# How to get 100% in your Leaving Cert Physics exam. Part 1: Section A

Section A counts for 30% of your overall mark and is the easiest section to pick up full marks. There are about 24 experiments but many of them are minor variations on each other. Stop wasting time trying to predict which ones will come up and just learn them all. Take one or two per night and make sure you can answer every question on each experiment from past papers. In particular you need to use the following as a checklist for each experiment.

(i)     Draw a fully labelled diagram which includes all essential apparatus (have you included the apparatus necessary to obtain values for both variables?).

(ii)   Be able to state how the two sets of values were obtained (this is a very common question).

(iii) Describe what needs to be adjusted to give a new set of data

(iv) Write down the relevant equation if there is one associated with the experiment.

(v)   Be able to state how the data in the table will need to be adjusted.

(vi) Know what goes on each axis.

(vii)           Know how to use the slope of the graph to obtain the desired answer.

(viii)         Be able to list three sources of error/precautions.

Misc Points

• The graph question is usually well worth doing.
• Learn the following line off by heart as the most common source of error: “parallax error associated with using a metre stick to measure length / using a voltmeter to measure volts etc”.
• Make sure you understand the concept of percentage error; it’s the reason we try to ensure that what we’re measuring is as large as possible.
• There is a subtle difference between a precaution and a source of error – know the distinction.
• When asked for a precaution do not suggest something which would result in giving no result, e.g. “Make sure the power-supply is turned on” (a precaution is something which could throw out the results rather than something which negates the whole experiment).
• To verify Joule’s Law does not involve a Joulemeter
• To verify the Conservation of Momentum – the second trolley must be at rest.
• To verify the laws of equilibrium – the phrase ‘spring balance’ is not acceptable for ‘newton-metre’.
• To measure the Focal length of a Concave Mirror or a Convex Lens.
Note that when given the data for various values of u and v, you must calculate a value for f in each case, and only then find an average. (As opposed to averaging the u’s and the v’s and then just using the formula once to calculate f). Apparently the relevant phrase is “an average of an average is not an average”.

I have a document here which containts exam questions on every experiment which has ever appeared on a past paper from 2002 to 2010 (Higher Level and Ordinary Level) – this should be your bible for Section A over the coming weeks. Solutions are also included.

Now get back to work.

More to come.

# Good luck to all exam students

Something to take with you into your Science exam tomorrow – the quotes are from Einstein:

Not everything that can be counted counts, and not everything that counts  can be counted.

Imagination is more important than knowledge.

Do not worry about your difficulties in Mathematics. I can assure you mine are still greater.

These exams are all about how good you are at learning stuff ‘off by heart’. Now no society in this day and age needs students who are good at learning stuff by heart so why do we do it – why don’t we spend time teaching students how to think for themselves?
Because that’s a lot harder to assess. So let’s teach something that we can assess and not worry too much about how useful that actually is. ‘That seem a little silly? Consider the following story:

A cop walking his beat one night finds a drunk on his knees, searching for something on the street.
The cop asks the drunk, “What are you doing?”
“Looking for my car keys,” says the drunk.
The cop asks, “Where did you lose your keys?”
“I don’t know,” the man answers.
The cop, a bit perplexed, asks, “Then, why are you looking here if you don’t know where you lost your keys?”
The drunk replies “Because the light is better here, under the streetlight.”

The following is a question on a physics exam at the University of Copenhagen:

“Describe how to determine the height of a skyscraper with a barometer.”

One student replied: “You tie a long piece of string to the neck of the barometer, then lower the barometer from the roof of the skyscraper to the ground. The length of the string plus the length of the barometer will equal the height of the building.”

This highly original answer so incensed the examiner that he failed the student who immediately appealed on the grounds that his answer was indisputably correct.
The university appointed an independent arbiter to decide the case.

The arbiter ruled that the answer was indeed correct, but did not display any noticeable knowledge of physics. It was decided to call the student in and allow him six minutes in which to provide a verbal answer which showed at least a minimal familiarity with the basic principles of physics.

For five minutes the student sat in silence, forehead creased in thought. The arbiter reminded him that time was running out, to which the student replied that he had several extremely relevant answers, but couldn’t make up his mind which to
use.

On being advised to hurry up the student replied:

“First, you could take the barometer up to the roof of the skyscraper, drop it over the edge, and measure the time it takes to reach the ground. The height of the building can then be worked out from this formula I have worked out for you on my text paper here.”
Then the student added, “But, Sir, I wouldn’t recommend it. Bad luck on the barometer.”

“Another alternative”, offered the student, “is this: If the sun is shining you could measure the height of the barometer,then set it on end and measure the length of its shadow. Then you measure the length of the skyscraper’s shadow, and thereafter it is a simple matter of proportional geometry to work out the height of the skyscraper. On the paper is the formula for that as well.”

“But, Sir, if you wanted to be highly scientific about it, you could tie a short piece of string to the barometer and swing it like a pendulum, first at ground level and then on the roof of the skyscraper. The height is worked out by the difference in a gravitational formula, which I have determined here this time on a long sheet of paper with a very long and complicated calculation.”

“Or, Sir, here’s another way, and not a bad one at all. If the skyscraper has an outside emergency staircase, it would be easier to walk up it and mark off the height of the skyscraper in barometer lengths, then add them up.”

“But if you merely wanted to be very boring and very orthodox about the answer you seem to seek, of course, you could use the barometer to measure the air pressure on the roof, and on the ground, and then convert the difference in millibars into feet to give the height of the building.”

“But since we are constantly being exhorted to exercise independence of mind and apply scientific methods, undoubtedly the best way would be to knock on the janitor’s door and say to him ‘If you would like a nice new barometer, I will give you
this one if you tell me the height of this skyscraper’.”

It’s a wonderful story, and when told is usually attributed to the Danish physicist Neils Bohr, but like all great stories is almost definitely apocryphal.

Good luck in the exam 🙂