We made a simple Vernier caliper and used it to recap on how Vernier scales work.
We then made some measurements using a ruler, a real Vernier caliper, and a micrometer.
You did some more of the sheets that I gave out last week.
Continue your revision for the EMPA.
More EMPA Revision
I gave you a strange graph task. I guess the idea with this type of question is that in the formula, measured quantities (in this case S and T) can appear in more than two sererate terms.
You job is then to make the measured quantites only appear in two separate terms, so they can be plotted on the x and the y axis. They can appear in any form you like (e.g. 1/T or S/T or whatever) as long as the resulting graph is linear.
You then did a past A2 task, and I gave you the Part B and the markscheme to go over at home.
Do the Part B paper, and then mark using the markscheme. We can discuss any issues on Tuesday.
You did the last part of this paper. We will go through it on Tuesday.
EMPA Past Paper
You continued with the past EMPA paper.
NOTE: we will do the final part next lesson on Friday. The paper is 1 hour 15 mins long, so if you want the full time, please arrive 5 minutes early.
EMPA Past Paper
You did a past EMPA paper - we will continue with this after the holidays.
I gave back your mock papers. On the whole these were very good, but a few of you will want to improve.
One of the biggest areas seems to be the multiple choice questions, and managing time effectively. If this was an issue then make sure you do lots of multiple choice questions.
If you are revising over Easter then obviously past papers are a good choice. However, there are only 9 available for unit 4, so don't do them all. You want to save some for later revision.
If you do a past paper, then make sure you do the whole thing under timed conditions, and keep an eye on how long you spend on each paper. You can convert your mark to a grade here.
If you don't want to use up all the Unit 4 past papers, there are lots of past paper questions here and answers here. I would recommend using these, and spreading the real past papers out between now and your exam.
I will also try to make some videos of how to approach the multiple choice paper.
Have a good Easter!
You did the mock.
I will mark it and put it in your pigeon hole as soon as I can.
Next lesson we will begin our EMPA practice by doing a practical, so you don't really need to do any preparation for that.
Eddy currents, back emf
You did a quick question about dropping a magnet through a coil, which we went through.
We agreed on mock timings - please arrive in time to start either at 8:20 to finish at 10:05 or at 9:55 to finish at 10:40.
I demonstrated eddy current braking in a spinning metal disk, and we discussed eddy currents in transformer cores. More here.
I started explaining back emf but we ran out of time. It's not that hard - see HW video.
If you like you can watch this video which I just made about back emf.
Finish the questions on Q1-2 P115 and check your answers in the back of the book.
Make notes on Hall Effect using P111-2 and this video.
Bring to next lesson.
Moving charges in fields
We looked at charged particles moving in electric fields and magnetic fields.
We did some calculations for a beam of electrons being accelerated by a voltage and then deflected by an electric field. They worked out really well.
Then we looked at the effect of magnetic fields on charged particles, and derived an expression for the radius of the circular path of a particle.
We will do some questions on this after half term.
Prepare a short presentation in groups of two or three on either cyclotrons or synchrotrons or mass spectrometers.
The focus of your presentation should be on the role of electric and magnetic fields in these devices.
Hopefully you can present on Tuesday after half term.
We went through the test in detail as there were some tricky parts.
I showed you an electron gun that fires a beam of electrons at a screen, and we deflected the beam with an electric field.
You did a test on fields and capacitors.
Have a go at this game that I made a few years ago.
Dots show magnetic field coming out of the screen; crosses show field going into the screen.
What happens when the charged particle passes through electric and magnetic fields? How do they change the partice's speed and direction? What shape is the particle's path through each field?
We looked at the variation of the torque on a coil on the angle of the coil.
We discussed how real motors reduce the variation of torque as the motor rotates, and watched this video which was really good. We stopped watching when he got to shunt/series motors - I imagine this will make more sense when we have learned about 'back emf'.
I gave you a labelled diagram of a DC motor and showed you a demonstration motor.
You did the questions on P109.
We discussed latitude, longitude, the longitude problem, compasses, etc. There is lots of intresting stuff to read about if you google it.
Revise for test on Friday.
Introducing Magnetic Fields
We did an investigation together to investigae the force on a current carrying wire in a magnetic field.
We observed that the force was in the direction predicted by Fleming's left hand rule, and found the effect of current, length, and angle on the force.
We looked at the formula F = BIl and then applied it to a coil of wire.
We developed numerous expressions for the torque on a coil of wire in a magnetic field, culminating with T = BIAn.
Next week we will look at the effect of the angle of the coil on the torque.
You used a student-friendly markscheme to mark each other's past paper questions.
You finished off the rest of the past paper questions in the book (Q6-8).
Use the markscheme I gave out to mark Q6-8.
Revise for test next week on fields and capacitors.
We went over the graphs I set for HW.
We showed why exactly half the energy is lost when charging a capacitor.
You collected data for the time constant of capacitor discharge with different resistances using a datalogging system.
We agreed to to a test on fields and capacitors on Friday 5th Feb.
Answer Q2-5 on P102-104.
Bring to next lesson.
Capacitor Discharge Calculations
We briefly looked at the formulae for capacitor discharge through a fixed resistor, and how they are derived.
You answered Q1-3 on P101.
We looked at an application of capacitor discharge for timing a collision. More on this next lesson (we can compare with video analysis).
We observed a capacitor discharging through a voltmeter alone, and compared the resistance of an ideal voltmeter (infinite) with a digital multimeter (1MΩ) and an analogue voltmeter (~5kΩ).
Read and make notes on "Charging a capacitor through a fixed resistor" on P100-101, and answer Q4 on P101 if you haven't already.
