Download Elementary Circuits

Physics 241 Lab: Elementary Circuits
http://bohr.physics.arizona.edu/~leone/ua_spring_2010/phys241lab.html
Name:____________________________
Section 1.
1.A. Circuit diagrams are simple ways to represent actual physical circuits. An example is
provided showing three unique ways to represent the circuit (the last diagram shown is the “circuit
diagram”). Note that the wide line for the battery represents the positive terminal of the battery.
(Note that most physicists are sloppy and write V for potential difference when they really mean ∆V.)
Draw a circuit diagram for the following physical system.
Quickly sketch your circuit diagram below:
1.B. A digital multimeter (DMM) may be used to measure constant (DC) voltage across a circuit
component while current is flowing.
Measure the voltage produced by several combinations of batteries using your DMM (make a quick
table below). Your table of combined battery voltages in SI units:
1.C. A D-cell battery and a AAA-cell battery are both 1.5 volts. That means that for a given circuit (like
a flashlight), each gives the same energy to each electron as it leaves the battery so that both produce
exactly the same circuit behavior (flashlight brightness). Explain what the advantage of the D-cell battery
would be versus the AAA-cell battery. Your explanation:
1.D. A DMM may also measure constant current through a circuit component.
Measure the current through a single light bulb powered by a 1.5 volt battery. Check your result with
your neighbors’ as some DMMs do not always measure current correctly .
Your measured current in SI units:
1.E. A DMM can also measure the resistance of a disconnected component.
Measure the resistance of a single unpowered light bulb. This is not a useful observation since a light
bulb is non-Ohmic: it’s resistance changes when used in a circuit (resistance grows with heat). You will
learn to calculate the resistance of a powered bulb later in this course.
Your measured resistance in SI units:
1.F. Measure the resistance of your finger:_________________ and your torso:___________________.
How would you expect these measurements to change while you exercised? Your observations (above)
in SI units and your response:
1.G. One way of thinking about a circuit with constant current is to use a sort of energy-circuit diagram.
The y-axis represents voltage (potential energy per unit positive charge) while the x-axis represents the
position of the circuit components. Here is an example applied to the first circuit shown in 1.A.:
This diagram is an attempt to convey that the battery lifts a unit of positive charge to 1.5 Joules of
electrical potential energy and that this unit of charge loses all this energy when flowing through the light
bulb (and lighting it). Another way to say this is that the battery provide 1.5 volts of electrical potential to
the circuit and that all this electrical potential is lost across the light bulb.
Note also that the wires are drawn nearly parallel to the x-axis. This is because wires are highly
conductive (very low resistance) and so there is no appreciable drop in electrical potential along a wire.
However, if you were to simply hook a wire to both sides of a battery without a light bulb, then you
would see that the entire voltage drop must be through this bare wire. Due to a wire’s low resistance, a
large current would flow from the battery and through the wire and things would heat up! (Note: always
use a resistance in your circuit when measuring amps or you will blow up your DMM).
1.H. Now imagine adding another identical light bulb in series with the first:
Explain what the voltage drop across each of the light bulbs must be. Your explanation:
Predict how the total resistance of this circuit would compare to that of the previous single light bulb
circuit. Your comparison:
Predict how the total current of the circuit (the current flowing through the battery) would be affected by
adding this extra bulb in series? What would the current be as a fraction of the single light bulb circuit?
Your answers:
Now compare the brightness of the bulbs. Explain if the brightness is affected by whether the bulb comes
first or second after the battery's positive terminal when placed in a series circuit. Your explanation:
1.I. Now imagine adding the extra identical light bulb in parallel with the first:
Explain what the voltage drop across each of the light bulbs would be. Your explanation:
Predict how the total resistance of this circuit would compare to that of the previous single light bulb
circuit. Hint: because the charge carriers have two paths to choose from, there will be less resistance than
if a single path were available. Your comparison:
Predict how the total current of the circuit (the current flowing through the battery) would be affected by
adding this extra bulb in parallel? What would the current be as a multiple of the single light bulb circuit?
