Jumper cables and ohm's law

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EXPERIMENT 10: Introduction to Electrical Circuits

Read the entire experiment and organize time, materials, and work space before beginning. Remember to review the safety sections and wear goggles when appropriate

Objectives: To build and understand the principles of a simple electric circuit, and

To learn to use the various functions of a digital multimeter (DMM), including ammeter, voltmeter, and ohmmeter.

Materials: Student Provides: Computer with spreadsheet software From LabPaq: Digital multimeter, DMM 5 Jumper cables 3 1.5V Batteries with holders 100 Ω Resistor Discussion and Review: Reading and understanding a circuit diagram and then building an electric circuit from it is a skill similar to reading a construction blue print. In this laboratory exercise, we’ll become familiar with the basic terms and symbols needed to build electric circuits. Some of the most common symbols used in the circuit diagrams you will be using are shown below. Name Symbol(s) Example DC Power (Battery) Flashlight batteries VDC AC Power, VAC Wall power outlet Resistor, R Heating element, lamp, etc Lamp Light bulb On/Off switch Light switch

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Name Symbol(s) Example Wire Any wire Capacitor Current storage Diode Digital thermometer Inductor Wire coil Meter Symbols: We will be using a digital multimeter (DMM) which can be used as a voltmeter to measure voltage, an ammeter to measure current, and an ohmmeter to measure resistance. The symbols for individual meters are: Voltmeter to measure voltage

Ammeter to measure current Circuit Drawings: The illustrations in this manual reflect the way circuits are traditionally drawn to show how components are connected. Your actual circuits will not look as nice and neat as the

diagrams since connecting cables will not be in perfectly straight lines and angles. At right is a photo of how an actual circuit might look. The left photo shows how two sets of jumper cables are connected to one resister in parallel.

Reading and understanding the color codes of resistors: To calculate the value of a resistor, use the color-coded stripes on the resistor and the following procedures, plus the table on the following page:

1. Turn the resistor so that the gold or silver stripe is at the right end of the resistor. 2. Look at the color of the first two stripes on the left end. These correspond to the

first two digits of the resistor value. Use the following table to determine the first two digits.

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3. Look at the third stripe from the left. This corresponds to a multiplication value. Find the value using the table below. Multiply the two-digit number from Step 2 by the number from Step 3. This is the value of the resistor in ohms.

4. The fourth stripe indicates the accuracy of the resistor. A gold stripe means the

value of the resistor may vary within 5% from the value given by the stripes. A silver stripe means the value of the resistor may vary within 10% from the value given by the stripes.

Resistor Color Codes: Read the code with gold or silver stripe on right end.

With a little practice you soon will be able to quickly determine the value of a resistor by just a glance at the color -coded stripes. Assume you are given a resistor whose stripes are colored from left to right as brown, black, orange, and gold. To find the resistance value:

Turn the resister to where its gold stripe is on the right.

The first stripe on the left is brown, which has a value of 1. The second stripe from the left is black, which has a value of 0. Since the first two digits of the resistance value are 1 and 0, this means this value is 10.

The third stripe is orange, which means to multiply the previous value by

1,000.

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Thus the value of the resistance 10 time 1000 or 10 x 1000 = 10,000 ohms. 10,000 ohms can also be expressed as 10 kilohms or 10 k Ω ohms.

The stripe in Step 1 is gold; this means the actual value of the resistor may

vary by 5%. Since 5% of 10,000 = 0.05 x 10,000 = 500, the actual value in our example will be somewhere between 9,500 ohms and 10,500 ohms.

DIGITAL MULTIMETER OPERATING INSTRUCTIONS: It is important that you read and understand the following instructions plus pay attention to the special cautions noted below or you could damage the multimeter and/or blow a fuse. Replacement fuses can be purchased at electronics stores. Direct any use specific questions to your course instructor. Digital Multimeters (DMM): It is important to familiarize yourself with the DMM’s operations now so you can take accurate measurements without damaging the meter. Multimeters are so called because they can measure three different qualities of a circuit. These qualities, their symbols, and their basic units of measurement are summarized in the table below: A different model of multimeter may be included in your LabPaqs, so generic DMM operating instruction as well as those specific to the Cen-Tech DMM are included here. Regardless of the DMM model in your LabPaq, you should thoroughly review its accompanying instructions in addition to the ones discussed below. Lead Wires (Cen-Tech): Lead wires must be connected correctly. The black lead is normally connected to the bottom terminal labeled COM for common which is also called ground. The red lead must be connected to the corresponding terminal for what you want to measure. For voltage, resistance and low DC current, use the middle terminal labeled VΩmA. (V=volts, Ω=resistance in ohms, mA=milli-ampere). For DC current above 200mA, use the top 10ADC terminal. Detailed instructions as to when to use the middle vs. the upper terminal for the red lead are given for each of the specific measurement instructions below. Always read instructions carefully to be certain you plug the leads into the correct terminals for the appropriate quantity of what you want to measure.