On another sheet of paper, copy figure 4a from P101, and sketch graphs to show how these things vary with time after the switch is closed:
a) voltage across the capacitor
b) voltage across the resistor
c) voltage across the cell
d) charge on the capacitor
e) current through the capacitor
f) current through the resistor
g) rate of energy transfer by the battery (i.e. power)
h) rate of energy dissipation by the resistor
i) rate of change of energy stored on capacitor
Sketch these and hand them in tomorrow morning. If you can work out the formula for each of these functions that's great - if not just sketch what you think it will be.
You created an iterative model for the discharge of a capacitor through a fixed reisitor.
We discussed one drawback of the model, and how it could be improved (i.e. by decresing the time between each iteration and using a computer to save effort).
You plotted graphs of charge, current and voltage against time for your model, and verified that they showed exponential decay.
You then did a practical to gather voltage and current data over time for the discharge of a real capacitor.
We discussed the potential benefits of using a datalogger to collect data - we will do this next week.
Read and make notes on P98-99.
Note that there are two ways to find the time constant. If you know R and C, the time constant is RC. If you have a graph of voltage/current/charge against time, then the time constant is the time it takes for the voltage/charge/current to fall to 0.37 of its initial value.
For the real data we collected in the lesson, calculate the time constant using both of these methods.
Capacitor charging at constant current
You did a practical where you charged a capacitor at constant current and determined the capacitance.
Unfortunately I connected the capacitors the wrong way round for you (oops...) so the results were somewhat non-linear, but they were still pretty good.
We then started to look at the energy stored on a capacitor, and I compared the situation to the energy stored in a spring.
Read P96-7 and add to your notes on energy stored.
Answer the questions on P97 and hand in tomorrow morning before registration.
We started off by looking at the idea of surface charge density, and the relation between surface charge density and surface electric field strength.
Then we started looking at capacitors - what they are, what they do, and what they are used for.
I demonstrated a few capacitors - some awful (that I made from bits of stuff around the lab) and then an amazing 1 farad capacitor.
We started to look at the formula for the voltage across a capacitor - more on this next lesson.
Read P94-5 and add to your notes - there are quite a few extra bits on these pages.
Answer the questions on P95 and hand in before registration on Monday morning.
I went over some stuff you have covered on your own about radial and uniform electric fields.
We did a tricky problem about a charged particle falling through an electric field.
Answer Q1-3 on P90-91 and either hand in on Monday or bring to the lesson on Tuesday.
If I don't see you on Tuesday, have a good holiday!
Read and make notes on P76-8. There is QUITE A LOT on these pages.
We discussed whether flying birds in a van would break a weak bridge, then I demonstrated it with a helicopter hovering over a balance.
Do some revision in preparation for a test next Friday.
Collect your answers to P46 from my locker (hopefully they will be there if you haneded them in on time) and make any necessary corrections.
Answer Q3-7 on P51-53 and bring your answers to Friday's lesson.
We've covered all the basic theory from these chapters, but there are a few more tricky applications of things you should know about.
Watch this video and see what answers you can come up with before next lesson.
If you still have time, do some revision in preparation for a test on Chapters 1-3 on Friday 16th.
Driving, damping, resonance
We went over the concepts of damped oscillations, driven oscillations and free oscillations.
I did a few demonstrations of various things, including resonance of an air column in a measuring cylinder (similar method to this), beating of two notes on a guitar, and damping a mass-spring system with water.
We started to look at a hard momentum question about a helicopter hovering.
Complete your graph, and work out your values for A and B.
Have a look at P41 which shows the formula for a mass-spring system - your value for B should be close to ½, and your value for A should be close to 2π / k½ (you will have to work this out for your k).
Watch this video, then answer the questions on P39.
You might also want to look at this video and this video which show working through some examples of SHM questions.
Simple Harmonic Motion
We watched a video of circular motion and plotted the y component of its motion over time - the y displacement, then the y velocity, then the y acceleration.
I then demonstrated that a pendulum swinging from side to side looks a lot like one component of circular motion, by swinging a pendulum above a turntable (see P38 of the textbook).
The motion of the pendulum is called 'simple harmonic motion' - we then thought of other oscillating motions that were or were not SHM.
After rec I demonstrated a trolley between two springs (see P40), and we saw that in this case the acceleration was proportional to the displacement, and in the opposite direction. This defines SHM.
I then baffled some of you with some differential equations, which you can largely forget. The main thing is the conclusion that x = Acos(2πft)
Watch the first three minutes of this video about SHM. You can wathc the rest if you like - it will be useful later in the topic.
Watch this video from 2:08 to 3:47 and make sure you are happy with the graphs of motion that are found on P36 of the textbook. Again, you can watch the rest of the video if you like, but that bit is enough for now.
By next lesson you should be happy with the definition of SHM, and be clear about the shapes of the graphs on P36. We will go over the formulae we did today next lesson, so don't worry too much about those.
More Circular Motion
We considered the centripetal force on three children at different positions of a merry-go-round.
This led to a reformulation of the formulae for centripetal acceleration and force in terms of ω.
We went through the last question of the HW, and then calculated the maximum speed for the car not to leave the ground in P27 Q1.
We did a practical to investigate the relationship between speed and centripetal force.
One person in each pair: calculate v² for each row, then plot a graph with v² on the x-axis and F on the y-axis. Find the gradient and the y-intercept.
The other person in each pair: calculate log(F) and log(v) for each row, then plot a graph with log(v) on the x-axis and log(F) on the y-axis. Find the gradient and the y-intercept.
Watch Part 1 and Part 2 of this video about log-log graphs - hopefully it will help you make sense of what is going on.
We will then go over what you have found next lesson.
Circular Motion in Context
We went through Q3 of the HW.
We did some calculations based on the 'long swing' (see P28) for a weight attached to a strip of paper.
We calculated the angle of bank of a velodrome bend that would require no friction at a certain speed.