"There has been a talk today with 25 teachers (enthusiastic) in a room"

Inductive breaking and AC motors

Inductive breaking used a fixed field and moving conductor.
Faraday laws = battery produced and current made.
Conservation of energy means that the train slows down.
Lenz's law means that resisting and drag soleinoids are made.
In a Tangara the drive motor is shorted over resistors in the roof.
The wheels are connected to gears that drive the motor at high speed.
Wires in the shaft winding carry current, due to the Faraday's Law.
The current creates a drag force due to the

Inductive Cooktops

This Choice magazine article gets the science wrong saying Al and Cu will not work with inductive cook tops. It also insists on close proximity. Both are wrong.

 
 

The following site covers the Pros and Cons of inductive cooker

1. By heating only the pan there is low thermal inertia, things get to temperature more quickly
   
2. There is less waste of energy, it is claimed it is more efficient than gas.
   

    
    

3. From our families experience, it has the problem of not working well for blind people who need to feel the heat.
   
4. It safer as there are no hot elements to start a fat fire if hot fat spills.
   

   
   

    
    

   Pasted from <http://thecoredump.org/images/stove_new.jpg>
   

    
    

Inductive Cooktops

This Choice magazine article gets the science wrong saying Al and Cu will not work with inductive cook tops. It also insists on close proximity. Both are wrong.

The following site covers the Pros and Cons of inductive cooker

1. By heating only the pan there is low thermal inertia, things get to temperature more quickly
   
2. There is less waste of energy, it is claimed it is more efficient than gas.
   

   
   

3. From our families experience, it has the problem of not working well for blind people who need to feel the heat.
   
4. It safer as there are no hot elements to start a fat fire if hot fat spills.
   

watch or link

Transformers

Summary

Motors and generators can have coils on shaft or casing or both. (both for universal).

Motors switch the field to be the wrong way with commutators.

Induction motors have designer eddy currents to create drag.

Energy is conserved, back torque and EMF with speed. High speed greater voltage.

 
 

 
 

Energy transforming devices can be compared:

 
 

They have different function:

 
 

Motors generate force from electricity

Generators make electricity from force

Transformers make electricity from electricity

 
 

 
 

They use different laws:

 
 

The motor effect describes current to produce force F = BilsinTheta

 
 

Faraday's Law v=-Dphi/DT describes changing magnetic flux to produce a voltage across a conductor. Either motion of the conductor or changing flux. Current flows if there is a circuit.

 
 

Conservation of energy applies to each:

 
 

When current flows in a generator, power is transferred, the motor effect resists motion W =Fxd, energy is conserved.

 
 

When a motor speeds up the Faradays law explains a back EMF

 
 

How do you describe a transformer?

 
 

A transformer is an electrically powered electricity generator.

A current in one circuit drives a magnet, that then drives the current in the next circuit .

 
 

The wires are drawn showing that electricity can not pass straight through the transformer. The primary is usually on the left or bottom of the diagram. The number of turns is N1. The more turns the stronger the magnetic field.

 
 

The core is drawn by a series of parallel lines to represent a laminated core. This core directs the magnetic field from the primary winding to the secondary winding. The iron core is permeable and intensifies the magnetic field inside the iron and reduces it outside therefore it acts as a channel for magnetic field. The energy transformation is electrical- magnetic-electrical.

 
 

N2 is the number of coils on the secondary. The voltage change across a transformer is given by

 
 

 

For AC currents. Faraday's law means that transformer time differentiates the input voltage. Having a rapid change in current, causes a voltage spike and spark. This is the principal of a cars spark plugs.

 
 

Step up transformers have many turns on the secondary coil. The wires on the secondary are small as the current for a particular power level is less. SUT are used for high voltages in long distance transmission, and in old TV sets, and street lighting.

 
 

Step down transformers have few turns on the secondary.

There is greater current in the secondary and the wires are thick to reduce resistive loss. SDT are used to get safe domestic voltage and again for Silicon Digital devices. They are also used for where high currents are needed in welding. Transformers are used to deliver high currents for speakers.

 
 

Managing energy losses in transformers.

 
 

There is ohmic or resistive energy loss as you can have kilometres of wire conductor in a transformer

 
 

The is mechanical vibration loss. The wires in a transformer are squeezed together at high current. This can be heard as transformer buzz. The mechanical vibration limit the matterials used for the core.

 
 

The are parasitic eddy currents. This is managed by using lamination to interrupt the eddy.

 
 

The is current leakage through the insulation of the wires. This is called dielectric break down. Special matterials are used PEEK and transformer oil.

 
 

The major problem of the losses is the increase in temperature leading to run away ohmic heating. Special transformer oil is cycled through the transformer similar to an oil heater. The oil moves through cooling fins, and if the air circulation is not enough fans are connected to force cooling. The oil is designed to expand when heated and so rise and push cool oil through the transformer. In large transformers only metal cores have the thermal conductivity. Transformer oil must withstand high voltages.

 
 

Major problems of failure in transformer is material degradation leading to break down. The oil channels can become clogged. Hot spots can cause run away failure. Dr Hill worked on optical fibre temperature sensors for inside transformers and optical fibre voltage sensors. You have sometimes as little as 1 second to shut a transformer down before it explodes.

 
 

 
 

 
 

 
 

 
 

 
 

 
 

 
 

 
 

Frustrated eddy current

 
 

Faraday's Law Phet

Run this simulation:

More Exam Drill

Homework: Complete 2007 exam (2hrs) for Wednesday. Email me a log of the time spent doing the project.


