Download Electic Circuits - Saddleback College

EIIIIIIII", EllcUic Oircuits
Saddleback College Physics Department
Purpose: To become familiar with the components of basic electric circuitry, electrical circuit behavior
measuring devices (digital and analog) and the wiring of electric circuits. Also, to compare the behavior
of electric circuit components to expected behavior as given by Ohm's Law, and explore equivalent
resistances and voltage and current behavior in parallel and series-wired circuits.
Sketches:
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Figure 2b
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Equipment:
Decade Resistance boxes (2)
2-1.5V batteries, wired as one voltage source
Connector leads
Alligator clips
Spade connector clips
Multimeter (5 digital & 1 analog)
Slide wire rheostat
Theory:
Definitions and Svmbols
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.
Resistor, measured in ohms; used to impede, or reduce the flow of current. It is also used to
"divide" voltage, as we shall see later in the lab. Symbol for resistance is Q, called an ohm.
.
1FElectromotive force provider, also known as battery, dry cell, or direct current source,
measured in volts. Device produces electric current by conversion of chemical energy into
electrical energy.
.
@ Voltmeter. This is a device which measures the potential difference, or voltage difference
between two points in a circuit. Measurements are provided in volts.
.
@ Ammeter. This device measures the current in a circuit in amps. An amp, or ampere as is its
formal name, is defined as 1 coulomb per second. Ammeters contain very low internal resistance,
so that their resistance does not adversely influence the measurement of current as a whole
throughout the circuit being studied. Because of this, it is imperative that they NOT be connected
in a circuit with onlv a voltage source present. Ammeters must always be used with additional
external resistance, to avoid internal damage.
.
.
M M'ultimeter.Can be digital or analog, although usually digital. Provides a variety of circuit
measurement instruments in one hand-held device. Instruments commonly found in a multimeter
inc:ludedirect current (DC) and alternating current (AC) voltmeter and ammeters, and an
ohmmeter, which measures resistance.
.
.
.
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Variable Resistor. Same definition as resistor, above, but is variable in its resistance
capability.
Wire or connecting lead. Oftentimes a copper wire contained in rubber insulating
material, usually red or black in color; contains very little resistance; also provides no voltage or
current to the circuit being studied.
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Slide Wire Rheostat. Acts as a resistor, but has sliding apparatus on top that allows
variable resistance to be obtained by sliding apparatus back and forth across set of coils on
resistor that comprise the resistor element.
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Series: when two or more circuit elements, ie. Resistors, are connected in series,
they are connected end to end or "in a single file line" with the bottom of one component
connected to the top of the next component, and so on. Circuit components can be any
combination of components and are not restricted to resistors.
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Parallel: when two or more circuit elements, ie. Resistors are connected in
series, they are connected side by side, with the tops of two or more components connected
together, and then the bottoms of the same two or more elements also tied together. Circuit
components can be any combination of components and are not restricted to resistors.
Formulas
. Ohm's Law: V = IR; perhaps the very simplest and most basic of the electrical circuit formulas, it relates the Voltage drop, V, across a resistor of Resistance R to the Current I through the
resistor.
.
.
.
Rs = R1 + R2
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=Rp
R1
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+... + Rn: Formula for equivalent resistance of resistors connected in series.
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Formula for eqUiva 1ent resistance 0f resistors connected in parallel.
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IValuetheoretical - Valueexperimental IxIOO% Formula for calculation of percent difference.
Value theoretical
.
.
Itotal = 11 + 12
+... + In Formula for calculation of branch current and total current for branches
of a circuitwiredin parallel.
~olal =
~ +V2 +...+ Vn Formula for calculation of total potential difference, or voltage, in a
circuit and voltage drops across individual devices, such as resistors, in a series-wired circuit.
Concept
By connecting circuit components in series and in parallel, it is possible to observe their behavior and
the effect of various component arrangements on measurements of voltage and current. From the
measurements of voltage and current, it is possible to plot a graph of current on the x axis vs. voltage
on the y axts yielding, by the definition of slope, a value for resistance in the circuit. This value can
then be compared with the stated value of resistance in the circuit, also known as the theoretical
value, to determine the quality of measurement techniques, quality of equipment, and degree of
compliance with known circuit formulas, ie. Ohm's Law and the laws for equivalent series and parallel
resistances.