Symbol: V I and A R Measurement:

Units: Voltage

Volt Current Ampere

Resistance Ohm or Ω

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Basic Operations: The CEN-TECH digital multimeter (DMM) has a circular range dial knob and a separate On-OFF switch. The central dial must be in the appropriate position for the operation you want to perform. The dial has the following positions starting with DCV and going clockwise:

DCV - To measure DC voltage: settings 200mV, 2000mV(2V); 20V, 200V, 1000V

ACV To measure AC voltage: settings: 200 & 750V

1.5V(4.0mA) & 9V(25mA) - To measure battery charge for 1.5V & 9V batteries only

DCA – To measure DC current – settings: 200µA, 2000 µA (=2mA); 20mA, and

200mA (= 0.2A)

10A – To measure DC current greater than 200mA

hFE – To measure transistor values

To measure diode voltage drop

Ω - To measure resistance – settings: 200 Ω, 2000 Ω, 20K Ω, 200K Ω, 2000K Ω Use of the DMM as a DC Voltmeter: To measure voltage (V) difference, the DMM leads are connected to the ends of the component(s) while the circuit is energized. Connect the positive red lead close to the + end of the battery and the negative black lead to the – end of the battery.

1. Turn the center dial to the appropriate DCV setting. The setting selected must be higher than the quantity of expected volts or the DMM fuse may blow out! For a 1.5V system, set at the 2V setting, for a 9V system set it at the 20V setting, etc. If you do not know the range of your value, start with the highest range and switch down to lower ranges as necessary. This will prevent damage to the meter that might occur if you select a range too low for the voltage you are measuring.

2. Plug the red cable lead into the center VΩmA jack. Plug the black cable lead into

the bottom COM jack.

3. Switch the multimeter on via the ON-OFF switch.

4. To measure the voltage, carefully touch the appropriate points in the circuit with the tips of the multimeter’s probes.

5. Read and record the measurement.

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6. When testing is complete, turn off the DMM; remove the test leads, and store your DMM.

Use of the DMM as an Ohmmeter: To measure the resistance (R) of a component such as a resistor, the component must be disconnected from the circuit. You will get an incorrect measurement if the component is in the circuit. You may also damage the meter if the component is in the circuit and the circuit is also energized. This is the only DMM reading that requires the circuit to be disconnected. You may measure circuit resistance up to 2000K ohms.

1. Turn the range selector switch to an appropriate Ω setting higher than the expected ohms. For a 100 Ω resistor, set the range switch on the 2000 Ω setting, etc.

2. Plug the red cable lead into the center VΩmA jack. Plug the black cable lead into

the bottom COM jack.

3. Switch on the multimeter via the On-OFF switch.

4. Touch the test leads together. The meter should read “0” Ω, (Ohms)

5. Carefully touch the appropriate points in the circuit with the tips of the probes to measure the resistance.

6. Read measurement.

7. If the reading is “1”, set the range selector switch to the next higher Ohm (Ω)

position.

8. When testing is complete, turn off the DMM; remove the test leads and store your DMM.

Use of the DMM as an Ammeter (Current meter): To measure current (I) the leads of the meter must be connected into the circuit. Wherever the meter is inserted into the circuit make certain that the red lead is closest to the + end of the battery along the circuit and that the black lead is closest to - end of the battery. It is very important that the multimeter be used in series as part of the circuit when measuring current instead of in parallel outside of the circuit as when measuring voltage differences. Improper use may damage the meter and blow the fuse making the multimeter inoperable.

1. Turn the range Selector Switch to the 10 A (amperes) position. Always start with the highest range if the amperage is unknown.

2. Plug the red cable lead into the top10 A jack. Plug the black cable lead into the

bottom COM jack.

3. Switch the multimeter on via the On-OFF switch.

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4. Insert the multimeter in series with the circuit to be tested.

5. Read measurement. If the reading is less than .2 A switch the red cable lead to

the center VΩmA jack and set the range selector switch to the 200mA.

6. Read and record measurement. If you need a current reading in Amps instead of milliAmperes, simply divide the mA reading by 1000.

7. When testing is complete, turn off the DMM; remove the test leads and store your

DMM. Here is an example of how easily you can blow a fuse if your DMM is used incorrectly. Assume you started in the 200mA (0.2A) position and used the DMM as a current meter for a circuit with a 1.5V battery and a 1ohm resistor; you would blow its fuse immediately! Ohm’s law, V = IR, is also stated as I = V/R. Thus I = 1.5V/1Ω = 1.5A or 1,500 mA. This is 7.5 times the limit of the 0.2A setting. For this example circuit you would need to use the 10A setting.

Always turn off your DMM when you have completed your measurements by moving the switch to the “off” position. Otherwise the DMM battery will be used up prematurely and have to be replaced. Maintenance:

Remove battery if not in use for long periods. Store unit in dry location Other than the battery and the fuse, this DMM has no replaceable parts. Repairs should be done by a qualified technician.

