Download DC circuits

Last updated Fall 2009
Building DC Circuitsi
Purpose: In this lab, you will build a variety of simple circuits and take measurements
of voltage and current to study series, parallel and combination circuits. You will learn
to use a multimeter to measure both voltage and current.
Theory: Circuits can be wired in two basic ways – series and parallel – or in ways that
use both series and parallel to make a combination circuit. In a series circuit, the
resistors (in this case, mini light bulbs) are wired such that they are all in a row. That is
one follows the other as shown in Figure 2. In a parallel circuit, shown in Figure 3, the
resistors are wired such that there are multiple paths for the charges to take as they flow
in the circuit.
E
F
G
D
Figure 1
Figure 2
H
Figure 3
You will be using a multimeter to measure both current and voltage. It is very
important that you know how to use the multimeter to make measurements. Failure to
use the meter properly could result in damage to the multimeter.
Measuring Current
When you measure current, you will set the multimeter up to be an ammeter. Make
sure that the leads are plugged in properly (the black one in “com” and the red one in
one of the current ports, designated by the “A” which stands for amps, the unit of
current). The two ports, labeled as 2A and 10A, indicate the maximum current that can
safely run through the circuit given how you have plugged in the leads. In this lab, you
can safely put the leads in the 2A port. In general, however, it is much safer to plug in to
the highest allowable current and readjust after you take your first measurement as to
avoid blowing the fuse in the ammeter. You will also need to set the dial properly. Turn
the dial to DC current (the A with a straight line and a dashed line above it) and set it to
the maximum current that you will be reading. It is critical that you put the ammeter
in series with the element that you wish to measure the current through. To do this, you
will need to break the circuit and close it using the ammeter.
Measuring Voltage
When you measure voltage, you will set the multimeter up to be a voltmeter. Make sure
that the leads are plugged in properly (the black one stays in “com” and the red one
gets plugged in where there is a “V” for volt, the unit of voltage). Turn the dial so that it
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is reading voltage for DC voltage (the V with a straight and a dashed line above it). It is
critical that you put the voltmeter in parallel with the element that you wish to
measure the voltage across. To do this, you will not need to break the circuit. Instead,
you place the leads on either side of the resistor and take the reading.
Materials: four light bulb sockets and light bulbs (spherical and cylindrical), wires,
batteries and battery holder (or other power supply), multimeter and probes.
Part I: Lighting a Light Bulb
1. For this activity, you will need one small light bulb, one wire and one battery.
Using these materials and only these materials, make the light bulb light up.
2. Draw a diagram below illustrating how you were able to light up the bulb.
Part II: Introduction to DC Circuits
1. Examine the two different light bulbs in your setup. The bulb filament (the very
thin conductor that glows) is made of tungsten, a metal that does not melt until
reaching a very high temperature. A glowing tungsten wire would rapidly
oxidize and burn up in air, so there is a vacuum or an inert gas such as argon
inside the bulb. The reason the filament glows when current passes through it is
that not all the electrical power (energy/second) dissipated in the bulb in
converted to heat energy (as is the case in most resistive elements), but some of it
is converted to the light energy that you see. Screw each light bulb into the base
plate holders provided, and use the multimeter to measure the resistance of each
bulb/base plate holder. (Be sure to check that the probes are plugged in properly and
that the dial is turned to measure resistance.)
2. Record
the
resistances
Rspherical
__________
Rcylindrical
__________
of
each
resistor
(bulb)
below:
3. Turn on the power supply. Using the multimeter in its voltage measuring
position (Be sure to check that the leads are plugged in properly and that the dial is
turned to measure DC voltage), measure the voltage of the power supply.
Vpower supply
_________
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4. Using the fact that you now know the voltage across the resistor and the
resistance of the resistor, calculate what the current should be for each bulb.
Ispherical (ideal) __________
Icylindrical (ideal) __________
5. Build a simple circuit for each bulb, spherical and cylindrical, as shown in Figure
1. Measure the voltage across the resistor. (Be sure to check that the leads are plugged
in properly and that the dial is turned to measure DC voltage.)
6. Measure the current through the resistor. (Be sure to check that the leads are plugged
in properly and that the dial is turned to measure DC current.)
Ispherical (actual) __________
Icylindrical (actual) __________
7. In which case is the bulb brighter? Why do you think this is so?
Part III: Series Circuits
8. Connect two resistors in series as shown in Figure 2 using two of the spherical
bulbs. Redraw the circuit diagram below so that you can record your
measurements on the circuit diagram.