When energy is delivered to a circuit, it is delivered to electric charges. These charges then, in
turn,
pass through the resistors that are placed in the circuit. When the charges encounter the resistors,
the resistors dissipate this energy in the form of heat. As such, a "voltage drop" exists over each
element in the circuit. This voltage drop correlates to the amount of energy expended by the charges
in the process of passing through the resistor.
As will be seen in Part III of this experiment, electric current and equivalent resistance behavior are
quite different between circuits wired in series and those wired in parallel. In parallel circuits, current
can branch and charges will follow each path when this happens.
3
Procedure:
PartI: Observationof the relationshipbetweenpotentialdifferenceV acrossa fixedresistorandthe
current.I. throuahit. First use the Diaital Multimeter to measure V. 1& R in steps 1-7 below.
.then use the Analoa Multimeter to measure V. I & R. Graph both results. thus YOUwill have 2
araphs for part I.
1. Usingthe voltmeteras connectedbelow,measurethe voltageof the battery(batteries)provided
as the powersourcefor the circuitandrecord.
2.
Using the Multimeter as connected below, determine the resistance of the R1 and R2 decade
resistance boxes when NO resistance is selected (as it will not be zero, like you may expect). You
will be adding the above value to your selected resistance when you record your resistances.
This provides a means by which to calibrate the resistance boxes.
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3. Connect equipment as in figure below:
A
While connecting components, do not connect the last lead to the negative side of the batteries
instead let it act as a switch to set current flowing. Once your instructor has verifiedthe
correctness of your circuit, connect the last lead to the negative end of the battery using an
alligator clip.
4. With R1set to 5 ohms, set R2to 25 ohms to begin with. The resistance of R2will be changed in
order to vary the current through R1. It is this current traveling through R1that we want to study.
5. Recognize that when using the Analoa Multimeter as an ammeter, voltmeter or ohmmeter each
function will have different scales to provide a wider range of measurement within one device. In
order to ensure that the voltmeter and ammeter have been connected on the correct scale for this
circuit, take the free end of the lead that will connect to the negative end of the battery and press
it lightly against the battery. Check for movement on the ammeter and voltmeter. If the needle of
either device "pegs," or jumps all the way to the right and stays there, you have connected the
device on a scale that is too small. After removing the lead from being held against the battery,
reconnect the ammeter or voltmeter on a higher scale so that readings can be obtained. For
safety considerations, it is important to always begin the connection on a high scale and gradually
move down to a lower scale in order to obtain more accurate readings. NOTE: Use the mirror
behind the.needle to eliminate parallax.
Using the Analog Multimeterto measure Resistance (flipthe switchto DC n and notethe log
scale reads right to left) requires zeroing the Ohmmeter for each resistance range used. To zero
the Analog Ohmmeter follow the steps below:
(a) Set theTange to "x1".
(b) Plug both ends of a lead into the meter to simulate -zero resistance.
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(c) Turn the" Q adj" wheel to zero the meter. If you are unable to get the 1 ohm setting
(on the dial) to zero, then you need to have the battery in your meter replaced. If this is
the case, please tell your professor.
(d) Now set the meter to the desired range and zero it.
6. The term "close the circuit" is used to mean complete the circuit. This means that there are no
"openings" or gaps between circuit components. It means that current is flowing, since current
cannot flow until the circuit is complete. Complete the circuit by connecting the last lead to the
low electric potential or low voltage (negative) end of the battery. Because voltage is potential
difference, the voltmeter is measuring the voltage drop across R1. Record this measurement in your data table: Also record the current I, as measured by the ammeter. Reduce the resistance
in R2by dropping it in increments of 3 ohms at a time, and record the current in the circuit and the
voltage drop across R1 each time the resistance in R2is changed. Once completed, disconnect
the lead from the negative end of the battery in order to preserve the batteries.