Battery/fuse Replacement:

Remove the test leads form the multimeter Turn the unit over and remove both screws with Philips screwdriver. Remove the back cover Remove the battery or fuse and replace with a new 9V battery or 250mA fast-

acting fuse. Replace cover and retighten screws.

Your DMM may also be used to make AC Voltage Measurements; Transistor (hFE) measurements, battery charge measurements, and diode measurements. However, these measurements will not be used in any of the physics experiments in this LabPaq.

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PROCEDURES: Set up the following data table to use for these experiments:

DATA TABLE: Resistance based on color bands: ____Ω; % uncertainty ____ (from color band) DMM-Measured Resistance ______ Ω Measured V

(V) Measured Current

(A) Calculated R = V/I

1.5V battery 3V battery 4.5V battery

Part 1: Before assembling the following circuits, set the DMM as an ohmmeter. Slide the function switch to Ω and the dial range to 2 kΩ. Check the resistance of the resistor by touching the two DMM leads to the two wires extending from each side of the resistor. Record this value. A. Your first circuit will consist of a 1.5V battery in its holder and a 100-ohm resistor.

You will use 3 separate jumper cables to set up the circuit as shown below in the illustration below.

1. Set the DMM as an ammeter. Slide the function switch to A and the dial range to mA.

2. Connect the first jumper cable

(1) to the negative end of the battery holder and to the 100 Ω resistor. Do this by simply opening the jaw of an alligator clip at one end of a jumper cable and firmly clasping that jaw around the metal tail, wire, or extender of the item to be connected into the circuit. Metal must touch metal in all connections.

3. Connect the second jumper cable (2) to the other end of the resistor and the

black lead of the DMM.

4. Connect the third jumper cable to (3) the red lead of the DMM and the positive end of the battery holder.

Remember that it is very important when measuring current to use the multimeter in series, which means that it is inside and part of the circuit as shown above. When measuring voltage differences the meter must be in parallel, which means it is outside of the circuit as shown in B. below. If you confuse these procedures you will blow out the DMM’s fuse!

5. Take the mA reading and record it in the data table.

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6. You measured the current at only one point, call it point “P.” Is the current the same everywhere in a simple circuit like this? To find out, rearrange the jumper cables and meter to measure the current at a second point, call it point “Q.” Discuss your findings in your conclusions.

B. Remove the DMM from the above circuit and close the circuit by connecting the jumper cables’ alligator clips that previously connected the DMM within the circuit.

1. Set the DMM as a voltmeter. Slide function switch to V , and the range dial

to 2V.

2. Set up the DMM as a voltmeter, which requires that it be parallel to and outside of the main circuit as shown in the next illustration. Touch the DMM’s positive red lead to the jumper cable connection at the positive end of the battery holder (A) and touch the DMM’s negative black lead to the jumper cable connection at the negative end of the battery holder (B). Now take the V reading between points A and B and record in the data table.

3. Reverse the DMM leads at points A and

B by moving the black lead to point A and the red lead to point B. Observe, then record and explain your observation.

4. Reposition the voltmeter to take a voltage reading between A and C, first with

the leads in one position and then with the leads reversed. Record and explain these voltage readings.

5. Reposition the voltmeter to take a voltage reading between C and D, first with

the leads in one position and then with the leads reversed. Record and explain these voltage readings. Note: You will want to thoroughly discuss these observations in your lab report summary.

Part 2:

A. Again set up the DMM as an ammeter within a circuit as in the previous Part 1 A-1, but this time you will add a second 1.5V battery in series with the first 1.5V battery as shown at right.

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1. To do this you will simply insert a second battery holder and jumper cable next to the original battery holder so that two batteries are in the circuit between the ammeter and the resister.

2. When the circuit is again complete, take the mA reading and record in the

data table.

B. Remove the DMM from the circuit shown in Part 2A above and close that circuit.

1. Set up the DMM as a voltmeter parallel to the circuit with the leads attached around both batteries as shown at right.

2. Set the function switch on V and

the range dial to 20V.

3. Take a V reading and record in the data table.

Part 3:

A. Again set up the DMM as an ammeter as in Part 2A, but this time add a third 1.5V battery in series with the other two 1.5V batteries.

1. To do this you will simply insert a third battery holder and jumper cable next to

the original battery holder so that three batteries are now in the circuit between the ammeter and the resister.

2. When the circuit is again complete, take the mA reading and record in the

data table.

B. Remove the DMM from the three batteries and the resistor connected in Part 3A above and close that circuit.

1. Set up the DMM as a voltmeter parallel to the circuit with the leads attached

around all three batteries. 2. Set the function switch to V and the range dial to 20V.

3. Take a V reading and record in the data table.

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Calculations and Graphing: For each of the three previous procedures calculate the resistance from the measured current and voltage: R = V/I.

1. Use an xy scatter graph to graph voltage on the y-axis versus current on the x-axis.

2. Use the linear fit trendline function of Excel® to add the slope of the line to the

graph.

3. What is the significance of the slope?

4. How do the graph and the slope of the line relate to Ohm’s law?

5. Can this series of experiments be considered a verification of Ohm’s law? Why or why not?

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