Physics

treefingers

In the experiment to measure the wavelength of monochromatic light:

Q) what would be the effect of replacing the diffraction grating with one of fewer lines per mm?

Experiment to find the resistivity of a nichrome wire:

Q) explain why the accuracy of the experiment improves as the lenght of wire is increased?

cheers!

eirebhoy

Delphi,
I wasn't quite following all your messages because there is so many, there so long and I have a lot of studying to do. Anyway, your saying start the graph with the first number being the first one in the data and theres no point onleaving a gap?


This graph is not like ones on the Leaving Cert, but it still should look like the one on the left?

eirebhoy

Originally posted by treefingers
In the experiment to measure the wavelength of monochromatic light:

Q) what would be the effect of replacing the diffraction grating with one of fewer lines per mm?

Fewer Lines=dots closer together=harder to see

PHB

Experiment to find the resistivity of a nichrome wire:

Q) explain why the accuracy of the experiment improves as the lenght of wire is increased?

Well, simply, I think theres another reason but,

Longer length of wire
-> Less percentage error when measuring length
-> less experimental error

Sev

Originally posted by Boston
Sev, i can start my scale from any point and the x and y axis with still intersect at zero. Juast say your dong the veriation of resistance with temperature for a metallic conductor experiment and you us cuso4 sultion instead of water, then you can start from a minus temperature, and work your way up and your graph would still be right.

After re-reading ths post again. Im, I'm afraid you are completely confused.

And still intersect at zero? Eh, you mean still intersect at the points you deemed to be the beginning of your scales. If you try to tailor your graph to fit through the 'origin' (which you cant with that kind of graph) when doing this experiment, then you are COMPLETELY WRONG. This experiment is not even supposed to produce a line passing through the origin. Yes, you are able to produce a line that will pass through the intersection of your two truncated axes, thats all.

Originally posted by Boston
This is clearly an example os someone who ahs learnt off the book with out understanding the why.

Speak for yourself.

Sev

Originally posted by Delphi91
You've advocated this book a number of times. Don't refer to it like it's the bible. It isn't, and it has mistakes.

It wouldn't surprise me if this book is a major factor taken into consideration when the exam questions are written and when the marking scheme is decided. After all its the book most students are using right?

pseudonym

i'm only really adding to whats been said already, but ive always been told to start the graph at (0,0), especially if it's a proportionality question and to extrapolate back to the origin from the closest point to it (unless its obvious ur not to) and to scale the axes so that they take up most of the graph page. Also been told that a particular scale SHOULD be obvious, as the examiners wud have worked this out while setting the figures.

Boston

Originally posted by Sev
After re-reading ths post again. Im, I'm afraid you are completely confused.

And still intersect at zero? Eh, you mean still intersect at the points you deemed to be the beginning of your scales. If you try to tailor your graph to fit through the 'origin' (which you cant with that kind of graph) when doing this experiment, then you are COMPLETELY WRONG. This experiment is not even supposed to produce a line passing through the origin. Yes, you are able to produce a line that will pass through the intersection of your two truncated axes, thats all.



Speak for yourself.

you replied three times to my post, and i'm confused, never said that the graph went next nor near the orgin? just stating that the scale can start from a minus temperature so, no not all graphs start at the origin.

eirebhoy

I've been looking through the folens sample papers and every on every Q7 was sound/heat/light, except for the The Revised Sample Paper (February 2002). I am relying a question to come on sound/heat/light in Section B (excluding Q12), so, what are the odds on one of these not coming up?

Sev

Originally posted by Boston
Sev, i can start my scale from any point and the x and y axis with still intersect at zero.

What exactly was that supposed to mean then?

Delphi91

Originally posted by Sev
Ok Delphi, im gonna drop it now. But I'll be making very sure I have zero marked clearly on my scales tomorrow. You might want to ask around and get opinions from other physics teachers, but I know for certain that the two teachers in our school would have a serious exception to your methods (for experiments like boyle's law, focal length, fundamental frequency of stretched string, especially)

I'm bowing out too at this stage. But I leave you with two final points:

1.

A suitable scale should be chosen...such that the graph covers most of the page rather than being confined to a small corner of the page.