Your answers:
1.J. Use an energy-circuit diagram to investigate what would happen if two 1.5 volt batteries were placed
in series and discharged through a single bulb. Quantitatively compare the power consumed by the light
bulb in the two-battery circuit to the single battery circuit. Note that doubling the potential of this circuit
will cause twice as much current to flow. (Hint: doubling the potential energy per unit charge should
double the current; then use the formula: P=IV.) Your calculated power comparison:
1.K. Explain which would last longer and which would be brighter: powering a single light bulb with two
batteries in series or powering a single light bulb with two batteries in parallel. Your explanation:
Section 2.
2.A. You will make predictions about the following circuits and then build and test them.
Predict the order of brightness of the bulbs from least bright to brightest and then explain why you
think this will be the case. Your predictions and explanations:
2.B. Now set up each of the circuits and test your predictions. What is the actual order of the bulbs from
least bright to greatest? Your observations:
2.C. For each of the battery arrangements shown below, make the described voltage measurements. Be
sure to apply the positive and negative leads of your DMM correctly (as shown) to obtain the correct sign
of the voltage (compare the sign of the voltage in circuits a and b).
Your observations in SI units:
2.D. Relate your observations of the voltage in part 2.C. to your observations in part 2.B. Explain how
the amount of applied voltage affects bulb brightness. Your discussion and answer:
2.E. Imagine (don't set up) the following circuit that uses two 1.5 V batteries in series to power three
identical light bulbs. Marbles are shown to represent the charge carriers that produce the current in the
circuit. If every second 9 marbles flow from the top of the battery, we say that ITOTAL= 9 amps. The
marbles must push each other out of the way to proceed through the circuit (ultimately pushed by the
battery). Predict the current of marbles for each of the five delineated parts of the wire. Then explain
what is meant when an electrical engineer says that they will lower the total resistance of a circuit by
adding a component in parallel. Your predictions and explanation:
2.F. In the simple circuit below, four locations along the circuit’s wires are labeled. A second picture is
supplied to help you think about the circuit from an energy perspective:
If the ground of your DMM was placed at location d, predict what the voltage readings would be on the
DMM screen if the positive lead was placed at each other locations in turn. Be sure to include the sign of
the electric potential (voltage difference). Your predictions in SI units:
If the ground of your DMM was placed at location a, predict what the voltage readings would be on the
DMM screen if the positive lead was placed at each other locations in turn. Be sure to include the sign of
the electric potential (voltage difference). Your predictions in SI units:
Now set this circuit up and test your predictions. Record each of your results and if some of your
predictions were wrong, explain the mental misconceptions you held. Your observations in SI units
and any explanations of misconceptions:
Compare the magnitude of the current at each of these locations in the circuit. Your comparisons:
2.G. Now imagine the previous circuit with a section of wire removed (again a second picture is given to
help you think about the circuit from an energy perspective):
If the ground of your DMM was placed at location d, predict what the voltage readings would be on the
DMM screen if the positive lead were placed at each other locations in turn. Be sure to include the sign of
the electric potential (voltage difference). Your predictions in SI units:
Now set this circuit up and test your predictions. Record each of your results and if some of your
predictions were wrong, explain the mental misconceptions you held. Your observations in SI units
and any explanations of misconceptions:
Section 3:
3.A. Below are three light bulb configurations made with identical bulbs. Imagine that each light bulb
carries 1 Ω of resistance regardless of its temperature (unrealistic). Answer all the following questions
without making observations. Note: the energy-circuit diagrams of sections 1.G.-1.J. may be useful.
Calculate the total resistance for each circuit. Your answers in SI units:
Calculate the voltage across each light bulb. Your answers in SI units:
If circuit A is known to produce 3 amps of current through the battery, find the currents through the
batteries in circuits B and C. Your answers in SI units:
Use your previous answer to find the current through each single light bulb in the three circuits.
Your answers in SI units:
Use your previous answers to find the power dissipated as heat and light by each light bulb in the three
circuits. Your answers in SI units:
Use your previous answer to compare the brightness of each light bulb in the three circuits.
Your answers:
Finally, use your previous answers to calculate the total power output by the batteries in each circuit.
Your answers in SI units:
3.B. Below are three light bulb configurations where all light bulbs are identical. Imagine that each light
bulb carries 1 Ω of resistance regardless of its temperature (unrealistic). Answer all the following
questions while making observations.