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9. Read and record on your sketch the voltage across each resistor. (Be sure to check
that the leads are plugged in properly and that the dial is turned to measure DC voltage.)
10. Read and record on your sketch the current at each of the following positions: a)
between the power supply and the first resistor, b) between the two resistors,
and c) between the last resistor and the power supply. (Be sure that the leads are
plugged in properly and that the dial is turned to DC current.)
11. What happened to the brightness of the bulbs as compared to when there was
only one bulb? Is this consistent with your observations from Part II?
12. Replace one of the spherical bulbs with a cylindrical bulb. Draw the circuit
diagram for this circuit making sure to indicate which bulb is spherical and
which is cylindrical. Read and record on your sketch the voltage across each
resistor and the current through the circuit. How does this compare to when you
had two identical resistors?
13. What happened to the brightness of the spherical bulb when you replaced the
second spherical bulb with a cylindrical bulb? That is, did it get brighter, dimmer
or stay the same? Why do you think this is the case?
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14. Now take an extra wire and connect it across the two terminals of one bulb. This
is known as a “short circuit.” What happens to that bulb? What happens to the
other bulbs? Is this consistent with your prediction from your prelab?
15. Now remove the extra wire that caused the short. Now unscrew one of the bulbs.
What happens? Why is that the case?
Part IV: Parallel Circuits
16. Connect two resistors in parallel as shown in Figure 3 using two of the spherical
bulbs. Redraw the circuit diagram below so that you can record your
measurements on the circuit diagram.
17. Read and record on your diagram the voltage across each resistor. (Be sure to
check that the leads are plugged in properly and that the dial is turned to measure DC
voltage.)
18. Read and record on your diagram the current at each of the following positions:
a) between the power supply and the first junction, b) between the first junction
and bulb G, c) between the first junction and bulb H, and d) between the last
junction and the power supply. (Be sure that the leads are plugged in properly and
that the dial is turned to DC current.)
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19. What happened to the brightness of the two bulbs in parallel as compared to
when there was only one bulb? As compared to two bulbs in series? Does this
make sense? Why or why not?
20. Add a third resistor in parallel. Draw the circuit diagram for this circuit. Read
and record the voltage across each resistor. Read and record the current in the
circuit. (Be sure to have the leads and dial set properly before taking any measurements.)
21. What happened to the brightness of the bulbs as compared to when there were
two bulbs? Does this make sense? Why or why not?
22. Now remove one of the bulbs in one of the parallel branches. What happens to
the other bulbs? That is, does it get brighter, dimmer or stay the same? Why do
you think this is the case?
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Part V: Combination Circuits
F
D
2
reFigure
1
E
A
F
G
G
H
B
H
C
Figure 42Figure 3
Figure
Figure 3
23. You will be building the circuit shown in Figure 4. Use the circuit diagram
shown above to record your observations.
24. Predict the relative brightness of the bulbs. That is, which ones will be bright (if
any)? Which ones will be dim (if any)? Why do you think this is the case?
25. Build the circuit. Did your prediction match the actual result? If not, please
explain.
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Questions
1. What happened to the voltage across each of the resistors when you added a
second and third resistor in series? In parallel? Why do you think that is?
2. How do the voltages across E and F as shown in Figure 2 compare with the
voltage of the power supply? Can you write this as an equation? What about for
Figure 3? In general, what can you say is true about the voltage in a closed loop?
3. Do the resistors dissipate energy? How do you know?
4. What happened to the total current in the circuit shown in Figure 1 when you
added a second and third resistor in series? In parallel? Why do you think that
is?
5. Do the light bulbs “use up” the current? How do you know?
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6. What can be said about the independence of branches in a parallel circuit? Does
this make sense? Why or why not? (Think back to when you took one of the
bulbs out in the parallel circuit. How did that affect/not affect the other
branches?
7. How do you suppose your house is wired – in series or parallel? Why would this
be true?
8. When you use the multimeter as an ammeter, you put the meter in series in the
circuit. If the goal is to not disturb the circuit in making a measurement, do you
suppose that the ammeter has high or low resistance? Why do you think this is
true?
9. When you use the multimeter as a voltmeter, you put the meter in parallel with
the component that you wish to measure the voltage across. If the goal is to not
disturb the circuit in making a measurement, do you suppose that the voltmeter
has high or low resistance? Why do you think this is true?
i
This lab meets Massachusetts state frameworks in physics items 5.2, 5.3, and 5.5.
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