Electrical Circuits
Purpose
To learn how to measure resistance, voltage, and current using a multimeter. To become familiar with the basic components of simple electrical circuits and Ohm's law.
Equipment
One (1) electric circuit kit with three identical lights (#52) and one (#50) bulb, one (1) resistor, connection wires, two (2) banana lead wires with alligator clips, extra 1.0 amp fuses, one (1) digital multimeter, one (1) burned out bulb.
Ammeter Light Bulb Ohmmeter
+ V
–
Power Supply Resistor Voltmeter
Figure 1 Symbols for Electrical Components. kilo (k)
Mega (M) mlli (m) micro (µ) nano (n)
1000 (103)
1000000 (106)
0.001 (10-3)
0.000001 (10-6)
0.000000001 (10-9)
Table 1. Common prefixes for the metric system thousand million thousandth millionth billionth
Part 1 Resistance Measurement
1.
Set the multimeter up to act as an ohmmeter. Plug the red wire lead into the V-Ω receptacle and the black into Com. Turn the dial to the section marked Ω. The Greek letter omega (Ω) is the symbol for ohms which is the unit of electrical resistance. An ohmmeter is a device to measure electrical resistance.
Note: Whenever you use an ohmmeter make sure that the resistor is disconnected from the circuit. This will insure that your measured value of resistance is accurate and that the ohmmeter will not be damaged by inadvertently connecting it to a power supply (or battery).
2. Place alligator clips on the ends of the leads and connect the ohmmeter to the resistor by clamping one lead on each side of the resistor.
3. Turn on the multimeter. Measure the resistance of the resistor. Adjust the ohmmeter range to get the best measurement. A reading of
1 .
(1 followed by blank spaces and a decimal point) is

Electrical Circuits an out of range indication. This means that the resistance is larger than the dial setting of the meter. Turn the dial to the next setting up. If the range scale includes a prefix (k or M), remember that for recording or calculations. (0.200 k  = 200  ) Record the value. Remember to include units with all numbers.
R resistor = ___________

4. Connect the ohmmeter to the one of the light bulbs in the electric circuit set and measure its resistance. You may have to adjust the ohmmeter range to get a good measurement. Record the value. R bulb
= __________ Always turn off the multimeter when you’ve made the measurement.
Questions
1.
What would be the resistance reading on the ohmmeter for a light bulb that is burned out. That is, a portion of the light bulb filament has burned up. If you are not sure ask the instructor to loan you a burned out bulb. __________  What’s the resistance of a copper wire? __________ 
2. With an appropriate meter each member of your group could also measure their body resistance by holding a lead from the ohmmeter in each hand. The meters we use in lab are not appropriate for this measurement but for the sake of discussion assume that each member of your group performed such a measurement and recorded the values shown below. Assign a name for each measurement in the table. The values in the table are typical of people that are relatively dry.
1.
2.
3.
4.
Name Resistance (Ω)
150,000
100,000
75,000
50,000
What does the resistance of your body tell you about the possibility of you receiving an electrical shock?
To answer this question you should wait until you do parts 2 and 3 and understand the relationship between electric current, voltage, and resistance. A current of 0.001 Amps (1 mA) or more creates a sensation that we call a shock. Since the most common household voltage is 120 Volts, use this in your estimation.
Current (A) Effect
0.001
0.005
Can be felt
Is painful
0.010
0.015
Causes involuntary muscle contractions (spasms)
Causes loss of muscle control
0.070 Goes through the heart; serious disruption, probably fatal if current lasts for more than 1 s.
Table 2. Effect of electric currents on the body (from Conceptual Physics 7th Ed. by Paul Hewitt)
3.
Who in your group is least likely to receive a shock? Most likely? Again, wait until you do parts
2 and 3 and understand the relationship between electric current, voltage, and resistance.

