Practice make Perfect Pracs!

Experiments or Practical Activities (called pracs for short) are a critical part of school science, not (just) because they are fun, but because they form the basis of science in the world. Theories are one thing (and an important part of science) but they are dependent on experiments – without the proof that real world experiments provide, theories would be nothing more than opinions – and have no more validity for understanding the world than any number of other belief systems.

The scientific method developed in ancient Greece, but has continued through a great deal of  “western” science. The basic idea behind the scientific method is that nothing is ever proven conclusively and forever. This means something different than most people interpret it to mean – not that nothing can ever be known (after all ideas like evolution are almost universally accepted amongst the scientific community), but that any theory is always subject to challenge if new information or evidence is uncovered – there is no precedence of authority or ranking. Anyone can look into a theory, and if they find evidence that would seem to challenge the prediction of a theory, present that evidence to the world and the scientific community for evaluation – and if others confirm the evidence, the theory that it challenges may be reconsidered. If the required change is significant enough, it is called a paradigm shift. The most classic example of a paradigm shift is Einstein’s update of Newtonian dynamics to take speeds approaching light  into consideration.

Regardless of what the data is, the development of theory is dependent upon the accuracy and precision of these measurements. Accuracy is how closely the measurements reflect the actual object being measured, Precision is how closely the individual measurements are grouped. Imagine the situation that a group of six people are estimating the height of a tree (which is later measured to be 2.46 m tall) . They make the following estimates:

2.1m, 2.9m, 3.0m, 2.3m, 2.8m, 2.8m

These estimates are neither precise nor accurate. Another group guesses:

2.7m, 2.6m, 2.8m, 2.75m, 2.8m, 2.7m

These estimates are not accurate, but are precise. A final group guesses:

2.4m, 2.45m, 2.35m, 2.4m, 2.3m, 2.5m

These estimates are both accurate and precise. Can you imagine a set of measurements accurate, but not precise?

Measurements can be off for any number of reasons, but there are two main categories of error:

  1. Systematic Error: this is the error that results from poor experiment design or poor choice of measurement strategy.

Here is an example of systematic error – if you were measuring air temperature, you want to position your thermometer in a way such that it is not being affected by other heat sources (such as hot asphalt, aircon exhausts, or BBQs)  unlike the thermometer below:

Lots of other examples of poorly sited instrumentation at the surface stations website

2. Random Error: Errors due to inaccuracy in measurement – when repeated measurements of the same phenomenon show different results.

OK, on to the videos:

Another good video.

WordPress doesn’t support playlists, so you can view the rest of the videos here.

See you in class.

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27 Comments on “Practice make Perfect Pracs!”

  1. Loughlin Says:

    I take it this means we’ll be dealing with significant figures in these experiments? The idea that all of our accepted values are slightly wrong… It hurts me, it really does.

    • CyberChalky Says:

      Well, get ready for the real pain when you first understand Heisenberg’s Uncertainty Principle and discover that it is actually impossible to know the precise location of an object at the same time as you know how fast is is going…

  2. Patrick Says:

    reading this, does this mean, that we have to assume some margin of error? so for example if we were to record a reading of say 2.4mA in a prac that requires us to record the current,would we need to assume that the actual figure maybe either be lower or higher than the figure we recorded?? and if so, how much of a margin should we give?

  3. Patrick Says:

    with the video play list we only watch from video 3 through to 6 right?

  4. Patrick Says:

    umm sir..i watched the video playlist…and umm….the video, Units, Measurements and Theory of Errors – Problem Solving 2….yeah….it was a bit to hard to understand. can you maybe help me understand it a bit more?

  5. Patrick Says:

    btw sir, i was reading the book that you lent me, about Albert Einstein, and i was reading the page that explains the speed of light, and its relation the principal of relativity, and one question popped into my mind, if i, theoretically was traveling at 99.95% the speed of light, and i threw a rock, away from me, not in front but to the side, would that rock, still be traveling at 99.95% the speed of light? :S

    • CyberChalky Says:

      Actually a tricky question. The simple answer is no – but that’s not quite right.
      We will be doing vector addition in the next unit (motion in 2 dimensions, first part of unit 2). In short, given an absence of friction, newtonian mechanics would imply that it is going to be travelling faster by an amount dependent on it’s “sideways” velocity. Einteinian Relativity also requires you to take into consideration the fact that it is still speed limited to c, but also that it is far harder to accelerate objects travelling near the speed of light.

