—If I owed you money how many different ways could you pay?

Information—Can you hold it in your hand? No could you put it in a book, oral, computer . . .

MP3—is it an actual physical thing?  No  but can you store it ?

Energy is

1.        Stored

2.       Transferred

3.       ?  Filled in later with conserved

Visual representation

Use a pie chart to represent 100% of the energy.

Group those that are released to the environment.

What about a cart going up a ramp and stopping.  Where did it go at the end?  The gravitational field

  

Adding the agent of change to the pie charts.  Then you can add the question what is doing work (the agent of change)

The dropper popper discrepant event was discussed.  This has a lot of potential I think

Fenyman had the same problem with the idea of energy it is just like an accounting system

Go back to the 3 things that are important about energy

1.       Stored

2.       Transferred

3.       Conserved 

Bar charting  is limited to beginning and end of an event

  

Between the two sets of bar charts show the system

 Break your bar into pieces

 Start with no friction.  Now add friction maybe change the zero for the gravitational field

If something is not included in the system then it still needs to be accounted for so put it outside the system and put the number of blocks associated with it.

Paradigm lab horizontally stretching a spring

Discussion  F=(slope)x+0

How does the slope change the farther you stretch it?

Elastic limit.

 Slope=spring constant  F=kx  Hooke’s law

If you stretched the spring did you store energy?  If you did then the graph probably shows it somehow?

Things to examine on any graph

1.       Axes

2.       Slope

3.       Intercepts

4.       Values/ points

5.       Shape

6.       Area

Derive the formula and then calculate the energy stored in the spring.

 We then did a couple of labs involving this concept of energy stored in a spring.

1.        Compare the energy in the spring to the speed of a cart (have different groups use different masses)

Finish the 3 lab E vs v

E vs h

And E vs d

The leap from F-d graph for a spring to area under the curve requires a certain amount of hand waving as does a number of things from this section.

Looking at E vs v white board

 Square root.  What would you have to the Energy to double the v?  Quadruple  Does your data support that?

Is it possible that the mass matters?

How does the elastic energy of the spring compare to the kinetic energy of the cart?  We know this because energy is conserved

 It does appear that mass makes a difference.  So look up on google what the accepted value for kinetic energy is.

 Now look at the E vs h graph.

First of all we can say that the Elastic energy is equal to the gravitational energy.

Is It reasonable that the gravitation field energy would be related to the gravitational field strength.

 Now E vs sliding

 

Lets work the magic  How does the elastic energy in the spring compare to energy lost to surrondings.

They have to be the same.

D=( ) El

D=( )ELS

 Units for Energy

Joule (J)  it is a derived Unit which is a Nm Pushing a cart 1m with a 1N force gives the cart 1 J of energy

Relate the lab of E vs skid to the energy lost to surroundings

 How does slide distance relate to speed?

Possible practicum  block and spring so that the block slides to the end without going over then end.

Problems  This has been the hardest unit so far, conceptually for me.  I think that the students will understand the energy flow a lot better than what I have done in the past due to the pie charts and or bar graphs.  I'm not sure yet which representation I will use.

Implementation:  I think these labs can be done more easily with my short tracks so there wont be a group size problem.  I just ordered an assortment of springs so that will help as well. My lab stations are just around the edge of the room so I will need to come up with a way to mount the cord so that the carts can be launched with it.  Perhaps I can make something that will mount onto the sides of my tracks?

Extras

@ brybatt  physicsmodeler.blogspot,com about standards based grading.

Phet simulations skate park show pie chart