DC Circuits and Ohm s Law
Contents Part I: Objective Part II: Introduction Part III: Apparatus and Setup Part IV: Measurements Part V: Analysis Part VI: Summary and Conclusions
Part I: Objective In this experiment, you will measure electric potential as a function of current through two different objects. You will use the data to determine which object is ohmic and which is non ohmic. For the ohmic object, you will use your results to find resistance.
Part II: Introduction DC Circuits A DC (direct current) circuit is a circuit in which electric current flows in one direction only. Current is the time rate of flow of electric charge. The direction of current flow is from positive to negative.! Current has symbol I! Current is measured in amperes (A) Electric potential (or voltage) is a measure of the electric potential energy per unit charge.! Electric potential has symbol V! Electric potential is measured in volts (V) Resistance is a measure of the difficulty of the flow of electric charge.! Resistance has symbol R! Resistance is measured in ohms (Ω)
Part II: Introduction Ohm s Law The class of materials referred to as ohmic follow Ohm s Law which states The electric potential V across a conductor is proportional to the current I through it. The constant of proportionality is the resistance R. In equation form: V = IR Materials which do not follow Ohm s law are referred to as non-ohmic. Examples of ohmic materials are copper and aluminum. Examples of non-ohmic elements are diodes and transistors.
Apparatus You have been provided with:! Power suppy! Multimeters! Electrical leads (bananaalligator and bananabanana)! LED light bulb! Alligator clips! Resistor
Setup circuit Begin to assemble the circuit: Turn off the power supply and both multimeters. Connect the negative terminal of the power supply to the COM port of the first multimeter. This multimeter will measure the current through the circuit and can be called an ammeter.
Setup circuit Connect the VΩmA port of the first multimeter to one end of the resistor.
Setup circuit Connect the remaining free end of the resistor to the positive terminal of the power supply.
Setup circuit Connect the COM port of the second multimeter to one end of the resistor.
Setup circuit Connect the VΩmA port of the second multimeter to the other end of the resistor. This multimeter will measure the potential across the resistor and can be called a voltmeter.
Completed circuit The circuit is now complete.
Power on Turn down both the voltage and current by turning both the voltage and current control knobs on the power supply all the way counterclockwise. Turn the power supply on. Turn the first multimeter (the ammeter) to 200m DCA. Turn the second multimeter (the voltmeter) to 20 DCV. It is OK if the readings are negative.
Power up Turn the current control knob on the power supply about halfway up. Turn up the voltage control knob on the power supply until the power supply reads 20.0 Volts. The multimeters should read approximately 67 ma and 20 V. Have an instructor check your setup and initial your lab report.
Caution! CAUTION! The resistor may get very hot! Turn the power supply off whenever you are not collecting data.
Lab Report Lab Report 1: Write the objective of your experiment. Lab Report 2: Write the relevant theory of this experiment. Lab Report 3: List your apparatus and sketch your setup.
Part IV: Measurements Turn the power on the power supply. Record the voltage and current in a table in your lab report. Read voltage and current from the multimeters. Turn down the voltage slightly so that it decreases by about 1 V. Record the voltage and current in your lab report. By turning the voltage slightly, continue to decrease the voltage in increments of about 1 V recording the voltage and current in your lab report. When you have finished recording data, turn off the power supply and the multimeters.
Part V: Analysis Click the icon to launch Graphical Analysis. Use Graphical Analysis to plot voltage as a function of current for the resistor.
Part V: Analysis Lab Report 4: Are the data linear? Lab Report 5: Is the resistor ohmic or non-ohmic? How can you tell? In Graphical Analysis, click Analyze then Linear Fit to obtain a linear fit to your data. Double click on the pop-up box and turn on Show Uncertainties. Print your graph and include it with your lab report. Lab Report 6: What does the slope of a voltage vs current graph represent? Explain. Lab Report 7: Using the information from your graph, record the resistance of the resistor with its uncertainty. Compare this value to the value stamped on the resistor. Do they agree? Comment on any differences.
Using the LED Do not disassemble the circuit. Next we will replace the resistor with the LED-resistor combination. Instructions for the circuit are on the following slides.
Using the LED Carefully remove the resistor from the circuit without disassembling the remainder of the circuit.
Make sure the power supply and both multimeters are turned off. Connect the lead from the ammeter to the LED bulb side.
Make sure the power supply and both multimeters are turned off. Connect the lead from the positive terminal of the power supply to the resistor side.
Setup circuit Connect the COM port of the second multimeter to the LED bulb side. Connect the VΩmA port to the resistor side.
Completed circuit The circuit is now complete.
Power on Turn down both the voltage and current by turning both the voltage and current control knobs on the power supply all the way counterclockwise. Turn the power supply on. Turn the first multimeter (the ammeter) to 20m DCA. Turn the second multimeter (the voltmeter) to 20 DCV. It is OK if the readings are negative.
Power up Turn the current control knob on the power supply about halfway up. Do not adjust the voltage control yet. The multimeters should read approximately 0 ma and 0 V.
Part IV: Measurements LEDs are strange Record the voltage and current in a table in your lab report. Read voltage and current from the multimeters. Turn up the voltage slightly so that the voltage increases by about 0.5 V. Record the voltage and current in your lab report. You will notice that as you initially increase voltage, the current remains at zero. Once current begins to flow, the LED will light and the current will vary with voltage. Continue to increase the voltage in increments of about 0.5 V until the LED lights or the current is non zero. Record the voltage and current in your lab report.
Part IV: Measurements LEDs are strange Once the LED lights, increase voltage in increments of 0.1 V. Record the current and voltage in your lab report. Read voltage and current from the multimeters. Continue to increase the voltage in increments of about 0.1 V until a maximum value is reached. Do not exceed 3.5 V or 15 ma. Turn off the power supply and the multimeters once you are finished collecting data.
Part V: Analysis Click the icon to launch Graphical Analysis. Use Graphical Analysis to plot voltage as a function of current for the LED. Lab Report 7: Are the data linear? Lab Report 8: Is the LED bulb ohmic or non-ohmic? How can you tell?. Print your graph and include it with your lab report.
Part VI: Summary and Conclusions Lab Report 9: Outline briefly the steps of your experiment. Lab Report 10: List your experimental results and comment on how they agreed with the expected results. Lab Report 11: List at least three sources of experimental uncertainty and classify them as random or systematic.
Wrap it up! Check that you have completed your Lab Report. Your report should include copies of the voltage vs current graphs.