Electrical Circuits
Make sure that the electric circuit kit is unplugged ( see below) before starting each part of the experiment. Plug it in only after you have double-checked all connections to make sure they are right. This procedure will prevent injury to the meter.
Note : The power supply in the electric circuit kits is technically a transformer and supplies AC volts and current. However, for this lab we’ll continue to use the generic battery power symbol.
Disregard the + and – symbols and set your multimeter to the AC position.
Part 2 Voltage Measurement
1. To set the multimeter up to act as an voltmeter, leave the leads where they are and turn the dial to the section marked V (V is the symbol for volts). A voltmeter is used to measure electrical potential difference or voltage.
2. Set the voltmeter on its highest voltage scale so that if for some reason the voltage is higher than you expect you will not damage the meter.
3.
Connect the voltmeter to the electrical kit power supply
+
–
4.
+ V
–
Measurement of voltage Figure 3
Vary the voltage range on your meter until you can accurately read the voltage. Record the voltage below. Remember to include units with all numbers.
V = ____________
5. Unplug the electric circuit kit (this is for safety). Turn off the multimeter.

Electrical Circuits
Part 3
1.
Current Measurement
Set the multimeter up to act as an ammeter by turning the dial to the section marked A (amps),
200 mA position (reads up to 200 milliamps) and changing your red lead wire to the mA receptacle. An ammeter is used to measure electrical current.
2. As a safety precaution, make sure the electric circuit kit is unplugged until everything is connected exactly as it should be. Check with the instructor if you are unsure about your circuit.
3. Using the resistor from Part 1 and the power supply from Part 2, set up the circuit as shown below. Insert the ammeter into the circuit so that the electrical current will flow through the ammeter as shown (the ammeter measures the flow of electrons through the circuit).
Do not connect the ammeter directly to the power supply, current must also flow through the resistor.

Measurement of Current in a Resistor Figure 4
4. Plug in the electric circuit kit. Turn on the multimeter. Adjust the ammeter scale so that you can accurately measure the current in the circuit (i.e., use the lowest scale possible for the most precision). Record the measured value of the current. Remember to include units with all numbers.
(And remember, you’re in the milliamp range. Record as mA and for calculations remember 1 mA = 0.001 A.)
Imeasured = ______________
5. Unplug the electric circuit kit (this is for safety). Turn off the multimeter.
6. According to Ohm's Law the electrical current, I, that will flow through the circuit is,
I = V where R is the resistance of the resistor you measured in Part 1 and V is the voltage of the power supply that you measured in Part 2.
Calculate the current that should flow through your resistor using Part 1 and 2 values.
Icalculated = ______________
7. Compare the calculated value of current and the measured value of the current.

_________________

Electrical Circuits
Questions
1. If you left the resistance the same, but doubled the voltage of the circuit what would happen to the current? Is the current proportional or inversely proportional to the voltage?
2. If you left the voltage the same, but doubled the resistance in the circuit what would happen to the current? Is the current proportional or inversely proportional to the circuit resistance?
Part 4
Section A
1.
Series Circuits
Using the electric circuit kit assemble the following simple circuit (Leave the electric circuit kit unplugged until the circuit is all ready to go). The kit contains a set of sockets and wires with clips to make it easy to connect the lights to the power supply.
2.
3.
4.
Figure 5 Simple Circuit
Leave the ammeter on the 200 mA amp scale.
When everything is set, plug in the electric circuit kit and turn on the multimeter.
Record the brightness of the lamp (bright, dim, etc.) and the current from the power supply. The subjective brightness is actually hard to estimate until you have completed the next section.
Lamp brightness
Current from power supply
________________
________________
Power used by circuit (P = IV) ________________
5. Unplug the electric circuit kit. Turn off the multimeter.

Electrical Circuits
Section B
1. Add a light bulb in series to the first as shown below. Set up the wires and lights to look like the sketch.
Figure 6 Simple Series Circuit
2. Record the brightness of the lamp (bright, dim, etc.) and the current from the power supply.
Lamp brightness
Current from power supply
Power used by circuit
________________
________________
________________
3. Unplug the electric circuit kit. Turn off the multimeter.
Section C
1. Finally add a third bulb in series to the circuit as shown.
Figure 7 Series Circuit
2. Record the brightness of the lamp (bright, dim, etc.) and the current from the power supply.
Lamp brightness ________________
Current from power supply
Power used by circuit (P = IV)
________________
________________
3. Unplug the electric circuit kit. Turn off the multimeter.

