Back emf in motors an transformers. Syllabus now finished.
Revise for mock next Thursday.
We looked at transformers and started to clear up gaps in work.
Ensure you have completed Q.2-4 on P.126 and Q.1-4 on P.132.
Answer Q.1 on P.133 and bring to next lesson.
Eddy currents, AC generator
We spent an inordinate amount of time investigating eddy currents (lots of videos on youTube e.g. here and here and here).
We derived the emf output of an AC generator and discussed peak and RMS values (see P.127).
Watch this video about how transformers work. Make notes using P.130-132 of the AQA textbook (this video may help - I haven't watched the whole thing). Make sure you learn the various parts of the national grid - there will be a quiz on this next lesson.
Answer Q.1&3 on P.132 and hand in on Monday.
Next week we will finish off AC and DC generators, transformers, and back emf, and then we will have completed the syllabus.
Moving coils in magnetic fields
We applied ε = Blv and ε = -N ΔΦ/Δt to coils moving and rotating in magnetic fields.
Read and make notes on Example 2 on P.125 of AQA. It would be worth reading through that whole of Chapter 8.2 again and adding to the notes we made in class.
Answer Q.2-4 on P.126. Check your answers in the back of the book. Hand in tomorrow morning.
In class we worked out the peak emf of a spinning coil - to see the graph of emf over time have a quick look at P.127 of AQA.
We looked in more depth at the force between two current carrying wires, and tried to show that when a current is induced in a coil by a moving magnet, the magnetic field produced by the current opposes the motion of the magnet.
We also looked briefly at the idea of flux and derived an expression for the emf induced in a wire moving perpendicularly to a uniform magnetic field.
Watch this video about flux. You can skip the boring bits, and also you can stop when he starts talking about Faraday's law.
Read P.123-5 of AQA and check you are happy with the derivation of ε = Blv.
We marked the past paper questions, and compared sunchrotrons with cyclotrons.
We induced current in coils using a magnet and looked at Lenz's law.
I forgot to mention Fleming's right hand rule in the lesson (you have done this before anyway) - read P.120-122 and check you're happy with it. I'll demonstrate this next lesson.
Watch this video about Lenz's law and use P.123 to add to your notes. IMPORTANT: in this video the guy uses a left hand grip rule - he does not mention it, but this works for ELECTRON CURRENT. We use either the right hand grip rule or the rule on P.123 for electromagnet current, because AQA always seems to deal with CONVENTIONAL CURRENT.
Mass specrometer and cyclotron
We looked in more detail at cyclotrons and mass spectrometers
We watched the cloud chamber, although the magnetic field was not strong enough to see beautiful curved tracks like these:
Make detailed notes on the mass spectrometer and bring to next lesson. Include:
how the ions are formed
how the ions are accelerated
how a particular velocity is selected
how the ions are separated based on their mass to charge ratio
how the ions are detected
Use your textbooks and this video to help.
Note that this video is far from perfect - be prepared to discuss the video next lesson in terms of what you found useful and if there is anything you would improve.
Do some research about one of the following things. Be prepared to speak for 2 minutes about it in tomorrow's lesson.
I mainly want to know about the motion of the charged particles and how it is achieved.
cyclotron: CB, AG, CD
mass spectrometer: LH, AP
cathode ray tube: GdR, AM, TS
Northern lights: EM, CC
Moving charges in a magnetic field
Read and make notes on P.110-112 of AQA and watch this video from 11:28 to 15:00 (when he starts talking about circular motion).
The derivation in the video is a bit longwinded and you probably won't need to derive anything like that in the exam, but if you manage to follow it, it's nice to see how F=BIL and F=BQv are related.
This video gives a quick explanation about the Hall effect - the diagram gets a bit messy, but hopefully it will help.
Answer Q.1-2 on P.112 of AQA and hand in on Monday.
Motor effect + electric motor
Do last lesson's homework if you didn't do it.
Write a review of a youTube video about the DC motor here. Have a look at other people's reviews and see if you agree. Make sure you put your name by your review / comments.
Read and make notes on P.106-7, especially the formula F=BIL, then answer Q.1+2 on P.109. Bring these to the lesson tomorrow.
Electromagnets and the motor effect
Watch and if necessary make notes on this video about electromangnets, this video about electromagnet polarity, and this video about the motor effect. You have covered this before, although you would have used Fleming's left hand rule for the motor effect rather than your palm as this guy does.
Can you think of any risks associated with the experiment in the first video?
Prepare for end of topic test tomorrow on chapters 4-6 (fields and capacitors).
If you missed the lesson, the answers to the past paper questions in the book are here.
Capacitors in timing circuits
Please use the measurements made in class to determine the average force of the impact of the ball on the table.
I need one report to be handed in by each group, which clearly shows the measurements you took, and how you used them to determine the force.
In order to calculate the time of the impact you need to know the initial voltage, final voltage, resistance and capacitance.
To calculate the force you need to know the mass of the ball and the velocity of the ball before and after the impact. You can work this out from the height it was dropped from and the height it rebounded to.
Reminder: there is a test on fields and capacitance next Thurs.
Past paper question practice
Please complete Q.1-4 on P.102-3 of the AQA textbook and bring to tomorrow's lesson.
N.B. there will be a test on Thursday 30th which will cover Chapters 4-6 (fields and capacitors).