Physics Study thread - M&G (2 Viewers)

[acmilan]

Hmmm...I thought that as well. I looked at the hsc.csu.edu.au site, and it doesn't state anything about nBIlsintheta, only nBIAcostheta and BIlsintheta.

The syllabus confirms Ms Tennille's thoughts.

 

[helper]

The only time you would put an n in the Force formula, if you are looking at the force on the side of a coil. Then each length of wire has the same force. It isn't part of the formula normally.

 

[3G'sMt5:16]

hmmm.. slow progress here... neways... la la la bla bla bla

T = Fd... hmmm... d would be the distance of the coil where the forece is acted upon... yar... i think this was covred already... lol... re-emphasis is goot

hmmm... for those usin jacaranda and seekin the answers i found it!

--> http://www.jaconline.com.au/physicsnsw/

Cheers big ears! hahaha (its for all topics)

soooo... boring..... lol... physics is never borin... lol.. it is

cyaz

NOTE: this is so in-effective... hmmm

 

[3G'sMt5:16]

Explain why, using physics principles, many motors have resistors which need to be switched on when the motors are warming up.

intelesting... I (current) = V\R right.. hmmmsss... back to the basics for theory:

ya'll know that when any motor is starting... it requires a large current (I) yar.. cuzz of da BACK EMF being zero firsts as any motor starts (since all motors USE MAGNETS!!! however the BACK EMF gradually incleases... yar).

also... EMF = - minus the differential of magnetic flux (not density nor phi) \ time x N (number of turns) and where - indicates the direcsion yar)... which decreases the need of E.P.D as the motor is readily operating normally or spinning (Electrical Potential Diffo... or V (voltuge...yarrr).

OR to explain quantitatively and qualitatively why EPD is not needed when the motor operates is seen below from jacarandaaaa (pg 126):

qwestionn: say a motor has a

a) 240V supply and when it is operating normally and with a high angular rotational velocity it would yield a voltage of b) 232V (due to BACK EMF...)...

answars:

a) ya'll know that I = V \ R sooo...---> 240V \10ohms = 24A yarrr... or Amps (I)

b) BUT when its 232Volts \10 ohms (R) its ---> = 23.2 AMPS!!! (I)

therefore... when the BACK EMF increases as the motor rotates continously... Current needed decreases.., yar? 'yeahh you say'... since da voltage drup or need of a voltuge is minimised as the motor runs normally...


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... so to GO TO DA POINT:

1. It requires a large current (I) .. therefore resistors should be turned on... OR the motor should be LEFT ON so that the back emf is increased so that the current induced by the magnet and supplied by the source is at its minimum!

2. da BACK EMF being zero firsts as any motor starts.. therefore resistors are useful to decrease heat that may be built up gradually

3. BUT induced current is also supplied by the supply of EMF caused by the ineteraction of the magnets huh... soooo yarrr..... if theres a supply of EMF... theres a BACK EMF!!! (which is a ELECTROMAGNETIC FORCE THAT OPPOSES the normal\main current flow)

SOO --> we say back emf supports the cooling... and supply of emf supports heat (since it induces a certain amount of current in the motor)

PHIEW>.... HOPE YOU UNDERSTAND... but yeh... those are my points! (phys aint easy sometimes hay) LOL!!

 

[3G'sMt5:16]

its equation of the y' means that we can find the m at any point along a curve...

hmmm

soOOoO

mmmmk? lalala

a normal equation means.. that, it is the ORIGINAL curve

 

[3G'sMt5:16]

for indefinate integrals or PRIMITIVE FUNCTIONSSS.. the constant CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC MUST ALWAYS BE ADDED to da answerrrrrr =)

 

[sunjet]

That was the most weirdest answer.

Simple terms, they need a resistor because the back emf is low when the motor is starting up, which can cause the motor to heat up and be destroyed. When the motor is running there is a back emf, as the motor is cutting magnetic flux and in accordance with lenz's law a back emf is created which opposes the supple emf, and reduces the currect and thus no resistor is needed when it is running only when it starts up.

 

[3G'sMt5:16]

ahh thnkx thats what i was looking for in my answer.. lol.. could nt explain it in more of simplisitic terms thnx