Notes for Experiment #1 -- Introduction to Electric Circuits and Measurements


Notes & Hazards

Random helpful notes for your experiment:

  1. Big bars and extra stuff on ends implies the Positive terminal; e.g., in a battery's representation, the longer bar is the positive terminal. In a cell, the end with a knob sticking out is the positive terminal.
    Positive terminals are traditionally red and negative (or ground or common) terminals are labelled black
  2. When connecting leads, please be sure to connect positive --> positive and negative --> negative
  3. The fluid analogy they give in the book is pretty good but here's another perspective on it. This image will be a nice one to keep in mind to develop your intuition. And this is something you definitely want to do as early as possible. Luckily, simple circuits lend themselves readily to such mechanical analogies (which more complex circuits, with more complex circuit elements, don't).

    The above image shows a rectangular cylinder that's filled completely with water. Since water is essentially incompressible, pushing the water on one side automatically moves the water everywhere in our mechanical circuit. This mechanical circuit has a pump, which simply pushes around the water that already fills the cylinder. Our simple mechanical circuit also has a wire mesh; when water flows through this wire mesh, friction comes into play and some of the energy of the flowing water is lost to the mesh and is released as heat, etc.

    Now, this mechanical circuit has a equally simple electrical analogue. The pump in our water circuit is equivalent to a battery in an electrical circuit. Just as the water pump pushes around the "sea of water" (that was already there), the battery pushes around a "sea of electrons" (that's also already there). This flow of electrons is what we call a current. The wire mesh also provides a resistance to the flow of water; similarly, a resistor in an electrical circuit provides a resistance to the flow of electrons. This flow of electrons is what's called electricity (please note that electricity is just the flow of electrons (the current) and is measured in Amperes, whereas elecrical energy is measured in Joules). Electric current is a very slow local flow of charges but electric energy is a very rapid movement of electromagnetic fields.
  4. When using the Ammeter, be sure to connect it in series. This just means that both leads from the ammeter are placed after the circuit element you want to measure. When using the Voltmeter, however, you have to connect it in parallel. This means that the positive lead from the voltmeter is placed near the positive end of the circuit element and the negative lead is placed near the negative end of the circuit element (see Figs. 6 and 7 in the manual). Just remember that voltmeters are connected across the circuit element whose voltage you want to measure and ammeters are connected after the circuit element you want to see the current flowing out of (you might want to refer to the waterfall analogy in Lab 3 to get a better grip on this).
    Finally, it might be helpful for you to know that an ideal ammeter has zero resistance while an ideal voltmeter has infinite resistance.
  5. Some other quick, random, stream-of-consciousness notes (these will probably make more sense when you actually start the experiment so be sure to jot them down and refer to them when you get stuck in lab):
    1. Make sure you connect the negative terminal to the common or ground input on the multimeters. Also, remember to use appropriate settings on the multimeters; this means that you have to make sure that the dial is set to DC or AC, to Voltage or Amperes or Ohms, and to the correct range (i.e., if the current is 180mA don't expect to see it at the 2mA maximum setting, etc.).
    2. The battery is the only motive source for the electrons; it might help to see the positive or negative ends by imagining the battery as pushing from both ends: i.e., in Figure 6, the positive ends pushes to the left while the negative end pushes to the right so that at the bulb, the left side is still positive and the right side is still negative. The picture you should try to keep in your mind is that of the battery pushing out simultaneously from both sides.
    3. Ohmic materials have constant resistance (linear I-V graph) but non-ohmic materials have non-constant resistance (screwy I-V graph).


Corrections

  1. Page 8, Figure 8: the positive and negative signs are transposed for the Goldstar Ammeter; please be sure to reverse the leads when you do the experiment itself.

Required Materials:

  1. Laboratory Manual (SGM 407)
  2. Laboratory Answer Book
  3. Calculator with statistical functions


Some Helpful Links & Miscellaneous Notes

  1. This lab should be relatively quick; everyone should be done in about 2 hours.
  2. Be sure to keep track of units; physics is one of those disciplines where the more precise and accurate your language, the easier it is to get through calculations and problems. It might seem to be nit-picking, but you'd be amazed how many times a careful accounting of units, etc. allows you to catch mistakes and see paths to solution that you might never have noticed if you'd hurried through some calculation.
  3. Some nice links for you to explore (especially for the more masochistic among you): lots of really cool (and really long) articles on electricity, an excursion into an encyclopaedia's understanding of electricity, and finally, an exploration into electromagnetism.


Ricky J. Sethi <rickys@sethi.org>
Last modified: Wed Mar 8 00:42:51 2000