)and
are marked in 3 color bands to denote their resistance. The color code follows:Introduction: In Physics 102
you will be constructing circuits for many of the labs. You will also use an
oscilloscope for several of the later labs. This exercise will introduce to the
idea of circuits. It may seem remedial for those who have some electronics
experience; however, ALL students should know how to build a circuit in lab and
not rely on the one partner who has some prior knowledge. Included with this lab
is a short electronic symbol dictionary. Bookmark this! Also, there is a lot of
jargon in electronics. I will try to explain the terms as I go along, but if you
have any questions, please be sure to ask.
Theory: A circuit, in electronics, plumbing or rally motoring, is a
closed loop. The general theory is that an item (electron, drop of water, or
sports car) begins at some starting point and proceeds around the loop, perhaps
with some choice of paths but never doubling back, and ends at the beginning.
There are two primary kinds of loops, series and parallel. These two can be
mixed to produce many variations on the theme, but these complexities can
usually be reduced to series and parallel elements.
The series loop is a path with no choices. The electron flow starts at the
source (battery, generator) and proceeds through each circuit element. A circuit
element is a resistor, capacitor, inductor, or any other electronic part or
device.
A parallel loop consists of choices. Does the flow (of water or electrons) go
this way or that? Each intersection is called a junction, and each path is
called a branch.
Sometimes we will wire circuits with open wires since the components are so big that "breadboarding" is impractical. In these cases terminals are provided on each component. Use spade or alligator clips on these. Avoid twisting wires together.
We may use real resistors this semester. Resistors
are measured in ohms ()and
are marked in 3 color bands to denote their resistance. The color code follows:
|
Black = 0 |
Green = 5 |
|
Brown = 1 |
Blue = 6 |
|
Red = 2 |
Violet = 7 |
|
Orange = 3 |
Grey = 8 |
|
Yellow = 4 |
White = 9 |
The fourth band relates to uncertainty, called tolerance, in these components:
| None = 20% |
| Silver = 10% |
| Gold = 5% |
One reads a resistor by the colors. For instance, if the first
two bands (opposite from the tolerance bands) are green and red, the first two
digits are 5 and 2. If the third band is yellow, follow the 52 with 4 zeros.
Hence the resistance has a value of 520000
, or 520 k.
If the fourth band is gold, this value may be 2.5% high or 2.5% low, a
manufacturing problem. Usually we will use decade boxes, but bookmark this chart
in case we use discreet components.
In theory, a wire is completely transparent, meaning it doesn't affect the circuit. The reality is that wires can break or make bad connections, so be prepared to replace suspect wires. In any case we need to know something about the voltage and the current in these circuits, and that means measuring.
Measuring Devices. A digital voltmeter, or VOM, is the device
we use to measure most voltages and currents (amperes or amps). Traditionally
the black lead is ground or common and red one is positive or hot. Plug the
black probe wire into COM (negative) and the red probe wire into VmA (positive).
Of course, the wires don't know what color they are, so if you use black for
positive nothing will blow up. The polarity (plus or minus sign) is taken care
of in the meter. We will use a VOM to look at Ohm's Law. In some
instances, like in today's lab, we will use an analog meter. You will
perform the connections the same way as you would do with a digital meter.
One ALWAYS measures voltage in parallel, meaning one
probe is placed on each terminal of a single component. Set the dial to DCV (V
with a line over or under it) when using the meter to measure volts. If the
number flashes the voltage is off scale. Switch the dial to a higher range.
In some cases you will encounter auto-ranging meters. The meters will
adjust the scale automatically for you.
One ALWAYS measures current in series, meaning the circuit is broken and the meter is inserted in series with the component. Set the dial to DCA (mA-A) when using the meter to measure amps. Move the red probe wire from V to 10A, then connect the circuit. If the number flashes the current is off scale. Switch the dial to a higher range.
TASK:
To learn about the current/voltage relationships in simple Series and Parallel DC circuits.
PROCEDURE:
Check the continuity of all wires using the digital VOM. Set the meter to Ohms, then connect each one of the meter's probe to one end of the wire. You should be getting a very small reading, close to zero. There is a problem wit the wire, if the meter reads "OL" (over load) or a very high value. Let the instructor know about this.
Determine the exact resistance of the two
resistors in your cup using the VOM. The resistance of one of them will be
around 100 ohms; call this resistor R1.
The resistance of the other one will be around 220 ohms; call it R2. Notice
that they have colored bands so that you can tell them apart (and read the
resistance).
Set up a series circuit. Notice that in each part the circuit is the same, the only difference is the type of meter and its placement. The resistors should be mounted on the breadboard. Set the voltage of the black power supply to 12 volts. However, keep the power off until you are ready to start taking data.
Set the analog ammeter to the 0-50 mA range. You will now make several current measurements on that circuit. Record all your measurements.
Measure the current between the positive
terminal of the power supply and R1; call this la.
Measure the current between R1 and R2;
call it Ib
Measure the current between R2 and the
negative power supply terminal; call it Ic.
Now you will take several voltage measurements on this circuit using the digital VOM set to DC volts. Record all your measurements
Measure the potential difference across
R1; call it Vab.
Measure the potential difference across
R2; call it Vbc
Measure the potential difference across
the terminals of the power supply; call it Vac.
Take apart the series circuit and set up the parallel circuit. Notice that in each part the circuit is the same, the only difference is the type of meter and its placement
Reduce the voltage on the power supply to 3.0 volts. Keep the power supply off until you are ready to take measurements. You will now make several current measurements with the analog ammeter. Record all your measurements.
Measure the current through resistor R1;
Call it lab
Measure the current through resistor R2;
Call it Icd
Measure the current leaving the positive
terminal of the power supply; Call it I
Using the digital VOM set on DC volts, make the following voltage measurements. (Record all your measurements)
Measure the voltage across R1; Call it Vab
Measure the voltage across R2: Call it Vcd
Measure the voltage across the terminals
of the power supply; Call it V.
CALCULATIONS:
Series Circuit:
From step #4 Calculate the average of the three current values. Compare each of the three current measurements to the current average value by calculating the percent difference.
From step #5 add the Vab to Vbc and compare
this sum to Vac by calculating a percent difference.
Parallel Circuit:
From step #7 add current Iab to Icd and compare this sum to I by calculating a percent difference.
From step #8 calculate the average of the three voltage measurements. Compare each of the three voltage measurements to the voltage average value by calculating the percent difference.
QUESTIONS: Use only two to three sentences to answer the questions. Pay attention to the percent difference values to answer the questions below.
Comment on the behavior of the current in a series circuit.
Comment on the behavior of the voltage in a series circuit.
Comment on the behavior of the current in a parallel circuit.
Comment on the behavior of the voltage in a parallel circuit.