Dr Blurg: March 2012

Hi - I've just finished checking and correcting (not marking) the notes you submitted for Part 2A. WOW - some very good ideas and a lot of hard work went into those notes. You should be proud. However, there were some frequent areas to improve and a few misconceptions. Your notes will be annotated to show which of these relate to you:

ai)
(A) For all diagram of motors make sure you show the current flow in the coil and the correct direction of torque produced by the motor effect.

aii)
(B) Make sure you say that because the current flows in opposite directions on either side of the coil the motor effect is in opposite directions on either side via the LHR, thus resulting in a torque about the axis.

(C) You need to relate the torque to the equation t = nBIAcosq and should note that momentum carries the coil past 90 degrees.

aiii)
(D) Increased magnetic field strength can be achieved by increasing the stator magnet strength AND by using a laminated soft iron core. Also - don't say move the magnets closer together - moving them closer to the coil may work (but not as important as the other two mentioned) but suggests that you are going to decrease the size of the coil - which would actually decrease torque.

(E) To increase the current you can decrease the resistance of the coil or increase the supply voltage.
NB in a resistor (any device that cases a voltage drop because electrical energy is being converted to heat, kinetic, light, etc) Ohm's Law applies ie I = V/R. Thus increasing the voltage drop (potential difference) = increased current flow. A transformer is different - it is close to 100% efficient and electrical energy is both input and output - there is no transformation of energy, so the electrical power in must equal the power out VpIp =VsIs thus increasing the secondary voltage causes the secondary current to decrease.

aiv)
(F) Laminations are thin layers of soft iron separated by ferrite (an iron oxide). The ferrite is an insulator but is ferromagnetic so it still enhances the magnetic field strength. The laminations run perpendicular to the COIL. They cannot run perpendicular to the field as they are rotating in the field with the rotor.

av)
(G) You MUST refer to the LHR when talking about a speaker. You must explicitly say that changing the direction of the current changes the direction of the motor effect force and thus AC causes the cone to move in and out - producing a sound wave.

(H) See (B) above and make sure you explain the production of the torque if asked a question about galvanometers.

(I) The radial magnets make the coil always at zero degrees to the field. THUS torque is INDEPENDENT of angle and only PROPORTIONAL TO CURRENT. Thus a LINEAR SCALE can be used to measure the current.

bi)
(J) When drawing generators always show the applied torque and the direction of induced current (and make sure you get it right!)

bii)
(K) The rotating coil in a generator causes an AC emf to be induced in the generator as the direction of flux in the coil is constantly changing as it rotates. The induced emf changes diction when the coil is at 90 degrees to the field. This is when the split ring commutator changes the brush connection so the output current is alwayss in one direction

biii)
(T) Generators work on the principle of induction - thus you must refer to Faraday's law when discussing how to improve them - everything that increases the induced emf ( and thus the induced current) is based on increasing the flux change or decreasing the time over which the flux change occurs. Laminated iron cores and stronger magnets, and more turns and bigger areas increase flux change. Spinning the generator faster increases the rate of change

biv)
(L) When comparing DC motors and generators you MUST talk about the supply of current (motor) vs the supply of torque (generator). You need to talk about the input vs output energy for both. You must talk about the role of the split ring commutator in either changing the input DC to AC in the coil (for a motor so the torque is in constant direction) OR the coil AC to output DC (in a generator). Finally, you MUST say that motors use the motor effect and generators use Faraday and Lenz's Laws.

bv)
(M) You must explain why there is an AC induced in the coil of a DC generator - see (K) above. Drawing the graphs of flux, rate of flux change, -rate of flux change are great for explaining this as you refer to them. Ie "Faraday's Law and Lenz's Law says the induced emf opposes the flux change - thus an AC emf is induced as shown by the graphs..."

ci)
See (J) above

ciii)
(N) BE CAREFUL. Unless you are an Autobot or Deceptacon AC generators are never 'easily transformed'. What you MUST say is that "The AC PRODUCED by an AC Generator is easily transformed...". Also be aware that the Q asked you to compare the pros and cons of the generator. Transformation is one advantage of AC power (and thus generators) but don't neglect the actual pros and cons of the generator structure - ie DC split rings wear & spark etc.

civ)
(O) Apart from the operational details the other things that you can/should compare between an induction cooker, EM brake and AC generator are:
The source of the flux change (ie AC coil vs motion between conductor and field)
The physics principles in place - all use Faraday's Law and Lenz's Law but EM brakes also use the motor effect and induction cookers also use resistance heating.

cv)
(P) You must be able to explain exactly what Westinghouse did to win. He undercut Edison's bid to power the CHICAGO World Fair - thus showing the success of his system to a global market. he was able to harness hydroelectricity from Niagara and transmit is over 20km away to Buffalo - all because AC was more efficient and ie it could be transformed and sent with low power loss unlike DC.

cvi)
(Q) You must be able to detail the STEP UP voltages at the power station and then each STEP DOWN transformations (and voltages) to the household THEN give at least one example of step up and step down in the household.

cvii)
(R) Don't forget that 'assess' means positives, negatives and JUDGEMENT. It is critical that you examine the effect of CO2 on global environment and that you have at least ONE VERY developed argument (for the top marks they base it on your longest chain of argument ie Fossil fuel burned = CO2 = enhanced greenhouse effect = global warming = sea level rise = loss of habitats = disruptions of ecosystems and displaced people. You MUST provide a judgement - it would be difficult to argue that AC generators have been positive for the environment (society yes, environment no).

cviii)
(S) Many were missing a critical link. Most linked transformers to transmission of power over long distances and thus ready access to electricity and electrical appliances BUT then when it cane to categories like medicine, business etc the link as gone. You can't just say transformers have been 'good' for medical diagnosis. You need to say that "electrically powered medical diagnostic & treatment devices have increased health of society..."

Please don't assume that because these comments are long that the answers were poor. Quite the contrary - they were deep and broad so that a number of issues came to light. Overall the Physics you communicated was fantastic.

BTW - very soon these awesome pics will make sense:

Chem study notes (with an extra page of advice), my ridiculously long essay on PHB and one of my 'best ever' students essays on 922* can be found at this link. *NB The essay question they had to do required them to basically summarise the key concepts from the whole 9.2.2 focus area into one 2-page essay. For your own study, you should be able to convert this to a ~1pg essay focusing on PHB (Biopol) only.

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