Teaching the photo-electric effect

The photo-electric effect is one of those demonstrations that can be tricky to get to work, so for my first few years teaching I used to just demonstrate what should happen. Somewhere along the line I got it to work properly and have been gradually building up ways to make it more fool-proof and more interesting.
This can all be done in a single class:

  1. Make sure electroscope is charged negatively
  2. Make sure to use a zinc plate
  3. Sand the zinc to remove the zinc oxide surface layer
  4. Use a normal lamp to show that this has no effect.
  5. You need to use a UV source of the required frequency – I bought a cheap hand-held battery-powered one and it doesn’t work, so presumably it’s emitting in the lower end of the UV spectrum. The one which does work is a larger plugin model bought from a science supplier for this purpose.
  6. Use two normal lamps to show this still has no effect
  7. Use a normal lamp with a higher power rating than the UV lamp to show that this has no effect (but surely this should work – more power means more energy per second, so why wouldn’t this work? – There’s more energy per sec because there are more photons, but each photon of visible still has less energy than a photon of UV. At least that’s my interpretation).
  8. Put a piece of glass on top of the zinc and now shine UV. Leaves do not collapse because glass absorbs UV light – can’t get a suntan in a glasshouse (well, some UV gets through, but relatively little).
  9. Get every student to do the demo. While they’re waiting get them to repeat the diffraction grating demo; it means that in the one class they have proved that light is both a particle and a wave – they will never do anything else in Science as significant or awesome (in the original sense of the word) as what they have just done in this class.

Note that in terms of ‘wow’ factor almost nothing happens; divergent leaves collapse – that’s it. I sell it as the most significant, least impressive demonstration in all of Science.

It’s definitely worth sticking with. If you want to try again do let us know how you got on.

And then show then show this one again – it looks like it’s exaggerating what’s happening, but it’s not.
It really is that strange

I remain convinced that those responsible for making out the science syllabus at junior cert and leaving cert level end each and every meeting with the following question:
“Right, are we absolutely sure that we have removed every possible reference, no matter how indirect, to wonder in this document? We are? Okay then let’s publish it.”

Analogy for the Photoelectric Effect
The energy of the incident photon is like the money in your pocket at the start of a night out.
The work function energy is analogous to the admission charge at the nightclub.
It’s an all or nothing thing; even if you’re only just one penny short, you will be refused entrance.
Different clubs may charge different entrance fees, which is analogous to the differing work functions of different metals.
The maximum Kinetic Energy bit represents the money left over after paying for entrance, which can then be spent on lemonade 

A nice analogy for work function, but as with all analogies it’s worth discussing the limitations.
For instance if one of the group does not have quite enough money, but the rest have some to spare, they can all get in.
Photons, on the other hand, cannot re-distribute energy between themselves.
Even if a photon with insufficient energy arrives at the same time as others with excess energy, it still cannot eject an electron.

You can find more resources on thephysicsteacher website here: http://thephysicsteacher.ie/lcphysics29theelectron.html

and my notes used when teaching The Electron are here:


Changing habits/ reluctance to innovate

Q. How many traditionalists does it take to change a lightbulb?
A. Change??!! That lightbulb has always been there. And now you just want to throw it out?? Over my dead body!

I have been interested in the issue of how people change habits for quite a few years, both from the perspective of students and teachers. I have come to the conclusion that the following steps are a useful checklist. For now let’s  just apply it to teachers:

  1. Teachers need to want to change (or at the very least appreciate that change is necessary). See note at the end of this post for a different perspective.
  2. Teachers need to know what they must do in order to bring about change
  3. Teachers need to change their habits

Each of first two steps can be enormous obstacles but can pale into insignificance compared to the third.

Teachers need to appreciate that change is necessary
Teachers are very busy, They are right to be sceptical of dictats which filter down from on high about how they need to change. Much of it is political and not very well thought out. So if you are coming in with your idea you need to be able to convince staff that this time it’s different; this one is worth taking on board. How you do that depends on the context. But don’t be fooled – there are no easy answers.

Teachers need to know how to change
I have heard advice along the lines of ‘get one person in each department on board and they will act as tutors for their colleagues within the department’. It’s not necessarily bad advice if it gets incorporated into a larger plan, but if it’s presented as a standalone ‘magic bullet’ then it’s doomed to failure.
For starters, how does it work if the department in question consists of just one teacher; plenty of departments fall into this category.
What happens if there’s a personality clash between the designated teacher and other teachers in the same department?

The solution has to be seen in the context of a whole-school approach. And this is where the infamous Croke Park hours come in.

“What do you discuss at staff meetings? Busy stuff or impact on student learning?”
John Hattie

Too often staff meetings across the country fall into this ‘busy’ category. And this is where you need buy-in from management. If they agree that the cause is worthy of support then you’re half-way to solving the first step in changing habits. If they’re not on board then you can pretty much throw your hat at it right there and then.