  6. Patrick Says:

    hi sir
    i was looking for a way to use breadboards and i found an app, but rather then use it and assume its good, i was hoping that you’d test it and see if its a valid application to use, and if its not do you know of any simulators that can be used to give accurate breadboard models?

    ps. today’s prac was brutal…to say the lease Lmao

  7. Patrick Says:

    oh the download link for the app is here:
    http://www.megaupload.com/es/?d=N93AZQDU

  8. Loughlin Says:

    Hey guys, I’m looking at doing Practical 4 with my group, is anyone interested in sharing information?

    And Sir, for the internal resistance of wires, we determine it by resistance = (resistivity ^ length)/ Cross Section Area (to a certain extent)

    as in, the resistivity of the wire to the power of the length divided by the Cross Section Area?

    And, when measuring Resistance with a multimeter, do you make it in series or parallel?

    • Patrick Says:

      measuring resistance of the resistor? or the entire circuit?

      • Loughlin Says:

        I was actually looking for a reminder on how to do both. I’ve researched a bit on how multimeters work, but I couldn’t find a definitive answer.

      • Patrick Says:

        id take a look at youtube.
        i found some of my examples through some video clips.

        but as for measuring the resistance of a resistor with the multimeter

        set the multimeter to Ohms
        connect both probes via crocodile clips to the resistor and that should give a close approximation of the resistance of the resistor

    • CyberChalky Says:

      No, it is much better if you use the ohm-meter function of the multimeter to determine resistance, and compare it to the resistance calculated by finding the voltage drop and current through the wire.

      Use this information to determine the resistivity of the wire; rather than using the resistivity to determine the resistance.

      • Patrick Says:

        so we use the multimeter to determine resistance of the entire circuit? then compare it to the resistance calculated once we find current and voltage drop?

        from that do we subtract the resistance we have calculated from the resistance we measured?

        and how do we calculate the overall resistance of a circuit with the multimeter?..

  9. Patrick Says:

    btw lock, heres a plan, you guys are gonna do prac 4 next right? our group will finish off doing prac one, as i think i have a general idea of how to go at it.

    we’ll swap data for both pracs

    prac 2 we’ll divide half the task between our groups and same for prac 3

    gives everyone a chance from both our groups to gather and record individual data

    sound good?

    • Loughlin Says:

      Yeah that does sound good, takes a lot of pressure and STRESS off!

      I figured out how to measure current in series again if you’re having troubles (or like I was, and having a major freak out, I guess!)And I think I’m taking off my SRC meeting, I’ll hand it to someone else to run so I can keep doing the SAC stuff. Anyone else is welcome to design some resistance paths and compare notes too. What was the other Prac on again?

      • Patrick Says:

        not 100% sure
        but Lari has pracs 2 and 3 written up
        i think if you want you take prac 3, and break down how to do that,
        while ill take prac 2 and break down how to do that

        how do you measure current agian with a series circuit? refresh my memory ><

        this time around i think a very systematic and methodical approach would serve us better then today where we rushed around like panicked chickens… ==

  10. Loughlin Says:

    Hey Sir, for practical four, where we are measuring the cross-sectional-area of the wire, is it safe to use a breadboard? Patrick and I hypothesized that it would not be safe, as the wire would be exposed and current would flow through somebody if they touched it. Or, would it be safe as long as we follow the procedure of having hands on the desk?

  11. Loughlin Says:

    Hey guys, I’ve submitted the experimental results for number four to Mr. Grichting, but I didn’t take down the current readings for the experiment. Could anybody who has them either post them in comment form or to Mr. Grichting? You kind of need Current in order to complete the experiment (unless you measure resistance with a multimeter tomorrow, doing the experiment again, yourself!)

  12. Ellen Says:

    hey how do i post up our results from experiment two on here so everyone can see them?

    • Loughlin Says:

      You can either type them by hand on these comments (annoying and might align wrong when posted) or put it into a word document and give it to Mr. Grichting to put up.


  13. […] Measurement: Practicals, Significant Figures, Using Technology, Extended Practical Investigations, Error & […]


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