leaving cert physics

How to get 100% in your leaving cert Physics exam. Part 3: Short Questions

Question 5 on the paper is 56 marks and counts for 14% of the overall mark. The choice is to do 8 questions out of 10. It is one of the more popular questions on the paper.

I have compiled a list of all the short questions which have been asked over the last ten years,  together with their solutions. It is a detailed document (26 pages) but is worth knowing not only because it prepares you for Question 5, but also because it provides a comprehensive overview of all the topics on the course itself.
I have included the years with the questions because it helps to identify the questions which are particularly popular with the examiners.

State The Principle of Conservation of Momentum.
[2002][2009 OL][2008 OL][2007 OL][2005 OL][2004 OL]
In any interaction between two objects, the total momentum before the interaction is equal to the total momentum after the collision, provided no external forces act.

And to my mind the two most popular questions at higher level:

What is the Doppler effect?
[2008] [2007] [2006] [2003] [2002]
The Doppler Effect is the apparent change in frequency due to relative motion between source and observer.

Define electric field strength.
[2009][2007] [2005] [2003] [2002]
Electric field strength is defined as force per unit charge.

There are a lot there so don’t try to learn them all at once; take a couple of pages each day, but then make sure you keep going over them (from the beginning each time) and in no time you’re confidence will be sky high!

Try to print two pages onto one sheet, and also back to back if possible to save paper.


Leaving Cert Physics: Unusual resonance demonstrations

This week we demonstrated resonance.

Apart from the standard demonstrations using tuning forks and wine glasses we also tried out variations

1. The Chinese Bowl

2. The aluminium rod

We are empty space

Having to pick out the singular most incredible concept in physics would be an interesting task (to say the least). Rarely a day goes by without me invoking the term ‘awesome’ in some context or other. I really do have a wonderful job.

Today however I came to what I think may be my favourite – the structure of the atom. You see the concept of solidity is merely an illusion. That table appears to be solid but actually it is 99.9999999% empty space.

“But it looks solid!”
Yup – but that’s (just) an optical illlusion caused by the interaction of electrons and incoming electromagnetic radiation.

“But it feels solid – when I bang my hand off the table I feel a solid surface!”
Again an illusion I’m afraid. This time caused by the interaction between electrons in your hand and electrons on the surface of the table repelling each other.

So next time you’re sitting in your car holding on to the steering wheel, just remind yourself that the steering wheel is hardly even there – neither is the car and for that matter neither are you (I’m trying not to have an exclamation mark at the end of every sentence here, but it’s not easy).

Now there is a (very, very) small amout of ‘matter’ in you. And here’s the thing; if you somehow managed to remove all the empty space in your body you would be left with a lump of solid matter which would be smaller in size than a grain of salt. How much would it weigh you ask? Why the very same as you do now, after all we have only removed the empty space (having a real tussle with the exclamation mark key here).

Okay, but this is all theoretical right?
Well maybe. It’s theoretical for humans, but there are objects out there which do not have empty space in them and which therefore are incredibly dense. They’re called neutron stars and to quote from Wikipedia:

this density is approximately equivalent to the mass of the entire human population compressed to the size of a sugar cube.

This gives them some other unusual properties. The radius of a typical neutron star is about 12 km, and just like a pirouetting ice-skater whose rate of rotation increases as they draw their arms in, so also does the rotation of a neutron star increase as its radius decreases. So how long does a full rotation last (remember that on Earth this is 24 hours)? On average there are somewhere between 100 and 1000 full rotations per second (see, if I can’t use exclamation marks at least I can use italics).

But back to the atom. To help students appreciate that it’s not just me who is bonkers, here I invoke the assistance of some more well-renowned experts.

Professon Brian Cox

Note that it took Rutherford two years to arrive at a correct interpretation of these results. It’s not like what you see in the textbooks – describe the experiment and then form an obvious conclusion.

It kinda freaked him out – Neil de Grass Tyson (audio link)

And for a bunch of other links see the related page on my website.

And finally, while you are expected to know that the atom is mostly empty (and be familiar with the experiment that ‘proved’ it), there is no sense in either the syllabus or any textbook I have come across of the wonder associated with this crazy idea. In fact it’s normally presented as just another piece of information to be learned off my heart. And I never hear anybody giving out about this, so I save my rants for unfortunate students and the odd blog post like this.

Now that’s mad!!