Calculate the voltage across each light bulb then set up the circuit and measure the result. Your answers
and observations in SI units:
Calculate the current through each single light bulb in the three circuits then set up the circuit and measure
the result. Your answers and observations in SI units:
Use your previous answer to compare the brightness of each light bulb in the three circuits. Your
comparisons:
What is the current flowing through the middle wire of circuit C? You answer in SI units:
3.C. In the following circuit diagram, use dashed lines with arrowheads to correctly draw the direction of
the electric field in each segment of wire in the circuit. Then use solid lines with arrowheads to show the
direction electrons would travel in this circuit. Explain the path an electron would take beginning from
the battery, though the bulb and back to the battery. Remember the convention that the larger side of the
battery in a circuit diagram denotes the positive voltage. Your two sets of arrows and your
explanation:
Section 4: (Authentic Assessment) Use only wire and a 1.5 Volt battery to make a single light bulb
light up (remove a light bulb from your board). Show another student once you are successful and have
them sign here:____________________________________________
I, the physics 241 laboratory TA, have examined this worksheet and found it to be thoroughly completed
excepting any sections that I have marked herein. TA signature: ______________________________
Elementary Circuits Report Guidelines: Write a separate section using the labels and instructions
provided below. You may add diagrams and equations by hand to your final printout. However, images,
text or equations plagiarized from the internet are not allowed!
•
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Title – A catchy title worth zero points so make it fun.
Goals – Write a 3-4 sentence paragraph stating the experimental goals of the lab (the big picture).
[~1-point]
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Concepts & Equations – [7-points]
o Discuss the following terms within several separate paragraphs:
o Battery voltage (FYI this is also called electromotive force sometimes.)
o How multiple batteries add or subtract (or neither).
o Current in a component vs. total current of a circuit.
o Resistance of a component vs. total resistance of a circuit.
o Power consumed by a resistor vs. total power supplied by the battery(ies).
o “In-parallel” components and “in-series components”.
o How to use the digital multimeter (DMM) (constant DC voltage, constant current, and
naked resistance).
o (In a separate paragraph) explain how the total current of a circuit is affected by the use of
parallel and series resistances. Supply enough examples to make your point.
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Procedure – Summarize the general procedure of this lab with a few sentences about each section.
I suggest statements such as “two batteries in parallel were placed in a circuit with three bulbs in
series to test the effects of multiple components on current.” This must be written in sentenceparagraph form SO DON”T USE BULLET POINTS. [2-points]
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Results – Summarize the quantitative or qualitative results of each section of the lab with a
separate, short paragraph. Be sure to reference any graphs as results in the appropriate part this
section (using the title of your graph). This must be written in sentence-paragraph form without
bullet points. [2-points]
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Conclusion – Write at least three paragraphs where you analyze and interpret the results you
observed or measured based upon your previous discussion of concepts and equations. It is all
right to sound repetitive since it is important to get your scientific points across to your reader.
Do NOT write personal statements or feeling about the learning process (keep it scientific). [6points]
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Graphs – All graphs must be neatly hand-drawn during class, fill an entire sheet of graph paper,
include a title, labeled axes, units on the axes, and the calculated line of best fit if applicable. [0points] NO GRAPHS IN THIS LAB
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Worksheet – thoroughly completed in class and signed by your TA. [7-points & mandatory for
report credit.]
Teaching Tips:
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Do NOT discuss Ohm’s law at any point in this lab.
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Discuss voltage difference as potential energy per unit charge (electric potential).
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Discuss current as electrical friction and compare what happens with 1 bulb vs. 2 bulbs in
series for the same battery.
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Discuss how lowering the total resistance of a circuit increases the current flowing through
the battery.
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Discuss the marble analogy of electrons to explain why current must always flow in a loop.
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Demonstrate use of the DMM: resistance of disconnected component, voltage across
components in powered circuits (approach with two leads) and current through components in
powered circuits with resistance (approach with one lead).
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Quickly discuss the difference between series and parallel arrangements of components.
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Remind students that a 9-V battery will blow the small light bulbs.
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Discuss that current times voltage difference gives work energy to move charge, W=q∆V.
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Discuss power output by voltage source vs. power converted to heat by resistor and the
created or
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.
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The applet http://www.falstad.com/circuit/e-resistors.html is very useful for demonstrating
current flow and helping students develop a general intuition about total circuit behavior.
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Demonstrate the resistance of steel using steel wool and a 9-V battery (over the sink!).