Physics

Delphi91

Originally posted by Sev
...Also, whats the deal with the experiment to measure the specific heat capacity of a metal? It's not mentioned in the physics book.

Is this a question in sample papers???

According to the syllabus, the experiment on SHC is listed as:

Measurement of Specific Heat Capacity, e.g. of water or a metal by a mechanical or electrical method.

If you go to the following site, you can download the methods for ALL the experiments on the LC course:

http://www.psi-net.org/physics/s3/s3index.htm

Hope this helps.

Mike

ella minnow pea

Originally posted by PHB
If I ran a college I'd want the average person who works hard instead of the super smart lazy person.

well if im in a car crash tomorrow I wanna be sure my surgeon knows everything, and wont just "work hard" to save my life....
"opps yeah, i probably could have saved her but i didnt really understand the procedure, i just learned it off a few years ago...!"

pseudonym

I think regarding the specific heat capacity of a metal, it would probably come up as :
"Describe an experiment to determine the specific heat capacity of a liquid OR a metal"

But then again the exam setters dont confer with me very often so you never know. I think as long as you know the mcÄè and ml formulae, the basic definitions (specific heat cap, latent heat), the basic principles behind the experiments (whats a calorimeter, where to put the insulation, not to stick body parts into the steam trap etc.) you should be fine for almost any question regarding heat they can chuck at you. As regards U-value, i can't see them asking anything but a definition.

But again, they dont talk to me very much about this.

pseudonym

ehm, dat should be mc(delta)(teta) and not whatever turned up, obviously this place doesn't support the greek alphabeth. philistines!

PHB

well if im in a car crash tomorrow I wanna be sure my surgeon knows everything, and wont just "work hard" to save my life....
"opps yeah, i probably could have saved her but i didnt really understand the procedure, i just learned it off a few years ago...!"

Who would you prefer?

The doctor who has worked his ass off and knows all the procedures.

Or

The guy who got into medicine because of an aptitude test and breezed his way through it, just passing each time cause hes lazy?

PHB

As for the specific heat capacity of a metal, it was in the educational company exam paper.

In an experiment to determine the specific heat capacity of a metal, a heating coil was used to impart heat to a well insulated block of metal. The coil is placed in a hole drilled in the metal and a thermoeter is place in another smaller hole. A little light oil is placed in each hole.
The following readings for recorder.

mass of metal block - 495g
Initial temp of metal block - 11 degrees Celsius
Current - 1.4A
Potential - 6V
Time - 8mins
Final Temperature - 20 degress Celsius

Determine the specific heat capacity of the metal [22]
Why was the oil used[6]
Why wasn't the thermometer placed in the same hole as the heating currect[6]
Sugget a suitable oil to use [6]

1. The first part is fairly simple, just work out the energy supplied and the change in temp, and then lash it into the equation

2. Oil was used to increase the amount of heat transferred into the metal or thermometer, so there is a more heat transfered or accurate reading respectivly.

3. The thermometer wasnt placed in the same hole as then you'd get a distorted reading from the heating coil, if you place it in a seperate hole it can only be from the metal.

4. Parafin Oil I'd guess[but thatsa stupid question that wouldnt come up]

Sev

Ok, well thanks Delphi. It's just the only exception I have to that glycerol/paraffin explanation, is that if it was just a matter of getting the wire to respond quickly to temperature changes, then 'because paraffin is a good conductor of heat' just doesnt cut it as an explanation. It's adding an extra layer of complication in the sense that the bunsen heats the water, which in turn heats the paraffin.

And since its probably not a good idea to have bare wires which are conducting a current sitting in a testube of water, the glycerol is used instead!

Thats what I thought could be the only thing that makes sense. But, em sure why not have bare conducting wires in water? Perhaps glycerol doesnt conduct electricity?

Sangre

But in the experiment for Joule's law, the heating coil is directly in the water?

Sev

Originally posted by Delphi91
Water is a poor conductor of heat but a good conductor of electrical whereas glycerol is the other way around.

Nevermind, I overlooked you saying that. Well theres my explanation then, glycerol is a bad electricty conductor. Being a good heat conductor then I guess is an added benefit.

Cheers for the help anyway, I might have some more questions for you over the weekend I hope your class does well.

Sev

Also, another point that always bugs me is graphs. If for example, in a question on boyle's law, you're asked to draw a suitable graph for the results of an experiment, does it matter what axis you put 1/v and P on? And if not, for which experiments does it matter?

pseudonym

what ive been told and observed is that its the quantity you are varying that goes on the x-axis, and the one that changes because of that on the y-axis. Generally if you are asked to graph something, the first thing mentioned goes on the y-axis, the 2nd on the x, ie y quantity vs. x quantity.

Although ive spotted one or 2 graphs in REal world physics that dont follow this.

PHB

Normally, I just graph whichever is in regular intervals on the x axis

Sev

Yes, thats what I've been told, that the quantity youre measuring is y, in respect to a value you can change, x. Although, in the physics book, that is not always the case. For example certain times the rules is reversed and the graphs are drawn in such a way so that the slope reveals a specific value, and I can understand the reasons behind that too.