One option which works very well is to get individual members of staff to take one idea that works well for them and present it to the staff as a whole. The advantage of this is that after a while a sizable number of the staff will have presented; this helps to get across the idea that this is a whole school activity and not just for the nerds. It also encourages discussion in later informal settings. Remember that teachers are very busy during the school day so if you can create an environment where teachers are happy to discuss this outside of school hours then you’re onto a winner.

If the cause is related to incorporating ICT into teaching and learning then another useful tool in your armory is the Whole School Evaluation. It’s quite possible that somewhere in your school’s WSE report there is a recommendation to the effect that this is an area where the school could/should look to improve. And I doubt if there’s a school in the country where that doesn’t apply.
If you’re looking to introduce modern ideas on teaching then you need only point to the new Junior Cert program.

Teachers need to change their habits
Steps one and two are very difficult – no point pretending otherwise.
But (and here’s the really bad news) they’re a walk in the park compared to point 3: there is no shortage of examples of where we know that we need to change and we  know how to change, but still don’t change. It’s because habits are the hardest things of all to change.

Any book I’ve read on this subject have been decidedly unhelpful. The following are two examples:
The Power of Habit: Why We Do What We Do, and How to Change
Better Than Before: What I Learned About Making and Breaking Habits to Sleep More, Quit Sugar, Procrastinate Less, and Generally Build a Happier Life

Both of these involve a lot of phrases like ‘my mom thought she could never change’ and ‘my friend used to . . . ‘ so they end up reading more like amateur self-help books than any synopsis of science-based research.

Changing hearts and minds is never easy. I’m a perfect example. I know that I need to change in order to become a better teacher. And I’ve a fair idea of how to go about it. I’ve even created a website to help me access other teachers’ experiences whenever I need to. Ironically some colleagues have taken these ideas from me and used them to change how they teach. And as a result they are now much better teachers. But I ‘m still struggling to get from point 2 to point 3 myself. I am enjoying the challenge however. And because I’m doing this on my own terms I can start with very small steps.

So where can you find resources on this?
I have mine here – it may act as a useful starting point: betterteaching.ie/reluctancetoinnovate.html
Some of the big hitters are there including cognitive psychologist Daniel Willingham on how cognitive dissonance can explain teachers’ reluctance to take on new ideas.

The half-man, half-myth that is John Hegarty once referred to my main website as web 0.5 so goodness knows what he would make of betterteaching.ie. Having that said, he did acknowledge that ‘content is king’ so hopefully this list of resources will prove useful to anybody else who is trying to encourage change in the staffroom.

Another useful option is to go the homepage of betterteaching.ie and join the Teaching and Learning discussion forum. It already has over 600 members and is open to teachers at all levels (primary, secondary, tertiary). The aim is to encourage sharing of ideas and resources on how to improve teaching and learning in all our classrooms.

Happy reading, and good luck.

Only after creating this post did I go to YouTube and TED for other perspectives. Here is an interesting sample of what I found:

BJ Fogg hits the nail on the head when he tells us that we have to start small, not big.
And we need triggers to constantly remind us. If this trigger is something that happens regularly throughout the day (in his case it was as simple as going for a pee) then we get reminded to make that small change throughout the day.

This links nice to my plan which to hang laminated instructions from the ceiling of my classroom with simple instructions on them such as “Praise the effort, not the work”, “Deliberate practice”, “Pose, pause, pounce” and “If you’re chatting you’re not learning“. There’s no point putting up posters on the walls; I need to be reminded of these points every single time I face my students.

Jeni Cross explodes some myths of behaviour change:

  1. Education will change behaviour
  2. You need to change attitudes
  3. People know what motivates them to take action (turns out social norms is what counts, though folk won’t even admit this to themselves)

Cross explains that we don’t need to change attitudes, which would contradict my point number 1 above. So I will happily concede this on the basis that science is indeed often quite counter-intuitive, but maintain that while changing attitudes may not be necessary, it should still help considerably to overcome steps 2 and 3.

Why it’s not okay to chat even if you’re “getting the work done”


I don’t know about your classes, but if I cited a student for chatting every time they did so in class I would have every student in my class on a punishment before the end of every class.
Quickly followed by mutiny in the science lab.

So a while back we agreed that we would go with constant chatting as the offence.
And this has helped with the worst offenders.

But there is another issue.
The ‘good’ students see no issue with chatting if they’re getting their work done at the same time.
And I don’t want to appear completely unreasonable so I usually go along with that.

It means that the work ‘gets done’ and there is a good atmosphere in the class.

