OCR Physics B G495 Field and Particle Pictures June 21st 2011 Exam Thread

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In the specification it says we should be able to describe and explain cases involving the action of a motor, including a simple induction motor;

How does a simple induction motor work?

Reply 421

41jms

can someone please run over how a motor and generator work please!! also do we need to know about the separate types??

Reply 422

Geekchic17

Original post by Jke

In the specification it says we should be able to describe and explain cases involving the action of a motor, including a simple induction motor;

How does a simple induction motor work?

Simple induction motor works altering the magnetic field around a free-moving loop/coil of (conducting) wire

The change in magnetic field induces a change in flux linkage with the coil, hence creating an emf in the coil, producing a current in the wire.

The current in the wire and the magnetic field (direction by flemings left hand rule - motion, field, current) puts the wire under a force, causing it to rotate.

Much like the simple motor really - where the current is changed every half turn by the split-ring-commutator, therefore changing force direction on the loop every half turn causing it to rotate steadily (in a fixed magnetic field)

:smile:

Reply 423

Mikkels88

yeh why is method 2 the most accurate?!

Reply 424

Geekchic17

Original post by 41jms

can someone please run over how a motor and generator work please!! also do we need to know about the separate types??

Generators work by rotating a conducting loop/coil in a magnetic field
The change in flux linkage with the coil induces an alternating emf, and hence alternating current in the wire

Why is it alternating current?
Due to the relative flux motion changing every half turn (from horizontal clockwise to vertical, the magnetic field - and hence flux motion - goes one way, say, from left to right, then from vertical continuing round clockwise to horizontal again relative to the coil the flux motion changes direction, now going from right to left) the emf produced changes direction, hence also changing direction of the current.

Reply 425

Summerdays

Original post by Geekchic17

Excellent, Geekchick :biggrin:

Reply 426

JoeCarr

Original post by Mikkels88

yeh why is method 2 the most accurate?!

yeh forgot about that one, good point.

Reply 427

Summerdays

Original post by Mikkels88

yeh why is method 2 the most accurate?!

Method 2?

Edit: Never mind lol

Reply 428

Unkempt_One

What they said, in a generator, an electromagnet spins in a gap in a magnetic circuit, causing an alternating flux hence alternating emf hence alternating current in the secondary. In a motor, a rotating magnetic field causes a rotor to turn.

An induction motor is where the rotor is a conductor, and hence currents are induced in the rotor. This induces magnetic poles in the rotor which causes it to rotate and align with the rotating poles of the magnetic field.

Another difference between an induction motor and a motor where the rotor isn't a conductor, is that whereas the rate of rotation of the latter is synchronous with the rotation of the magnetic field, the rate of rotation of an induction motor lags behind the rate of rotation of the field.

Reply 429

Mikkels88

Original post by Summerdays

Method 2?

Edit: Never mind lol

haha the most useful post youve had so far :tongue:

Reply 430

JoeCarr

Original post by Summerdays

Method 2?

Edit: Never mind lol

aha, fancy giving a signature thorough explanation of yours?

Reply 431

Alex.Stevens

Just done some research, well talking to people on the whole earth bulging scenario...
This is what Ive come up with:

Rotation produces centrifugal force, which partially offsets the gravitational acceleration downward.
larger force at the equator so offsets more gravity, so pulled in less, so bulges
at pole there is no centripetal force so no offset

any thoughts?!

Reply 432

JoeCarr

Original post by Alex.Stevens

it's as good as I will ever be able to explain it, though you'll probably get a barrage of rage now because you used the word 'centrifugal'.... :rolleyes:

Reply 433

JoeCarr

I'd also like to post a final challenge to the uncertainty answer in the section C answer booklet, as it says the implied uncertainty for 9.81 is 0.01, but surely it's half of that because you would be rounding up anything 0.005 below or down from above, so isn't the uncertainty +/- 0.005??

Reply 434

Sereni

Original post by Summerdays

Method 2?

Edit: Never mind lol

Original post by JoeCarr

yeh forgot about that one, good point.

Original post by Mikkels88

yeh why is method 2 the most accurate?!

It is most accurate because method one relies on the electronics being accurate, ie that the time at which the laser emits light is the same as the detector stopwatch starts, so wires that are different lengths, different components etc result in inaccuracies, remember we are talking many significant figures.

Method three relies on the values of a third party, and so is only accurate to the extent that they are, it may be more accurate to work out the wavelength yourself.

Method two is most accurate because it relies on the frequency of caesium, which is in the DEFINITION of a second and so contains NO error, then using a prism like method the light you are measuring is refracted and the relative distance from the caesium dot is then plugged into a load of equations which churn out the wavelength of the stabalised laser, which can then be used to measure out a metre.

Reply 435

Unkempt_One

Original post by Alex.Stevens

See this alternate explanation in terms of energy: http://www.cleonis.nl/physics/phys256/equatorial_bulge.php

Reply 436

Alex.Stevens

Original post by JoeCarr

it's as good as I will ever be able to explain it, though you'll probably get a barrage of rage now because you used the word 'centrifugal'.... :rolleyes:

Yeah I thought that as well, don't see whats wrong with the word! I mean its nearly centripetal just a couple of letters different!
But on a serious note, isn't centrifugal the right term as its acting away from the rotation axis??

Reply 437

Summerdays

Original post by JoeCarr

aha, fancy giving a signature thorough explanation of yours?

The first method is dependant on the distance, a smaller distance wil give a larger uncertainty. With method two you are using the interfermoter, which accurately tells you the distance from one maxima to another, and thus tells us the wavelength of the emitted photon. We can easily find the frequency of the emitted light because we know the EXACT value of the speed of light.

Method 3 is strictly used for calbration purposes.

I am not sure if my explanation is correct though :hmm: