This is a way to determine the Young Modulus as an individual, rather than group practical. For years I thought that you needed a pulley for the wire, but it turns out you don’t. Without that limiting factor, it becomes a pfaff reducing exercise.
Get a plank of “whatever you can find” width, about 1.2m long, drive a screw in about 5cm from one end. Photocopy a normal ruler at 100% then at 90%. Check the reduction is accurate! Use the 90% to make a vernier scale. Get about 2m of 32swg copper wire and tape the vernier scale to the wire at 1m from the nail. Line the vernier up with the 100% scale and gently hang 1N weights to the end of the wire that hangs off the end of the plank, measuring each extension. The diameter of the wire is done traditionally by using a micrometer or looking up the diameter of swg wire in mm.
Steps and Photos
For those of you who assess against CPAC, this is a good experiment for 3a (Identifying Risks) and 3b (Working Safely). I use the Hazard | Risk | Control approach to a risk assessment. e.g.
Flailing wire causing damage to exposed skin/eyes. Weight falling on feet.
Eye protection. Weights over wire for kinetic energy transfer. Sandbag on floor.
I realise that the weights on foot issue is not a massive one, and using copper wire makes the flailing wire unlikely too, but they do need practise thinking about these things for bigger challenges ahead.
Three graphs of real data showing how the pulley (middle) makes no difference, the first and third are with the wire simply hanging over the edge of the plank. The final one (green dots) is Hooke’s Law.
I hope you found this useful. Any comments, suggestions etc. Please contact us through twitter on @physicstp or use the form below.
I think I am getting a bit blasé about publishing, two fails in two weeks. ?
When I create a Podcast Post I have to do three main jobs for the podcast to release properly:
Upload and attach the mp3
Tell the Post when to go live (5:00am on the next term-time Thursday – an arbitrary choice that I have stuck to)
Tell the Post it is a Podcast Episode (rather than a blog etc.)
Last week I failed at #2, this week I failed with #3. Both cases were because I changed the auto-tweet text. For some reason you have to completely start again from scratch if you want to change the release auto-tweet. This means copying and pasting all the content in to a new page, usually late at night, usually a bit frustrated…
I am making a checklist now. #1 is
“Check the damn tweet doesn’t have a spelling mistake!”
Enough procrastination. I have a pile of lab books to mark. ?
We were contacted by Seamus Smith who told us about some more sites for Technicians. I have just slightly tweaked the site to have two more pages in the menu structure: Technician Sites and Physics Teaching Sites.
One of the sites Seamus mentioned is TecHKnow which has a huge directory of sites, that makes our pages look a little weedy. But on the basis that anything that links back to them helps, I am leaving our pages up.
I am still searching for the motherload on Physics Teaching sites. If you know it, please let us know using our twitter handle @physicstp or the form below.
PhysicsNet:“If you want to understand everything from the smallest building blocks of matter to the large scale structure of the universe then Physics is the subject for you. Good luck with your studies.”
If you want to suggest another link please tweet us @physicstp or email using the form below.
After sending out a few cannons, I decided to send out a “Tips” sheet in the cannon packaging to help people get started. I’m reproducing it here for those who didn’t get it:
Vacuum Cannon Tips
The pipe suppliers are used to providing for large scale filtration systems. They care not about little nicks and scratches on the pipes. I sand off the more obvious burrs (so the pipe might look a bit battered) but they will have no effect on the performance of your cannon.
Please don’t underestimate how loud it is close up; it makes your ears ring for some time afterwards if you fire it with no protection. The shooter DEFINITELY needs ear defence of some sort because they won’t have their hands free to cover their ears.
Flanges (pictured) do make it easier to prepare and fire. There are many ways to obtain some. You can use cardboard, old mini-whiteboards, 3D printing… Thomas found that his tech department were very happy to be involved, knocked out laser cut flanges within a day and enjoyed the test firing process!
The tapered holes for the valve make a very good seal, but if in doubt PTFE plumbing tape will make the seal even better. Thomas has not needed it apart from the early prototype.
Please warn your colleagues! You don’t want an evacuation because of a suspected shooter on the premises. This has happened. “Duds” are possible, but can be mostly avoided with the following approach:
Don’t forget to roll the ping-pong ball all the way down the tube. This is easier before you add the tape because of the ball’s tight fit.
It is worth taking care with the taping of each end. The rear just needs to be secure. The front needs to be secure but not too secure! Take care on the front, cut a square piece of tape and carefully fit it to the front of the tube. Try to fold back and stick as little as possible. You want the least folding and sticking that will support the vacuum. Trial and error will help you work out the optimum. We believe the tape blows off, so you want that to happen before the ball arrives. (If you have flanges, this step is much easier, just cut a square and make sure it is securely pressed all round the circle of the tube and gently stuck to the flange.)