But It’s a load of cobblers.
What I’ve only come to realise/appreciate in recent years is that when it comes to learning, you can’t multi-task. You can’t chat AND be learning.
The fact that you might have the question answered at the end of the class is beside the point; (practically) no learning took place while you were engaged in conversation, but in order to get the question ‘done’ the student was able to ‘switch’ focus from chatting to working and back again, and they’re able to do this so quickly that it seems that they’re doing both at the same time.

This is analogous to a computer which seems to be doing lots of things at the same time, but in actual fact the ‘brain’ (the CPU) is just switching (very rapidly) from one task to another.

The problem with this from a student’s perspective is that all this switching is very detrimental to the quality of the learning.

In John Hattie’s Visible Learning and the Science of How We Learn he cites research on typists in the 1950s which showed that when they had to answer phone calls as part of their brief, the cost was far more than just the time spent on the phone.

Similarly, responding to a (non-routine) email can cost a typical employee an average of 15 minutes.

Now if three quarters of the class time consists of students working on problems, then I know in my heart that I’m never going to get full compliance (no chatting) on that for 30 minutes or whatever.

And students would see themselves as being unfairly punished if I cite them as soon as they begin to ask a question that is not related to the problem at hand.

So I have started to compromise.

First of all it is vital that they see where I’m coming from with this; they need to appreciate that I’m not being draconian for it’s own sake. There is a logic behind the exercise.

Secondly, if they do buy into it, they my proposal to them is that we go for complete attention to task for an agreed amount of time, say just 5 minutes. They agree to stay completely on task (obviously allowed to ask a colleague (quietly) or myself a question if they need to, but otherwise there should be silence).

A couple of points:
It still calls for a certain flexibility; I usually find that I can give them a warning and that does the trick rather than citing them.

Five minutes is a reasonably short period of time in one respect; but for many of them it’s the longest they have stayed quiet all day.

I then allow them the next 5 minutes to chat while continuing to do questions, but the plan is to extend the period when they’re solely on task and reduce the period for which they are answering questions and chatting.

To monitor the five minutes you can either get an online countdown timer – just google it – or get a wall-clock. You can get a cheapie wall-clock from Argos for €5.
A ‘real’ clock also discourages a student taking out their phone, either for this activity or for telling the time in general (“what’s a wrist-watch?”)
It’s important (again) that they appreciate that if they’re not focused on the task at hand then no learning is taking place. This has obvious implications for how they do homework. It can also seem counter-intuitive in that they have tangible evidence of having ‘work’ done, so they may be slow to accept our reasoning. But they do seem to ‘get it’ and I’m delighted with how well it’s working in my own classes.
It’s also why we shouldn’t text and drive, or even use a hands-free set while driving; again Hattie points us this website on car safety.
Apologies to all teachers who don’t have this problem; I’ve just taken 3 minutes of your life which you won’t get back.

When it comes to improving teaching and learning, it turns out that most ideas don’t require any extra work on our part, just a change of habit, which unfortunately can be the hardest thing to change.

Listen to Keano: write only what the examiner wants to read

Back in 2013 Manchester United played Real Madrid in a Champions League match.
In the 56th minute Nani went into a tackle with his foot up high; the referee not only gave a free kick against United – he also sent Nani off.
United lost the game and as a result went out of the competition.

Afterwards the ITV commentary team were discussing whether or not the referee made the correct call. They replayed a clip of the action at normal speed, then in slow motion.
Roy Keane was on the panel but wasn’t saying much at this stage. Finally the host asked him what he thought. It’s very simple he said; the debate here shouldn’t be about whether or not the referee made the correct call – the discussion should be why Nani was daft enough to raise his leg that high in the first place. By doing so he was creating a situation where the referee was forced into making a call one way or the other. At that stage the damage was done. Nani should have known better.

Whenever I correct a test I usually get a couple of answers where it’s unclear whether or not they merit full marks.
My tendency now is not to award the marks. This highlights to the student the danger of putting the examiner in a situation akin to the referee in the story above.

And Keane’s point is just as applicable here – If at all possible avoid creating a situation where the examiner has to make a call as to whether or not to give you full marks. It may seem obvious, but if it’s the leaving cert then remember that you won’t be in the examiner’s house when he’s correcting your paper, so you won’t have the opportunity to explain to him (or her) what you meant by your answer. And even though your answer may make perfect sense to you, it may still not get full marks on the day.

The moral of the story? Give onto the examiner that which is his. If there is a standard answer to a commonly-asked question then just learn it. And make sure that this answer – and only this answer – is what you write down on the day of the exam. If you’re reading this as a parent then check that your child knows their definitions – and if they stray off course by putting things in their own words then don’t be afraid to give them a red card.
On a serious note, if their definition doesn’t make sense to you (if it doesn’t read as an english sentence should) then it probably won’t make sense to an examiner either.

As I mentioned in my last post this is all just a game.
And this is just one of the rules.
So if you want to play you have to learn the rules.
Make Keano proud.