Electrical Circuits
Questions
1. What happens to the brightness of each bulb when an additional bulb is added in series to a circuit?
2. What happens to the current when an additional bulb (or electrical device) is added in series to the circuit?
3. Does the effective resistance of the circuit increase or decrease when an additional bulb is added in series to a circuit?
4. What happens if you unscrewed one of the bulbs? (Try this.)
Part 5 Parallel Circuits
Section A
1. Connect the simple circuit you had in Part 4, Section A.
2.
Figure 8 Simple Circuit
The parallel circuit will draw more current and with three lights can exceed the 200 mA maximum of the ammeter as presently set up. Let’s change it to handle up to 10 A. Switch the red lead to the 10 A receptacle and switch the range to the 20 m/10 A position. The readings will
3. now be in amps but with less significant digits.
When everything is set plug in the electric circuit kit. Turn on the multimeter.
4. Record the brightness of the lamp (bright, dim, etc.) and the current from the power supply.
Lamp brightness
Current from power supply
________________
________________
Power used by circuit (P = IV) ________________
5. Unplug the electric circuit kit. Turn off the multimeter.

Electrical Circuits
Section B
1. Add an additional bulb to the circuit in parallel as shown. Set up the wires and lights to look like the sketch.
2.
Simple Parallel Circuit Figure 9
When everything is set plug in the electric circuit kit. Turn on the multimeter.
3. Record the brightness of the lamp (bright, dim, etc.) and the current from the power supply.
Lamp brightness
Current from power supply
Power used by circuit (P = IV)
________________
________________
________________
4. Unplug the electric circuit kit. Turn off the multimeter.
Section C
1. Add one more bulb to the circuit in parallel as shown.
Figure 10 Parallel Circuit
When everything is set plug in the electric circuit kit. Turn on the multimeter. 3.
4. Record the brightness of the lamp (bright, dim, etc.) and the current from the power supply.
Lamp brightness
Current from power supply
Power used by circuit (P = IV)
________________
________________
________________
5. Unplug the electric circuit kit. Turn off the multimeter.

4.
5.
3.
Electrical Circuits
Questions
1. What happens to the bulb brightness when an additional bulb is added in a parallel circuit?
2. What happens to the current leaving the power supply in a parallel circuit when an additional bulb is added?
Does the effective resistance of the circuit increase or decrease when an additional bulb is added in parallel to a circuit? The effective resistance is the voltage divided by the current.
What happens if you unscrewed one of the bulbs? (Try this.)
Your house is wired in parallel. Explain why a fuse will blow in your house if you turn on too many electrical appliances at once.

Electrical Circuits
Units
A multimeter can measure resistance, R, voltage, V, and current, I.
Resistance is the measurement of how difficult it would be to get an object or material to carry an electric current. This is measured in ohms,  .
Voltage is the measurement of the potential of an electrical source to create an electric current in a circuit. Voltage is measured in volts, V.
Current is the measurement of the amount of electric current in a circuit. Current is measured in amps, A.
Reading a Multimeter
The value that appears in the display of the meter is the measurement reading, but to get the unit correct you must look carefully at where you have the dial set.
There are three sections to the dial corresponding to resistance, voltage, and current.
Inside each of the sections are settings. At each setting a value is written that indicates the maximum reading that can be made at that setting. The letter following the number at the setting is a multiplier that you use to get the true size of the reading in the standard unit.
Example
Display shows 0.999. Setting is 2K 
The correct measurement is 999 
Multipliers
M – multiply by 1,000,000
K – multiply by 1,000 m – divide by 1,000
 – divide by 1,000,000
V
A
