13/12/07
Merry Christmas. Radioactivity
Snell's law revised as well as the daft practical exam looked over with mark scheme.
Revision of waves, particularly the em spectrum and ultrasound and bits.
NB RM - Snell's law question for next time.
HW Look at the practical exam paper I gave out to you. Don't do it, but peruse it.
RM = non functional.
No test yet.
We revised force and motion by looking at a bunch of questions.
You seemed entirely untroubled by them, so good news I suppose.
Well here I was again! I summarised transformers for you, then we dug into some more electromagnetic induction questions in preparation for a test on said topic next time.
Things to revise about are:
1. A wire moving across a magnetic field.
2. A magnet moving near/through a coil of wire.
3. A simple generator.
4. A microphone (perhaps).
5. Transformers (how they work, why they need a.c.)
6. How transformers are used to step up the voltage and reduce the current for long distance transmission of electricity.
I gave you some notes printed from here .
HW Revise for a test on electromagnetic induction next lesson.
Well I wasn't about for this week for you guys either due to Jury service.
You were ploughing through some questions on transformers and electromagnetic induction in general.
I have been away for literally hundreds of thousands of millenia.
However, you guys have been attempting some questions on electromagnetic induction in my absence. I seriously need to look at all of you books and fins out how that has gone.
We discussed transformers and their use in the national grid.
They have 2 coils joined by an iron core. AC (alternating current) sent into the primary coil causes an alternating voltage to be induced in the secondary coil. This is due to the changing magnetic field putting a force on the electrons in the second coil.
The alternating voltage can be larger or smaller than the input voltage depending on whther there are more or less turns in the secondary coil than the primary one.
We looked at a little mock up of the power distribution system.
Power is generated by turning coils in magnetic fields. It is then stepped up to very high voltages to be sent long distances around the country.
In a nutshell, you must learn the following points:
N1/N2=V1/V2
We sat an electromagnetic forces test.
RM leg = kaput
Electromagnetic induction
We saw a demonstration of electromagnetic induction. We already know that moving charges experience a force in a magnetic field. If a conductor is physically moved through a magnetic field, the free charge carriers inside the conductor experience a force. This force pushes them through the conductor, it is an induced voltage. An induced voltage can cause an induced current if the conductor is connected as part of a complete circuit.
Both positive and negative charges within the wire feel a force (in opposite directions), but only the free electrons are able to move.
The induced voltage is reversed if the wire is moved the other way. The induced voltage is made stronger by moving the wire faster, or having a stronger magnetic field.
Moving a magnet through a coil of wire also caused an induced voltage. There was no voltage induced if the magnet was held still, even if it was right inside the coil.
Reversing the direction of movement causes the induced voltage to be reversed. Also turning the magnet around the other way (reversing the magnetic field) will have the same effect.
To increase the size of the induced voltage you can do one of 3 things:
Importantly, it was possible to make the electromagnet induce a voltage in the other coil whilst keeping it totally still. This was achieved by turning it on or off.
A voltage is induced in any conductor which experiences a change in magnetic field.
This change can be caused by the magnet or conductor moving in relation to one another, or by switching the magnetic field on and off (if it is an electromagnet.)
We saw a simple generator connected up to an oscilloscope.
A magnet was rotated quickly near to a coil of wire. The ends of the coil were attached to an oscilloscope which recorded the voltage across the coil and how it changed over time.
Any conductor that experiences a changing magnetic field will have a voltage induced in it (the electrons inside experience an electromagnetic force along the wire.
Several designs are possible. You can rotate the coil of wire, or you can rotate the magnet, as long as the wire is passing through magnetic field lines, a voltage will be produced.
A simple generator like the one we used wil create a alternating current(ac). The frequency of the ac depends on how often the magnet rotates. Rotating the magnet faster increases the number of peaks and troughs per seond in the induced voltage and increase the size of the induced voltage (as magnet is moving faster, putting larger electromagnetic force on the elctrons within the coil.
HW Revise for a test on electromagetic forces (all stuff so far)
RM was late!
We then looked at a loudspeaker, which uses electromagnetic forces to work.
The loudspeaker uses a coil which can slide backwards and forwards over the central pole of a circular permanent magnet. Attached to the coil is the speaker cone.
Sending a current through the coil causes it to develop a magnetic field which means it feels a magnetic force of repulsion or attraction to the permanent magnet.