Let the pump run for a while. You know very quickly if you have a bad seal because you will hear cracks from the tape and then the frustrating hiss of air being sucked in. If it is working nicely it gets to a pretty good vacuum very quickly but if you wait 10 seconds or so it continues to increase.
Pierce the rear tape. Scissors are fine. It fails catastrophically and very very loudly. It is worth checking there is no tape hiding in the tube before loading the next ball.
The tube does not need to be super strong (the tape will fail long before the tube), but it does need to be close in diameter to a table-tennis ball (40mm). The right tube is known as Imperial 1½ inch PVC, common in the USA but not so much in the UK. I collected it from Koi Logic for £5.05 a metre since delivery is quite expensive and it is not too far away. CLEAPSS suggest the plastic pipe shop where it is about the same cost but you get a longer length (so more expensive and a lot of waste).
1m of tube is fine, it is easy to transport and store. You can get tremendous speeds from a 1m tube. CLEAPSS recommend that you do not go longer than 1.5m. I chose 1m because above that, the delivery charges get silly.
Being a bit of a perfectionist I thought that tapping it in to a tapered thread would allow for cheaper components, a better seal and for replacing components if needed. I found some cheap nylon fittings at Wreking Pneumatics, all they needed was a correctly threaded hole.
For pneumatics I learned you really should used tapered thread (BSPT) holes to get a good seal with no need for tape, glue or sealant. Tracy tools are helpful on the phone and sell reasonably priced tap, dies and drills. I wasn’t sure what would work best so bought a plug tap and the correct size drill (8.4mm for the ⅛” plug tap) for both ¼” and ⅛” tapered thread holes. The prototype has ¼” BSPT but I learned that the schrader valves suggested by CLEAPSS and potential vacuum gauges all use ⅛” BSPT so the ¼” tap I bought was redundant. (I actually bought another but it took so long to come in the post via amazon I gave up on it; Tracy Tools delivered within 24 hours of ordering.) Tapping PVC is a doddle compared to steel.
The tape was the cheapest “three inch” (78mm wide) tape I could find on eBay, just bought in bulk. The prototype used “two inch” and worked, but barely covered the width of the tube (see photo).
The balls are practise balls from eBay, bought in bulk. “Real” balls are much stiffer and about twice the mass. I bought 300. Softer balls expand a little in the tube I think, so possibly get more push, and being lighter can accelerate faster? This is worth investigating perhaps?
No Flanges ?
Flanges make a great stand for the cannon and provide a good surface for the tape to adhere too. The best I could find was £8 a flange, but that would almost double the cost of the cannon. Robin and I tested the cannon with no flanges with very good results. My tech department whipped two up for free with no problem and you can make basic ones out of cardboard or 3D print them. There are many options. Our tube’s diameter was 48mm (Officially 48.26mm according to the Standard). This is a good opportunity for the school to personalise their cannon!
Adding a Pressure Gauge
I found pressure gauges on eBay for £7 including postage. They come with a ⅛” BSPT (taper) male and screw straight in to another ⅛” tapped hole. Just search for: 1/8″ BSPT pressure gauge.
We are selling Vacuum Cannons in the shop. I am setting it up with some trepidation: if nobody buys one I will be several tens of pounds out of pocket. If everyone buys one I will be unable to make them fast enough to keep up with demand. Of course, the most likely thing is somewhere in-between. Being part-time I can respond quite fast, and I can limit the number I have to make and dispatch by setting the stock levels in our shop software. And through the piss-poor planning you have come to expect, we are releasing them a day before I go away for a week over half-term. Oops.
The pay-off is that more people will get to play demonstrate with this fantastic toy piece of equipment. I had never heard of it, nor had Robin, but CLEAPSS are all over it so it must be pretty well known. When I tested the prototype in the kitchen I didn’t really think through the consequences of a ball travelling at several hundred miles an hour vs my kitchen wall. There was ball debris everywhere; I found one shard 5m away on a high window sill two weeks later.
I have set up stock of 2 in the shop but with unlimited back orders. Once I know what the demand is (or indeed if there is any!) I will start making and shipping them in batches. I intend to keep doing them at £20 until I lose the will to go through all the guff of making, packing and dispatching them. Then I will either give up or raise the price. So buy now to avoid disappointment.
In the next podcast we inevitably talk about the rope model. I tried it a few times in the past and hated it ?. It was only in making the podcast that I finally understood how to do it and how good it is. Not knowing how to do it is as much a function of my comfort with the donation model as it is my being alone in my school with no other physicist to talk to. But talking to Robin and Stuart about electricity really got me thinking.
When I did the rope model it didn’t work because I had quite a few kids involved in the demo. I found it really hard to make the rope run smoothly through their hands (not least because the rope had a huge knot in it!) and it was a very static sort of demo. There was no fluidity at all, no flow. I had visions of attaching pulleys to the walls of the room to make it work (I like big scale). I chatted to Stuart about this and he was able to tell me a way to do it that worked. It works so well, and is so easy to do. I was bowled over.
my recipe for an effective rope model demo:
Have a rope that is in a 3-4m loop where the join is as smooth as possible. (Cutting and melting together a rope is ideal).
