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Don't forget your assessment

2010

This year I'm not teaching Physics but thanks for visiting.

I've got blogs set up for my other classes if you are interested.

• 13 biology
• 12 biology
• 11 science

Ray diagrams for lenses

This video shows the method of constructing ray diagrams for lenses

Physics for future presidents

Today in class we watched this you tube video lecture from UCLA Berkley.

Waves waves waves

Dear Students our last topic for 2009 is waves.

This is one of those topics where understanding the jargon is a key to success.

Some of the jargon (I mean key words and terms) you need to have some comprehension of include:

• light
• sound
• electromagnetic
• spectrum
• reflection
• refraction
• diffraction
• interference
• resonance
• constructive
• destructive
• curved
• mirrors
• lenses
• critical angle
• boundary
• frequency
• amplitude
• velocity
• wavelength
• pulse
• superposition

One of the most famous incidents involving resonance is featured in the clip below showing the collapse and ultimate destruction of the Tacoma Narrows Bridge.

The millennium Bridge another classic of engineering skill.

Getting towards the sharp end

The year has passed in a flash and the practice exams will be upon us from 10 September. The topics covered for us include Mechanics, Electricity and Nuclear physics. Along with the practice exam we also have two internal assessments these are the practical test and the Physics in an integrated context. On the basis that there is no point reinventing the wheel I have done a quick snoop on resources made available by other school's web sites. One of the best is from Wanganui high school:
Yr 12 Physics

Why is AC used rather than DC to supply electricity

Attention Mr G.

I hope you are regularly visiting the Physics blog. Here is some stuff that may help you answer your BIG question.

Stolen for Wikipedia

"Centralized power generation became possible when it was recognized that alternating current electric power lines can transport electricity at low costs across great distances by taking advantage of the ability to change voltage across the distribution path using power transformers. The voltage is raised at the point of generation (a representative number is a generator voltage in the low kilovolt range) to a much higher voltage (tens of thousands to several hundred thousand volts) for primary transmission, followed by several downward transformations, to as low as that used in residential domestic use, such as, e.g., 120 / 240 VAC at 60 Hertz in North America and 230 / 400 VAC at 50 Hertz in Europe."

Transmission loss

The advantage of AC for distributing power over a distance is due to the ease of changing voltages with a transformer. Power is the product current × voltage (P = IV). For a given amount of power, a low voltage requires a higher current and a higher voltage requires a lower current. Since metal conducting wires have a certain resistance, some power will be wasted as heat in the wires. This power loss is given by P = I²R. Thus, if the overall transmitted power is the same, and given the constraints of practical conductor sizes, low-voltage, high-current transmissions will suffer a much greater power loss than high-voltage, low-current ones. This holds whether DC or AC is used.

Transforming DC power from one voltage to another was difficult and expensive due to the need for a large spinning rotary converter or motor-generator set, whereas with AC the voltage changes can be done with simple and efficient transformer coils that have no moving parts and require no maintenance. This was the key to the success of the AC system. Modern transmission grids regularly use AC voltages up to 765,000 volts. ^[10]

Alternating current transmission lines do have other losses not observed with direct current. Due to the skin effect, a conductor will have a higher resistance to alternating current than to direct current; the effect is measurable and of practical significance for large conductors carrying on the order of thousands of amperes. The increased resistance due to the skin effect can be offset by changing the shape of conductors from a solid core to a braid of many small wires.

Links:

War_of_Currents

Alternating_current

Electric_power_transmission

New Link

Check out the Physics podcasts site I have added to the "other sites" list. Imagine you could download a podcast onto your Ipod and secretly listen to Oldish American Men lecturing about Physics. If you tap your fingers a little your friends will probably think you are listening to some of that wild hippidy hop young peoples music.

MIT Physics

One of the major Universities in the states has put all it's course online. Here is a lecture on electricity and magnetism.
Ideal to use up any spare band width you have on your broadband plan. Not bad for learning about Physics either.

Voltage Divider

voltage divider