How to get 100% in your Leaving Cert Physics exam. Part 2: Answering Graph Questions

The following can be downloaded as a word document here

Drawing the graph

  • You must use graph paper and fill at least THREE QUARTERS OF THE PAGE.
  • Use a scale which is easy to work with i.e. the major grid lines should correspond to natural divisions of the overall range.
  • LABEL THE AXES with the quantity being plotted, including their units.
  • Use a sharp pencil and mark each point with a dot, surrounded by a small circle (to indicate that the point is a data point as opposed to a smudge on the page.
  • Generally all the points will not be in perfect line – this is okay and does not mean that you should cheat by putting them all on the line. Examiners will be looking to see if you can draw a best-fit line – you can usually make life easier for yourself by putting one end at the origin. The idea of the best-fit line is to imagine that there is a perfect relationship between the variables which should theoretically give a perfect straight line. Your job is to guess where this line would be based on the available points you have plotted.
  • Buy a TRANSPARENT RULER to enable you to see the points underneath the ruler when drawing the best-fit line.
  • DO NOT JOIN THE DOTS if a straight line graph is what is expected. Make sure that you know in advance which graphs will be curves.
  • BE VERY CAREFUL drawing a line if your ruler is too short to allow it all to be drawn at once. Nothing shouts INCOMPETENCE more than two lines which don’t quite match.
  • Note that examiners are obliged to check that each pint is correctly plotted, and you will lose marks if more than or two points are even slightly off.
  • When calculating the slope choose two points that are far apart; usually the origin is a handy point to pick (but only if the line goes through it).
  • When calculating the slope DO NOT TAKE DATA POINTS FROM THE TABLE of data supplied (no matter how tempting!) UNLESS the point also happens to be on the line. If you do this you will lose beaucoup de marks and can kiss goodbye any chance of an A grade.



What goes on what axis?

Option one

To show one variable is proportional to another, the convention is to put the independent variable on the x–axis, and the dependant variable on the y-axis, (from y = fn (x), meaning y is a function of x). The independent variable is the one which you control.


Option two

If the slope of the graph needs to be calculated then we use a difference approach, one which often contradicts option one, but which nevertheless must take precedence. In this case we compare a formula (the one which connects the two variables in question) to the basic equation for a line: y = mx.

See if you can work out what goes on what axis for each of the following examples (they get progressively trickier):

  1. To Show Force is proportional to Acceleration
  2. Ohm’s Law
  3. Snell’s Law
  4. Acceleration due to gravity by the method of free-fall
  5. Acceleration due to gravity using a Pendulum


There is usually a follow-up question like the following;

“Draw a suitable graph on graph paper and explain how this verifies Snell’s Law”.

There is a standard response to this;

“The graph of Sin i against Sin r resulted in a straight line through the origin (allowing for experimental error), showing Sin i is directly proportional to Sin r, and therefore verifying Snell’s Law”.


If you are asked any questions to do with the information in the table, you are probably being asked to first find the slope of the graph, and use this to find the relevant information.



Imagine if the key-word in the Leaving Cert Physics syllabus was ‘wonder’

This is an image, courtesy of Wordle.net, of the current Leaving Certificate Physics syllabus. Wordle is a program that gives the most common words the largest font size:

This is a similar image of the proposed new syllabus.

Notice the new focus on the words ‘learners’ and ‘learning’.

Imagine if a syllabus had as its most common words the following:











If any of this was a priority then chances are that Particle Physics wouldn’t have been removed (and with it Pair Annihilation, Anti-matter, Neutrinos, Fundamental Forces, etc.).

Chances are that Cosmology would also feature strongly in the new syllabus (Black Holes, Quasars, Pulsars, Big Bang, Neutrinos (again), Dark Matter, Alien Life, etc. etc.). It doesn’t.

Maybe it’s just me.

Energy – the most poorly-taught concept in all of science?

It is important to realize that in physics today, we have no knowledge what energy is.

Richard Feynman

Sometimes when I’m teaching I have been known to go off on a tangent which may be only marginally related to what we’re doing in class; other times I manage to restrain myself and may just allude to the tangential concept in passing.

But then there are times when these tangents are actually necessary, and by leaving them out I am doing the students an injustice. Ususally I try to include this information in the students’ notes also but sometimes I just don’t get around to it. Energy is just such an example.

Georgina (this is where I’m supposed to say that Georgina is not her real name, but as far as I’m aware it actually is) is one of the top students in my fifth year class this year and I used to think that she was a bit of a ‘slogger’ – liked to work hard and liked to know that there was always a ‘correct’ answer. In fact I wasn’t even sure if she would adapt to my style of teaching, where I tend to ask questions and not always provide the answers. But Georgina showed what she was really make of when we were revising the chapter on Energy.

“I know this sounds like a silly question”, she said, “and I know we’re finished the topic now, and it’s not that I can’t do the problems and exam questions because I can, it’s just that I don’t seem to get what energy actually is

I realised that there was more to this student than I first thought.

You see nobody gets energy; in fact by pretending that it’s all straightforward we actually do the students a disservice. Not only are we ignoring the wonder associatied with the idea, we are also denying them the opportunity to engage with the concept at any level beyond the superficial.