7. Prepare a graph of voltage V vs. current I by plotting V on the vertical axis and Ion the horizontal
axis. Observe the relationship between V and I from the graph. Rearranging Ohm's Law into
V
- = R shows us that resistance is the ratio of the change in voltage.(A V ) to the change in
1
current ( AI). The slope of the graph, therefore, should be the resistance of the element.
Compare the resistance found in the graph to that indicated on the resistance box. Also compare
this with the measured resistance from Step 2 above.
Part II: The potential difference across different elements of a circuit resistance in series
).
Using the circuit shown below, set R1 and R2 as shown in the chart below. For each pair of
resistances in the chart below, record the drop in voltage, or potential, across R1 (Figure a), drop
in voltage, or potential, across R2 (Figure b), then record it across both resistors (Figure c).
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Figure a
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Resistance Values, Voltage Drops, and Current Data Values
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(ohms)
26
22
18
14
10
6
(ohms)
(volts)
(volts)
(volts)
4
8
12
16
20
24
5
.2
I (amps)
2.
From the data above, explore the relationship between V1 and R1. What is the nature of this
relationship? Does current (I) hold constant throughout? If so, why? By Ohm's law, the
equivalent resistance of the circuit can be obtained from the ratio of the combined voltage drop,
V1+2,
to the total current, I. Using this knowledge, calculate the total resistance in each
combination from the voltage and current numbers for that combination. Can this be developed
into a formula for the equivalent series resistance using R1,R2,RTotal
?
3. The type of circuit shown below is often used as a voltage divider, which provides a variable
voltage from a fixed electromotive force provider, such as the dry cells, or batteries, used in this
experiment. By replacing R1and R2,with a slide wire rheostat, voltage drop can be varied in
accordance with the resistance in the circuit. Be sure to choose a rheostat whose total resistance
is in the range
200 ::;R ::;1000 .
a
4. After connecting this circuit, move the sliding contact back and forth from point b to a and back
again. Watch the voltmeter gauge as you are doing this. What happens to the potential difference
between points band c as you move the sliding contact? What can be said about the voltage
drop between points a and b?
Data
Point
1
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3
4
5
6
7
8
9
R2
(ohms)
R1
(ohms)
Voltage
(volts)
Current,
I; amps
Computed
Resistance,
R, in ohms
5
5
5
5
5
5
5
5
5
Part III: VoltaQeand current behavior in a circuit with parallel resistors
1. Wire the circuit as shown below.
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2. With R1set at 15 ohms and R2set at 30 ohms, close the circuit and take readings of 11,12,and 13.Also
record the potential drop across R1and across R2by taking readings of V1 and V2, respectively.
3. What circuit behavior is observed? How do the voltages in V1 and V2,the branch voltages as they are
often called, compare? What happens with the current in each branch? How does the current in each
branch compare with the total current?
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4.
Using Ohm's Law, compute the equivalent resistance in this circuit from the applied voltage and the
total current. Now, compare this equivalent resistance with the resistances of R, and R2.What do you
see happening?
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,
5. UsingOhm'sLaw,again,andthe voltageand currentobservedabove,provethat R =
1
Rl
2
+ R2
,
Compare the R found here with your values for R, and R2.How does this value compare to the value
for total effective resistance found in Step 4?
Be sure ,to include your answers to the questions in your conclusion.
*"'Complete the light bulb activity below.
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7
PHYSICSLAB
R.N. Parsons
battery
3.
2.
11.
bulb
Hookup the circuits shown 2oove.Whathappens to the brighmessof the lampsas more are
a.ddedin series? Why? Removeone l13.mp
in circuit #3. What happens? Why?
6.
5.
12.
CD
CD
ro
What happens to the brighmessof each lamp as more lampsare added in paranel? Why?
Removeone lamp in circuit#6. What happens? Why? Is yourhome wire in seriesor parallel?
~replacethe
two dry cell batteries with the small hand crank generato!:(be gentle with it:!)
Try aUthe above circuitsand try it without a load. Why is it more diffic.ultto crank when it's
hooked to the lights? Which circuit usesthe most energyto ma.intainthe same brighmess?
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