This was the closest I could get to a video of the discussion. Unfortunately it kicks in just after Keane’s comment about Nani unnecessarily putting the referee in a situation where he had to make a judgement call.
But then again, Keane is always worth watching.

For what it’s worth, the clip also illustrates one of our inbuilt biases known in psychology as fundamental attribution error. It’s one of the most profound ideas in all of science because it tells us that our own view of reality is filtered in such a way as to make us seem to be better than we actually are. But maybe that’s for another day.

Irish company Havok makes waves with its Physics engine

Havok is an Irish company.
This gives us a pretty good idea of what they are about:

Take a look at their leadership team to see the interdisciplinary nature of their combined skillset.
Havok make the news recently and the following was posted on an american physics-teachers’ forum.
I am reposting it here with permission from the poster – John Denker:

Here is a news report that mentions physics in a real-world context:

Every video game has a module called the “physics engine”.
A good physics engine is worth a bunch of money.
The price that Microsoft paid has not been announced, but I reckon it is in the neighborhood of a billion dollars. That’s based on the fact that Intel paid 110 million to acquire Havok back in 2007.

This is useful as part of the answer when students ask what physics is good for in the real world. The number of physics jobs in the computer graphics industry is not super-huge, but it’s more than the number of physics jobs at (say) CERN.

It’s also worth mentioning that most of the high-paying jobs are interdisciplinary. Expertise in physics alone is not nearly so valuable as expertise in physics /and something else/. Even more valuable is the skill we call life-long learning, i.e. being able to come up to speed in a new area quickly.

Let’s remove voltage from the Junior Science syllabus; post #1


Dear Mr/Ms Junior Cert Syllabus writer,

The time has come to question why the concept of voltage is still on the Junior Cert syllabus.
It is by far the most difficult concept for students (and indeed teachers) to grasp.
Consider a relevant extract from the Junior Cert Science syllabus

Set up a simple electric circuit, use appropriate instruments to measure current, potential difference (voltage) and resistance, and establish the relationship between them

Let’s take a look at potential difference (commonly referred to as ‘voltage’):

The following extract has been taken from the minutes of a History of Science meeting, in 2002.

John Roche, of Linacre College, Oxford, opened the session after tea, speaking on the concept of voltage. He began by claiming that almost every concept in electricity and electromagnetism is ambiguous, and the concept of voltage is one of the most incoherent. Its evolution is difficult to follow.

 Abbé Nollet, in the 18th century, distinguished quantity and degree of electrification. Others made similar distinctions between quantity and intensity or tension or pressure – what we would call voltage.

 Roche showed how the term “voltage” had come to be used nowadays in three different ways; for electromotive force, potential difference and (absolute) potential.

Volta defined electrical tension as the endeavour of the electrical fluid to escape from a body. Volta’s tension was more akin to a force, unlike the modern definition of electromotive force, which is a misnomer, being defined in terms of energy.

Ohm carried Volta’s concept to closed circuits with the idea that voltage was proportional to the difference in tension between the ends of a conductor. For Ohm, it was the gradient of electrical tension that drove the current.

Poisson introduced an entirely different concept, of charge divided by distance to a point, which Green called the potential. This was an analytical device only, arising from an analogy with Laplace’s gravitational potential function.

Kirchhoff reconciled Volta’s tension with Poisson’s potential function through the concept of energy or vis viva introduced by Helmholtz. From Kirchhoff, current is driven by the electric field in a conductor and voltage is related to the energy supplied, but physicists and electrical engineers do not usually think of them in this way.

All the earlier interpretations remain current, but with different weights, and most of the time voltage is seen as a driving energy.

 IOP History of Physics Group Newsletter, Spring 2000, page 65

So what exactly should we be telling our students about potential difference? How many (non-physicist) science teachers can define or explain potential difference? Maybe most can, but if so I would be very pleasantly surprised.

Would it hurt anyone if we replaced the syllabus extract above with something more simple, like the following?

Set up a simple electric circuit using appropriate instruments to light a number of bulbs in series.
Understand that for current to flow a power supply and a complete circuit are required.

The other aspects of the syllabus on electricity could remain as they are, but no Ohm’s Law, no experiment to verify Ohm’s Law, no mathematical problems based on Ohm’s Law, and no more mention of potential difference.

Let’s put the swine flu in perspective

The total number of deaths in World War One was 16 million.
The number of deaths as a result of the Spanish flu which followed in 1918 was somewhere between 30 and 80 million.

In the 14th century the Black Death is estimated to have killed 75 million people (including anywhere from 30% to 60% of Europe’s population – including Ireland).

 Which isn’t to say that the current Swine Flu shouldn’t be taken seriously, just that when it comes to risk our ability to be objective tends to go out the window.

The Black Death by Philip Ziegler is a book well worth reading for the historical and in particular the social and political implications of this desease.