Download Lab 25 Electrical Resistance - Series

LAB 25 OHM'S LAW-SERIES
Monday, April 28, 2014
OVERVIEW
An electric current is a flow of charge (electrons). For Direct Current (DC) charge always
flows in the same direction. In Alternating Current (AC) the charge changes direction by moving
back and forth at a frequency (cycles per second) of the electrical system.
Nearly all substances fall into one or two categories: conductors, through which charge
can flow readily, and insulators, through which charge can flow only with great difficulty.
Metals, many liquids and plasmas (gases whose molecules are charged) are conductors.
Nonmetallic solids, certain liquids and electrically neutral gases are insulators. In addition, there
are several substances, called semi-conductors, which are intermediate in their ability to conduct
charge.
Electrons located in the outermost orbits around the nucleus are known as "valence"
electrons. Conductors have valence electrons which are free to move and thus charge is allowed
to move through the material. Non-conductors have valence electrons which are tightly bound to
the nucleus and are not free to move or allow charge to flow through the circuit.
Electrical resistance is the opposition to the flow of electrons or charge through a circuit.
The unit of electrical resistance is the ohm (symbol ). A resistor is a component of a circuit
which is designed to provide a specific resistance to the circuit. A series circuit is a circuit which
has two or more resistors but only one path for the flow of charge or electrons. The Total
Resistance (or Equivalent Resistance) of a series circuit is the sum of all resistors in the circuit:
RT = R1 + R2 + R3 + …
Ohm's Law gives the relationship between voltage, current and resistance in an electrical circuit.
𝑂ℎ𝑚′𝑠 𝐿𝑎𝑤: 𝑉 = 𝐼 · 𝑅
where V = voltage in volts,
I = current amps and
R = resistance in ohms.
In this Lab you will use three methods to find a resistor's value.
1. Find the resistance using Ohm's Law.
2. Measure the resistance using a multimeter.
3. Determine the resistance by reading the color code of the resistor.
You will also find the total resistance (RT)of a series circuit using digital and analog multimeters
as well as measuring current and voltage drops across each resistor.
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LAB 25 OHM'S LAW-SERIES
Monday, April 28, 2014
OBJECTIVES
A) Measure voltage and current values in a series circuit.
B) Read the color code of a resistor.
C) Find the total resistance of a series circuit.
D) Measure resistance with a Digital Multimeter (DMM)
EQUIPMENT REQUIRED
Mounted Resistor Set
Universal Lead Set
AC/DC Power. Supply
Digital Multimeter (DMM)
Hook-up Wire
SPST Knife Switch
Circuit Panel
Circuit Panel Easel
Analog Multimeter (VOM)
Wire Strippers
PROCEDURE
Lab Setup The basic setup for this lab is shown in Figure 1.
A) Identifying the Color Code Values
1. There are six color-coded resistors in your resistor set. Choose any resistor, and make a note of
each band color on a separate sheet of paper. Determine the color code value, using the table in
Figure 7. Continue with the remaining resistors, determining the color code value of each.
2. Assign the designation "RI" to the resistor with the lowest resistance. Record the band colors
and color code values for R1 in Data Table 2.
3. Continue recording the information for resistors R2 through R6, in ascending order of
resistance values.
B) Measuring Resistance with an Ohmmeter
1. Attach the red and black banana-spring tip leads to the digital multimeter (DMM).
2. Set the DMM to measure resistance. Choose the lowest range. Turn on the DMM.
3. Plug both meter leads into a single island on the circuit panel. See Figure 5. The DMM should
show a reading of zero. If not, consult the instructor.
4. Mount all the color coded resistors from your set to the circuit panel as shown in Figure 6.
Make sure to keep the numbers for the resistors from your circuit the same (R1 stays R1, etc.)
5. Set the DMM setting to its highest range. Connect the DMM leads across R1. See Figure 6.
6. Reduce the range setting of the DMM, until a reading appears that has the greatest number of
display digits. Record the measured resistance value in the far right column of Data Table 2.
7. Repeat steps 5 and 6 for the remaining resistors.
C) Series Circuit Construction
1. Mount resistors R1, R2, and R3 to the circuit panel as shown in Figure 2.
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LAB 25 OHM'S LAW-SERIES
Monday, April 28, 2014
2. Mount the SPST (Single-Pole, Single-Throw) knife switch to the circuit panel. Using your
lead set, build the series circuit of Figure 2.
3. Open the SPST switch.
4. Set the power supply for 0-24 VDC, and connect your circuit to the + and - jacks. See figure 2.
D) Collecting Data - Taking Series Circuit Readings
1. Turn on the power supply, and set the voltage to 10 V.
2. Close the switch.
3. Using the analog multimeter (VOM), measure the voltage drop across each resistor. Be sure to
observe proper polarity. See Figure 3. Record your readings in Data Table 1.
4. Adjust the VOM to measure current. Set the VOM range switch to the highest current range.
5. Open the switch. Connect the meter leads across the switch. Now the current path will be
through the ammeter. Adjust the meter range down, until the pointer reads nearest center scale.
This is the total current (IT) for your series circuit. Record the value in mA in Data Table 1.
Convert the current units to amps, and record in Data Table 1.
6. Turn off the power supply.
E) CALCULATIONS
1. You have measured each resistor with a resistance meter. Now you will compute the value of
the first three resistors using Ohm's law.
𝑉
𝑅 =
𝐼
The current is the same in each resistor in a series circuit:
𝐼𝑇 = 𝐼1 = 𝐼2 = 𝐼3
Compute the resistance of each resistor R1 through R31 using the voltage recorded for each
resistor in Data Table 1. Record your answers as "Computed Resistance" in Data Table 1.
2. Find the total color code resistance by adding the color code values. For a series circuit,
𝑅𝑇 =𝑅1 + 𝑅2 + 𝑅3
3. Find the calculated total resistance by adding the computed resistance values. Record the
answer in Data Table 1.
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