Choose one student only.
Hold the loop and pass the other end to the student. Tell them to grip it lightly (they don’t want rope burns) in one hand with the rope passing vertically down through their fist (this is the detail I never understood – one student, one hand).
Explain that the rope is the electrons and the grip is the resistance.
Pull the rope hand over hand through their (one, stationary) hand.
Question: What do you feel in your hand?
Answer you want: Warmer
Tell them to add another hand, holding with the same light grip. The rope will get harder to pull, make this obvious, it will naturally run more slowly.
Question: why is the rope moving more slowly?
Answer you want: because there is more resistance.
Question: how can I increase the current to make it move as fast as before?
Answer you want: Pull harder.
Explain that this means more energy being delivered by the cell to the rope, or greater pd.
Pull harder, to make the rope move at the original speed again.
Tell them to grip tighter with one hand, but not to tell you which hand that is.
Question: what has happened to the rope speed?
Answer you want: slower.
Question: does the rope/the pd know which hand is gripping harder?
Answer you want: no.
Question: what is happening at the tighter hand?
Answer you want: warmer.
Explain how this is energy transfer
Question: are the electrons in any way different before and after the hands?
Answer you want: no.
Remind them that electrons just go where they are pushed/pulled by the pd, they aren’t changed, they don’t make a choice, they just go where they go.
Get another rope, get them to hold one loop in each hand, but you pull them together. You can model parallel with this but I wouldn’t go too far as the model does tend to break down a bit.
You can show AC nicely too. Remove one loop then tell them to hold their fist horizontally, not vertically. Now grip the loop in both your hands and pull it backwards and forwards. The hand gets warm just the same.
Robin and I started with very limited ambitions, it wasn’t quite “more listeners than presenters” but not far off. We agreed to do episodes until the end of the academic year and then see where we stood. At that point we had not got much further than me telling Robin he and I should start a podcast and then months later him finding the statistic that about 50% of schools in the UK have one or zero physics teachers. We just wanted to try something, try to build a community and see what happened. I am a regular listener of The Cycling Podcast and modelled a lot of our approach on them.
As we started I had no idea about format, content, approach, social media, iTunes ratings, post production, how to talk to a microphone and had never considered a T-shirt or shop (or adding a forum) But I am top loaded with running web sites and general geekery, so I figured I would muddle through.
It was a revelation to me (physics teacher, but never an audiophile) to find that a microphone has preferred orientation. Listening back you can tell the episode where I worked out where to point my mouth when recording. I found a great T-shirt supplier when looking for somewhere to buy a jokey shirt for Robin. They actually make the shirts on demand and provide a WordPress plugin, so one thing led to another and now… a shop! Just a bit of fun of course, as Robin says in Episode 9, this is nothing about money… But if it helps build a community then that’s brilliant.
As I went to sleep on the night before episode 1 I was not convinced I wouldn’t grab my phone at 4am and cancel it. Now, as we approach the recording of episode 10 I feel more comfortable hearing my own voice. I’ve also been dragged kicking and screaming in to social media and I feel a community is growing. (We have had a few emails thanking us for doing it and the interaction on Twitter has been fun). It has also been hugely enjoyable talking to different people and learning from them: I recorded another PIM this week which will most likely be in Episode 11. Please do continue to send us ideas. We love hearing them all. You can use the form below.
Finally, to get some 5* ratings in iTunes is pretty stunning to be honest, so thank you very much, listener(s!).
I said in Episode 6 I would share what I came up with for the Jelly Chair lesson and refutation texts. I did put some thought in to this but think that there is room for improvement. I ended up writing a few sentences for the kids to do:
The ancient Greeks thought that arrows were pushed through the air by the god Apollo. We know this is wrong because…
Many people think that objects will slow down and come to rest if no resultant force acts upon them but really…
A typical GCSE level understanding of balanced forces allows a person to sit on a chair made of jelly. This is false because…
Galileo showed that two balls of different weights accelerate at the same rate when dropped.. Before this people thought heavier objects fell faster but this is wrong because…
Some people think the International Space Station has no forces acting upon it because it is in Space, the evidence that they are wrong is…
When a helicopter is hovering it is very common for people to think that the weight of the helicopter has an equal and opposite force that is the down-force from the rotors. This is wrong because…
I was particularly happy with the last one (though I have no idea if that is an actual misconception or not), and even happier when one of the students said something like:
The down-force is equal to the weight!
They had truly understood the situation, and was able to explain to their partner that the weight and down-force must be equal in magnitude and in the same direction as weight if the helicopter is hovering (not equal and opposite).
As I roamed the class and spoke to the students I was pretty confident the refutation sentences were having impact. The challenge is to use them again and again and embed them in my practice.