Physics 8.02T
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Fall 2001
Simple Circuits Experiment
Introduction
Our world is filled with devices that contain electrical circuits in which various
voltage sources cause currents to flow. We use radios, telephones, computers,
flashlights, irons, heaters, stoves, motors — the list is long. Each of these contains
circuits, which take electrical energy from some power supply and turn it into other
forms: the light of a lamp, the heat of a stove, stereo sound or mechanical work.
In general, electrical currents generate heat and magnetic fields and produce
chemical effects. Any of these phenomena can be used to measure current. One of the
simplest ways is to let the current flow through a coil of wire that is in a magnetic field
and to measure the resulting torque on the coil by observing the deflection of a torsion
spring.
This
is
how
your
multimeter
works.
See
http://web.mit.edu/8.02t/www/coursedocs/current/labs.htm for a link about multimeters.
In this experiment you will use meters, batteries, power supplies, and resistors to
make measurements to get some intuition and knowledge about circuit behavior.
Equipment:
You will use two size D 1.5 V batteries, the AC/DC Electronics Lab circuit board,
a multimeter; three 33 Ω resistors; and wire leads.
Figure 1: The AC/DC Electronics Lab Circuit Board
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AC/DC Electronics Lab circuit board:
Throughout the AC/DC Electronics Lab circuit board (see Figure 1 on previous page)
there are springs. These allow you to easily connect wires or various circuit elements:
• In order to maintain two different springs at the same voltage, connect the two
springs with a wire lead.
• In order to connect a circuit element (for example, a resistor), insert the element
between a pair of springs to ensure contact.
The circuit board has some mountings or elements already wired into place. In this
experiment we will use:
• On the left side of the circuit board: two mountings for batteries with + (positive)
and –(negative) terminals indicated (see “A” in Figure 1).
• On the top left of the board: three 3 Volt bulbs with two springs on each side (see
“B” in Figure 1).
• On the bottom of the board, two rows of four spring pairs with a banana jack at
the end of the row on the right side (see “C” in Figure 1). Each pair of springs in
a row is wired together.
Experiment 1: Measuring Resistance with the Multimeter
• Measure the resistance of each of the three 33 Ω resistors by placing the
multimeter clip leads on either side of the resistor.
Figure 2: Multimeter
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In order to use the multimeter (Figure 2 above) to measure resistance, insert the
red clip lead into the + V-Ω-A pin jack and the black clip lead into the –COM pin jack.
Set the range selector switch to the RX1 Ohms scale. Zero the scale by connecting the
ends of the clip leads together and adjusting the OHMS ADJUST dial until the needle
points to zero on the green OHMS scale.
Experiment 2: Measuring Resistance for Resistors in Parallel and Series:
• Measure the equivalent resistance of two 33 Ω resistors connected in parallel.
Two resistors R1 and R2 in parallel give an equivalent resistance of R1R2/(R1+R2).
•
Measure the equivalent resistance of two parallel 33 Ω resistors connected in
series to a third 33 Ω resistor. Two resistors R3 and R4 in series give an
equivalent resistance of (R3+R4).
You should now connect two 33 Ω resistors in parallel, labeled R1 and R2 , and
the third 33 Ω resistor, labeled R3 , in series with the other two as shown in Figure 3.
Figure 3: Resistors in parallel and series.
WebAssign Question A: Go to WebAssign either now or at the end of this experiment
and open the “Resistors In Series And Parallel” exercise. Calculate the equivalent
resistance of a pair of 33 Ω resistors that are in parallel and enter it in WebAssign.
•
Measure the equivalent resistance between the points A and B in Figure 3 using
your multimeter (across the parallel combination R1 and R2 ) and record it
here:__________________ ohms Does it agree with the value you calculated
above?
•
Measure the resistance between the points B and C in Figure 3 using your
multimeter (across the resistor R3 ). and record it here: __________________
ohms
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WebAssign Question B: Calculate the total equivalent resistance of the three resistors
and enter it in WebAssign.
•
Measure the resistance across the entire combination of resistors between the
points A and C in Figure 3 and record it here: __________________ ohms.
Does it agree with the value you calculated above?
Experiment 3: Measuring Current In A Closed Circuit:
• You will measure the current in each branch of a closed circuit containing a
resistor network and batteries.
Connecting the Batteries:
You will now make a closed circuit consisting of two 1.5 V batteries in series
with your combination of resistors (Figure 3).
Figure 3: Closed circuit with batteries and resistors
Insert the two 1.5 V batteries in the plastic battery mounts on the AC/DC
Electronics Lab circuit board. Use the long white wire lead to connect the positive
terminal of the battery on the upper left corner of the board to a component spring pair on
the left side of the upper row. Use a short white wire lead to connect the negative
terminal of the upper battery to the positive terminal of the lower battery. Use another
white wire lead to connect the negative terminal of the lower battery to a component
spring pair on the lower row. If you want to disconnect the battery, disconnect any of
these connections.
WebAssign Question C: When the batteries are connected, calculate the total current
that flows in the circuit and enter it in WebAssign.
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We now measure the current that flows in the branch of the circuit between the batteries
and the resistors, as follows.
Figure 4: Multimeter in series in the closed circuit
Always set the multimeter to the proper scale (voltage, current, etc) before you connect it.
In order to measure the current, rotate the range selector switch on the multimeter to the
250m DCA setting. Place the multimeter in series in the circuit (Figure 4 above). You can
do this by removing the wire lead from the negative terminal of the lower battery. Then
connect the black clip lead from the multimeter to the negative terminal and the red clip
from the multimeter to the spring pair on the lower row that also connects the end of the
resistor R3 (this is the point C in Figure 4).
•
With the batteries connected measure the current that flows in the circuit and
record it here: __________________ amps. Does your result agree with what
you calculate above?
WebAssign Question D: Calculate the current that flows through each resistor in the
parallel pair and enter it in WebAssign.
•
With the batteries connected measure the current that flows through one of
parallel pair of 33 Ω resistors and record it here: __________________ amps.
Does it agree with what you calculated above?
Experiment 4: Measuring Voltage Drops
•
You will measure the voltage drop across different resistor combinations for a
closed circuit containing batteries.
Remove the multimeter from your circuit and close the circuit again.
In order to measure the voltage across a circuit element with your multimeter, rotate the
range selector switch on the multimeter to the 5 V DC setting. Use the red and black clips
across any element to measure the voltage drop (this puts the multimeter is parallel with
the circuit element Figure 6).
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Figure 6: Multimeter in parallel with a circuit element to measure voltage drop
Question: Calculate the voltage drop across the parallel resistors R1 and R2 .
•
Measure the voltage drop across the parallel resistors and record it here:
__________________ volts.
Question: Calculate the voltage drop across the resistor R3 .
•
Measure the voltage drop across the series 33 Ω resistor and record it here:
__________________ volts
Question: (Discussion Question): If you now try to measure the resistance of any
resistor, will you get the same resistance measurements as you did when the batteries
were disconnected?
•
Try this out and see what you get. Can you explain your result? What is the
source of the current when the batteries are disconnected?