Sending an alternating current into the coil causes it to alternately be attracted and repelled, so it vibrates at the same fequency as the a.c. The speaker cone therefore generates a sound.
The other electromagnetic force devices you need to know about are:
1. Electromagnets (coils with soft iron cores)
2. Relay switches (to magnetically move an iron switch in electronics)
3. Circuit breakers ("resetable fuses")
4. electric bells (bells which are electric)
HW Those away etc. need to do the motor questions and diagrams in their books for next time.
We built little electric motors, which was cool.
They're surprisingly fiddly.
HW Finish all the HW questions from the photocopied handout in your books. Hand in books next time.
We learned about electric motors. They work by exactly the same principal as the one that caused the "kicking wire" to kick. However, a loop of wire is used which means that the current flows first one way, and then the other as it continues around the loop. An external magnetic field is created using permanent magnets the same as before.
Here is an animation which attempts to portray what is happening.
To keep the turning force on a DC motor from reversing every time the coil moves through the plane perpendicular to the magnetic field, a split-ring device called a commutator is used to reverse the current at that point. The electrical contacts to the rotating ring are called "brushes" since copper brush contacts were used in early motors. Modern motors normally use spring-loaded carbon contacts, but the historical name for the contacts has persisted.
Motors can be made to turn faster by increasing the external magnetic field, having more turns in your loop of wire or by having more current flowing through the loop.
If you reverse the direction of the current by switching your power supply around, the motor can rotate in the oppostie direction.
We looked at some demonstration electric motors although it was tricky to get them to work.
HW Copy P169 Fig. 20.6 with labels into your book, along with fig. 2 from the handout to explain how motors operate.
We looked at some uses of electromagnets, such as electric bells and relay switches.
Then we did the kicking wire experiment.
A current carrying wire experiences a force in an external magnetic field. The force is always at right angles to the direction of the current and the direction of the magnetic field lines.
The direction of the force can be determined by Fleming's left hand law.
The physical reason for this force is due to the combining of the magnetic field from the the wire and the external magnets. The fields agree on one side of the wire, but cancel each other out on the other. This is known as the catapult field effect.
A force is always felt from areas of strong magnetic field to areas of weak magnetic field. Strong fields are shown by the field lines being closer together.
The direction of the force will be reversed if either the current is reversed or the magnetic field is reversed (but not both). The force is made stronger by increasing the current (more moving charge to experience a force) or making the external magnetic field stronger.
HW Qs 1,2,3 P167 IGCSE text book.
We started electromagnetic forces by recapping simple magnetism.
Important points:
We went to the computer room in order to write up a report summarising the energy options available to mankind in the future.
HW Finish off your report for next time, including a conclusion based upon your opinion for the best way forward.
Yep, but depleted 'cos of Jewish new year.
I talked about the differences between nuclear fission and fusion.
A neutron is absorbed by a large nucleus causing it to become unstable and split into two. The daughter nuclei are more stable than the original one, so energy is released. (They have less mass, E=mc2, so mass has been converted into energy.)
If enough fissile material (uranium-235 or plutonium-239) is present, the neutrons released in one fission will hit other nuclei, causing them to split.
The energy can be released catastrophically as in an A bomb, or in a more controlled manner in a reactor. Control rods are used to absorb some of the neutrons in this case.
The daughter nuclei are radioactive with a long half life which causes problems as what to do with the spent fuel.
Nuclear fusion
2 small nuclei are slammed together at very high temperatures. They become more stable as a result, turning into Helium. This releases energy. Fusion is the reaction that powers stars, the above reaction is one that humans are trying to harness in order to have an almost inexhaustible supply of energy. (The fuel can be derived from seawater.)
Fusion does not leave radioactive waste and is much more efficient than fusion in terms of the amount of energy released per gram of fuel.
Humanity must harness this power effectively if it is to have any long term hopes of survival.
I evangelised about nuclear fusion being the only sensible long term solution for mankind, something that we will have to crack if we are to survive into the next millenium. Info here and here .
No. Stubbers. You saw a big old film on energy and energy resources and then did some questions from the textbook on the matter.
It was delightful to see all of you jolly little people again.
We got right back on with it with some questions on work, energy and power as a recap of what was going on at the end of last year.
I'm away on Monday morning with my 1st form so we'll I'll set something for you guys to do on the new topic of energy resources....