What annoys me is when they say, graph A 'against' B. There appears to be no system in that phrase "graph.. against...", most of the time in my physics book, that means to have A on the y axis, and B on the x axis. But in one or two experiments it says "graph A against B", but with A being on the x axis. It's all a bit stupid and confused. I guess thats the leaving cert for you.

ella minnow pea

Originally posted by PHB
Who would you prefer?

The doctor who has worked his ass off and knows all the procedures.

Or

The guy who got into medicine because of an aptitude test and breezed his way through it, just passing each time cause hes lazy?

id prefer the person who's intelligent enough to know what to do if my spleen is puntured in three places, instead of the doctor who goes "opps, only learned off what to do if there's two holes in your spleen, that was all that was on our course. ho hum."

PHB

Do you really think its possible for a person not to be able to adapt, after working your ass off and getting 550 points, then work your ass off through college?

You think its just directly proportional to how much work you do.

Anyone who gets 550 to get medicine is fairly bloody smart, no matter how much people claim its just cause he worked hard.

ella minnow pea

jebus im just saying that the leavin aint a true test of intelligence
for the majority of subjects except maybe, i dunno, accounting you can just learn em off to get your A, or go to a grinds school, or whatever
just pisses me off

and your giving out about aptitude tests, well i can guarentee that most Medicine students either got grinds or went to a grinds school in order to get their points
i know a girl who hasn't had any of that, because her parents cant afford it, she *omigawd* she has to make do with public schools, and therefore she wont get 550 or whatever, even though i know she'd be a great doctor

what the hell kinda sense does that make?

PHB

Unfortunatly its not the best system, but I know a lot of people who havnt gone to grinds and will get 550 points so

Anyway, off topic

Delphi91

Originally posted by pseudonym
what ive been told and observed is that its the quantity you are varying that goes on the x-axis, and the one that changes because of that on the y-axis. Generally if you are asked to graph something, the first thing mentioned goes on the y-axis, the 2nd on the x, ie y quantity vs. x quantity.

Although ive spotted one or 2 graphs in REal world physics that dont follow this.

Yes, that is correct. The "technical" explanation is that you put the independent variable on the x-axis and the dependent variable on the y axis. Think of it in maths - when you graph a quadratic equation like y = x^2 + 3x + 4, you put the y values on the vertical axis because they are "dependent" on the x values (the independent variable). Another way of phrasing it is that you put the value that you vary on the x-axis and the value that you subsequently measure on the y-axis.

It's always a good idea to then re-arrange the equation so that the independent variable is on the left. If you do it properly it will then be in the form "y = mx + c", from which you can read off the slope. N.B. If the graph goes through the origin, it will be in the form of "y = mx"

One other tip: when it comes to getting the slope of the graph, NEVER use two points that you have actually plotted (i.e. from the table given to you). Use two points from the "average" line going between all the points. This is because, due to experimental error, you cannot guarantee that these points are accurate.

One final tip: when it says "draw a suitable graph", 9 times out of 10, you have to do something to the figures first e.g. in Snells Law/Refractive Index, they usually give the values for i and r, not Sin i and Sin r. If you don't do the sin thingy first, you get a curve, and they don't usually give curved graphs, unless its radioactive decay or diode characteristic.

Mike

Bottle_of_Smoke

I'm gonna put the variable on the x-axis to keep them happy although it shouldn't matter which axis, all your showing is that they're proportional or whatever, it's the relationship that matters. I cant see how they could take marks off, just make sure ya know the formulae they're what really matter

eirebhoy

I know its a bit late to be asking a question like this, when your drawing a graph, for example "f" against "root t". If f starts at 256hz do I have to leave a big gap on the y-axis or do I just start straight away with 256 just above the origin?

PHB

Big gap!

Sangre

I'd start the graph at the 1st two points...is that bad? I still get the straight line.

Sev

As Dave said.. big gap!

Start the graph at zero, always. They demand it, well at least my physics teacher does anyway. Otherwise you're potentially screwing about with the slope or whatever values the graph might reveal. For example, when graphing 1/u and 1/v. You get the reciprocal of the focal length by taking the average of the points for which the line cuts the x and y axis. If your graph doesnt start from zero, all hell breaks loose.

Sangre

Why?
What difference will it make if my first point is 0,0 or 4,7?

If you have to start at zero thats fine, but with some things its very awkard like frequency.

PHB

Cause if you start at zero for one value and dont for the other, you'll get a messed up curve and it wont be through the origin, unelss you do some creative enginering.

Always start at 0!

Sev

Em, when I mean start the graph at zero, I mean have the axis originate from zero. Like, have the value zero well and truly on both axes of the graph.

Best to illustrate my ideas. Here goes.

Sev

Also, make sure to put in a proper title for the graph, and write explicity what each axis represents and what units it's measured in, dont just give a simple 't', write 'Time'.

There's a whole lot of marks going for something so retardedly simple as drawing a graph, so best do it good, and be very explicit and accurate about everything you put on it.

SOL

ROFL

phaxx

From those physics experiments Delphi91 linked,

"There is very good thermal contact between the glycerol and the thermistor since the glycerol does not contain dissolved gases."

and

"The boiling tube of glycerol is placed in a water bath to limit the maximum temperature reached to 100 °C."

DSMe

holy sweet mother of christ i hate this subject with such a passion .... its the only subject i have left to study and i have to get an a in it ... but i cant force myself to look at this ****e , especially electricity , what a pile of borin crap! jesus im ****ed