Bottom line – nobody gets energy because there’s nothing ‘to get’. Energy is not tangible (alghough it is ‘an indirectly observed quantity’ if you want to sound clever), you can’t hold it in your hand, you can’t weigh it on an electronic balance, you can’t see it, touch it, smell it etc. Yet when the universe was first created there was a certain amount of this put in to the mix (actually now that I think about it the mix itself was energy (with perhaps just a little dash of time)), and it’s all still there today. Its form can change, but the energy itself can’t ever disappear – no sirree bob.

It could be argued that it is in fact merely an accountant’s trick which enables him to ensure that all actions balance.

Consider the following analogy which I like to use.

If a child asks you ‘what is money?’ you could take a few coins out of your pocket and show them to the child and say ‘this in money’. Now fast forward a couple of decades; all transactions are now done electronically/online and all coins and paper money are no longer legal tender. Now how do you explain what money is?

Well it’s a means of payment for goods and services, right? Somebody sells you an orange and you agree to transfer into their account a set amount of this ‘money’. And now that the shopkeeper has this money in his account he can use it to buy something else. So it’s a bit like a transferrable IOU.

Now energy is a bit like this, but there is only a certain amount of IOU’s in the universe and this was set when the universe first came into being (I’m not sure if we know how much energy is in the universe – presumably we do?), although to complicate matters since the early part of the last century (thank you Albert) we now know that all matter (‘stuff’) is basically energy in another form.

Anyone still with me?

Now the point of all of this  is to highlight once again that there is no reference to this concept on either the Junior Cert or Leaving Cert syllabus. It could be argued that this is because it would be too difficult, but the obvious response to this is that nobody understands Energy as it is currently presented anyway. Students merely learn off the definitions and formulae and if they think about it at all will probably just assume that it’s just another example of Physics being ‘too hard’ to understand.

So what’s the actual point in asking students to learn the definition in the first place?

In fact I’d imagine many students who can give the appropriate definition for Energy (“Energy is a form of Work”) couldn’t follow up by explaining what Work is (it’s a mathematical product of force and displacement to give the simplified version).

At Junior Cert level students are expected to be able to show how light and sound are forms of energy – again most students should be able to give the correct demonstration but if you ask them how this demonstration verifies that it is is form of energy few will be able to give a convincing answer. In fact while Energy is the single greatest unifying concept in all of science, even that idea alone is not worthy of mention in the sylllabus; as a consequence Energy is seen as just another chapter to be learned off, with (once again) no emphasis on how it ties together everything else.

Now if teaching Junior Cert Science and coming from a Biology or Chemistry background what are the chances of your students developing an appreciation of this all-encompassing concept? – were you ever told about it?

I thought I’d find something on YouTube to illustrate this, but I couldn’t find anything.

What is it about this idea that we want to avoid?

Below are some quotes from other more prominent commentators on this elusive concept:

When Feynman wrote,

“It is important to realize that in physics today, we have no knowledge of what energy is,” he was recognizing that although we

have expressions for various forms of energy from (kinetic, heat, electrical, light, sound etc) we seem to have no idea of what the all-encompassing notion

of “energy” is.

The various forms of energy (½mv2, mgh, ½kx2, qV,mcT, ½I2, ½CV2, etc.) are abstractions not directly observable.

2007 American Association of Physics Teachers

Feynman’s quote in context:

 There is a fact, or if you wish, a law governing all natural phenomena that are known to date. There is no known exception to this law – it is exact so far as we know. The law is called the conservation of energy. It states that there is a certain quantity, which we call “energy,” that does not change in the manifold changes that nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says there is a numerical quantity which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is a strange fact that when we calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same. It is important to realize that

in physics today, we have no knowledge of what energy “is.” We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. It is an abstract thing in that it does not tell us the mechanism or the reason for the various formulas.

The Feynman Lectures on Physics Vol I, p 4-1

The origin of the decibel scale

A standard leaving cert physics exam question is “why do we have the decibel scale”?
The standard answer is that the range of sound intensities is so large that a second, much more compact scale is required to make the numbers more manageable, and for sound this scale is based on multiples of ten and is called the decibel scale (and what it measures is called sound intensity levels).

The old syllabus included a detailed analysis of this scale so that the numbers actually meant something. For the new syllabus (2002 onwards) it must have been decided that the maths was too difficult so this part was scrapped, except for one very odd ‘fact’; the student must know that a doubling of the sound intensity results in an increase of sound intensity level of  3 dB. Now needing to know that piece of useless trivia is ridiculous and is probably only there as a sop to some university professor who was horrified that the detailed analysis was dropped:

At least that’s my best guess, which doesn’t seem too dissimilar to what the author of a recent book on Physics and Music  entitled How Music Works thinks about the decibel in general.

I think the decibel was invented in a bar, late one night, by a committee of drunken electrical engineers who wanted to take revenge on the world for their total lack of dancing partners.


How Music Works: The Science and Psychology of Beautiful Sounds, from Beethoven to the Beatles and Beyond
by John Powell

Now what’s the betting that students will remember this explanation and forget all about the technical one?