Notes for Experiment #1 -- Introduction to Electric Circuits and Measurements
Notes & Hazards
Random helpful notes for your experiment:
- 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
- When connecting leads, please be sure to connect positive
--> positive and negative -->
- 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.
- 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
- 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):
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.).
- 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
materials have constant resistance (linear I-V graph) but
non-ohmic materials have non-constant resistance (screwy I-V
- 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.
- Laboratory Manual (SGM 407)
- Laboratory Answer Book
- Calculator with statistical functions
Some Helpful Links & Miscellaneous Notes
- This lab should be relatively quick; everyone should be done in
about 2 hours.
- 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.
- 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 <firstname.lastname@example.org>
Last modified: Wed Mar 8 00:42:51 2000