Question

"Compare DC, AC and Induction motors, and discuss their application"

Step 1: Copy and Highlight key words.
Step 2: Write "Comparison"
Step 3: Write out table, Property, DC, AC, Induction, Comparison
Step 4: Fill in rows: Power in , Commutators, Process, Back EMF eddy current, Application
Step 5: Summary

Question

"Explain Einstein's similtanety"

Similar steps, c constant, inertial frames, lighten bolts on space ship.

Question

"Describe a step down transformer"

Copy and highlight. Draw a diagram.

Thursday Lesson

Coached Exam Question.


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Copy this in your book.

Explain how Einstein used a thought experiment to show that Inertial Frames experience time dilation

Highlight verb and important terms

Note: A thought experiment is one done in your head, Inertial frames means moving frames etc and time dilation.........

Start to answer question with:

Explanation:

First sentence, define or mention important terms. Then state Einsteins postulate, about c and the laws of physics. Then draw bounce diagram and conclude.

This video explains the concept

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Copy and Highlight

Discuss factors that influenced your measurement of g with a pendula

Write

"Discussion:........"

First sentence: Key terms, diagram, and equation.
Talk about mass, effects of location on planet.

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"Compare Edison and Westinghouse's Power war"

"Comparison:.............

Table of Property/ Edison/ Wesinghouse/ Comparison

Time and Place/ New york 1900/ New York 1900 / Same time and place
Method / DC / AC / Different methods
Distribution/ generators everywhere/ step up- travel- stepdown/ Westinghouse less expensive
Danger/ DC safer / AC Kills you/ Edison was safer.

Summary statement.

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Describe the magnetic field induced at a point half way down a copper tube when a magnet is dropped down it.

"Description: " + Diagram + Lenz law + Right hand wrap + For example

Remember the field oppose entry and exit.


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Read the Chapter on Induction motors to work out the difference between DC, Universal (AC) , and induction motors

past papers

2001
2002
2003
2004

2006
2007
2008

absolutely must see site

this is an excellent site showing animations for this unit

lightening strike

If link doesnt work try

Edison's Elephant

Link to article on Edison vs Westinghouse

Monday Lesson

Homework: Hand in Coached Exam - Last warning
Write a summary the advantages of AC over DC for power transmission. Summarize the conflict between Edison and Westinghouse.

Section One Generators and Power distribution.

Context:
Electricity authorities use high-voltage transmission lines and transformers to distribute electricity to homes and industries around each state. Voltages can be as high as 5 x 105 volts from power stations but by the time this reaches homes, the electricity has been transformed to 240 volts. While it is relatively economical to generate electric power at a steady rate, there are both financial and environmental issues that should be considered when assessing the long-term impact of supplying commercial and household power.

1. describe the differences between AC and DC generators
2. discuss the energy losses that occur as energy is fed through transmission lines from the generator to the consumer
3. assess the effects of the development of AC generators on society and the environment
4. § gather secondary information to discuss advantages/disadvantages of AC and DC generators and relate these to their use
   § analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities
   § gather and analyse information to identify how transmission lines are:
   – insulated from supporting structures– protected from lightning strikes
   

Section Two Exam Practice

Exam Practice answer these questions.

Compare Centripal Force and Kinetic Science.
- Copy this question, Highlight the verb, Write Heading Comparison table, Write in Header, Property, Centripital, Kinetic, Comparison.

Discuss the statement "Michelson Morley believed in Aether and the Scientific Method. Their experiments lead to the abandonment of Aether"
- copy this question, highlight verb, write heading "Discussion", State what happened, comment on the statement.

Explain how the relativistic space travel could allow us to reach Stars 1000Ly away but there is little prospect of reaching these speeds.
-copy question, highlight verb, Write heading explanation, Write the time dilation equation, explain with an example, Write the mass dilation equation and the Energy equivalence equation, with an example. Finish

Compare Generators and DC Motors.
-copy and highlight, produce a table.

Corrected Homework

There is now in and out trays inside the science staff room.

Place your homework on the top one, and take the corrected work out the bottom. There is a sign in book where you can write in what you wanted corrected, and will log comments.

From the Practice Half yearly here are some hints:

Good and Bad eddy currents- the bad ones are from coiled extention leads, losses in anything AC electrical.

When you have a question, draw realistic diagram. If you draw a 20 deg triangle you can see that the horizontal velocity has to be bigger than the vertical

Torque is a vector quantity

Work done on an object is the amount of work put into an object, done by an object is the amount of work out.

It is now a requirement to circle the verb in a question. Then use it the answer. If possible use a table.

A comparison table has Three columns Object one, Object two, Comparison of Object one to two.
Four columns are even better where you list the concept in the first.

Compare Earth and Mars gravitational effects

G Effect Earth Mars Comparison
g 9.8 3.6 Earth is bigger
Atmosphere Large Small Mars has lost is atmosphere
escape V 4000 1000 Mars is smaller

Generators

Revision

Force on currents = motor

1. wire in B field


2. parrallel wires


3. loop creating torque - DC motors make the inner rotation solenoid fight the field

Faradays Law

1. Back voltage = - time differential of B*a

2. wire cutting a field is a battery

Lenz's law - practical Faraday - Eddy currents make a solenoid that resists motion. Motion turns into electrical energy and heat

1. dropping magnet - solenoids are made that arrest fall turning energy into heat


2. spinning disk - two solenoids are made opposing entry of a magnet, and dragging it back


3. inducation furnace for cooking, eddy current heats metal

Homework SUMMARIZE CHAPTER 8 finish half yearly for correction.

Generators

1. Direction of current through (1) + charge right hand slap (2) lenz law

2. Slip AC- Split DC

3. Rotating faster = higher frequency and higher voltage

Transformers

more loops- more volts

Step up for efficient power transmission. P = I x I x R

eddy currents reduced by lamination. layer of insulators.

Step down to make voltage Kid friendly.