"Senior Physics" - George Porter, Folens (who now works for the Dept of Education)

2.

Chose the range of the scales on the axes so that the points are suitably spread out on the graph paper and not all cramped into one corner. In some cases, this may mean excluding zero from the axis: for example if lengths measured in an experiment varied between 5.2m and 7.7m it would be better to allow the scale to run from 5m to 8m rather than from zero to 8m

"Handling Experimental data" - Mike Pentz & Milo Shott Open University Press

Mike

Raptor

Ok, jumping in at the middle of the conversation here, but quick question bout the graph for 1/u+1/v=1/f

The only question i could find in my papers is one sample paper. In this they ask to draw a suitable graph, for the values of u and v given, and write (1/u against 1/v). Then they ask to find the focal length from your graph.

Firstly a general question. If they suggest a suitable graph, do you HAVE to use it?

What i mean is, you COULD go and plot these against each other and get on funky graph, and then get the slope from the intersections with the 2 axis as stated above.

Wouldn't it be much easier to draw a graph so that the slope is f, and just calculate it?

Ie 1/u + 1/v = 1/f
can be rewritten as
f=(u+v)/uv

Then you could graph u+v against uv and the slope is f.

So to summarize,
1. Do you have to do the graph they suggest (which they rarely do)?
2. If they had suggested nothing in this question could you graph u+v against uv?

Sev

I guess at the end of the day, the appearance of my graph will be dependent on which experiment it is, what I'm trying to show, and if I feel I'm showing it best in light of the question. But I'll take that what you said into consideration.. and we'll see I Want to apologise if I seem overly stubborn, but I'm just trying to hold firm to all the values that my hard working physics teacher has tried to instill in us over the past 2 years.

And to raptor, personally I think doing that would be very dangerous. Best stick to the the general guidelines of the curriculum and give them what they want to see, as I've been taught.

PHB

You're just making extra complications for yourself Delphi.

Take for example, a calibration curve for a termometer, if you didnt start at 0, but started at 40 as those were the lengths found, if they asked you to find the temperature at 0 degrees, you'd have to go into negative terrority, which is just adding extra complications.

People need a method when making graphs, and its just good sense to start at the same point each time, since the majority of graphs you will start at zero from.

PHB

Also, in the 1/u + 1/v graph the slope gives magnification, which I think its meant do give

Dont be at yourself

Originally posted by PHB

Take for example, a calibration curve for a termometer, if you didnt start at 0, but started at 40 as those were the lengths found, if they asked you to find the temperature at 0 degrees, you'd have to go into negative terrority, which is just adding extra complications.

But equally, if you did start at 0, even though the lengths were given from 40cm to 100cm, and were asked to find the temperate at (for example) 57cm, your graph would be less accurate than had you started at 40cm, as the scale you'd be using would have to be smaller. Common sense should obviously prevail and people should read the whole questions before they answer any part of it, allowing one to draw their graph from the origin, or not, as the case may be.

Also, "if they asked you to find the temperature at 0 degrees" - that'd be a handy one tomorrow, eh?

Sev

Originally posted by PHB
Take for example, a calibration curve for a termometer, if you didnt start at 0, but started at 40 as those were the lengths found, if they asked you to find the temperature at 0 degrees, you'd have to go into negative terrority, which is just adding extra complications.

Well, I can understand there not being a need to start a calibration curve graph at zero. The graph is not intented to go through the origin and if it were to, that origin would be -273 degrees celcius (which is just silly), and it certainly would not be a straight line. Either way, they would never do that, ask you to get a reading for a range outside the given range of the results, that is just cruel. If they did, you could always read over the question before drawing up your graph and take that into consideration.

Delphi91

Originally posted by Raptor
...The only question i could find in my papers is one sample paper. In this they ask to draw a suitable graph, for the values of u and v given, and write (1/u against 1/v). Then they ask to find the focal length from your graph....

If they ask you for 1/u against 1/v, then draw it that way!! Another other way will gain you no marks.

When they ask you to use your graph to find a value for f, they will be looking to see that you have made some sort of mark on the graph where you picked values of u and v.

subway_ie

Just resurecting the thread because it contains some interesting pointers from last year.