NORTHERN ILLINOIS UNIVERSITY PHYSICS DEPARTMENT Physics 211 E&M and Quantum Physics Spring 2018 Lab #4: Electronic Circuits I Lab Writeup Due: Mon/Wed/Thu/Fri, Feb. 12/14/15/16, 2018 Background The concepts of current flow, voltage and resistance in actual electrical circuits are confusing when first introduced to students. There are several misconceptions about electricity that may be the source of student s general confusion about how an electrical circuit actually works. The purpose of this lab is to explore the concepts of series and parallel electrical circuits by applying Ohm s law, resistor addition rules and current junction rules to circuit diagrams and then to actual circuits 1. Overview Current is defined as the rate at which charged particles move through a material, copper wire as an example. Voltage is defined as the electromotive force that pushes the charged particles through the conducting material. Resistance is defined as the conducting material s ability to slow down or limit the flow of current. Comparing an electrical circuit to a pressurized water system helps relate these concepts to something we already understand. Think about the water system in a typical home. The water supply that comes into the house has a certain pressure (created by pumps or a water tower) so that when you open a faucet you get a nice stream of water. This water pressure is comparable to the voltage supplied by a battery or power supply to a circuit. If all the water outlets in a house are closed there is no water flowing through the pipes, but the pipes are still full of water due to the pressure. Again, this is comparable to an electrical circuit that is full of electrons, which are ready to light a lamp as soon as the light switch is turned on. Finally, the water system in a house is made up of different sized pipes that offer different resistances to the flow of the water, e.g. a small pipe restricts the water flow more than a large pipe.
Let s start with a simple series circuit, which means all the components of the circuit are connected in a single loop, or end-to-end. The required components are a voltage source (battery or power supply); a resistor, something that the will carry the generated current, and a switch to open and close the circuit. A small light bulb will be used for the resistor because it gives a visual indication that current is flowing in the circuit. Ohm s Law relates voltage, resistance and current in a circuit at a constant temperature: V IR The resistance of a specific component in a circuit is determined primarily by the material it is made of and temperature. An extreme example is an incandescent light bulb, which radiates light due to high temperatures of the tungsten filament in the bulb. The increase in temperature of the tungsten filament can increase the resistance of the bulb by a factor of 10-15 compared to its cold resistance value. Ohm s law implies that if we have a constant voltage, which is usually the case in direct current circuits, then a higher resistance will result in a lower current and vice versa. In a series circuit, current will be constant throughout the circuit, and the voltage will be different through each component but sum to the total voltage of the power source. The total resistance in a series circuit will be the numerical sum of the resistances of all components. In a parallel circuit Kirchhoff s Rules tell us that current branches at junctions in parallel circuits just like water flows when pipes split at a junction: I I I 1 2 3 Parallel circuits contain these junctions or branches, where current is split, but the voltage is constant. The equivalent resistance of resistance in the parallel branches is calculated as follows: 1 1 1 1... R R R R Eq 1 2 3 2
2. Procedure The TA will explain how to use the voltmeters and ammeters to measure the voltage and current in the circuits. Apparatus includes:. Power supply (black). 3 Bulbs. Voltmeter. Ammeter. Rheostat (variable resistor). Wires. Simulation software on lab computer A. Activity #1 How do you Think Circuits Work? (no equipment used) Write all answers to questions underlined in blue into your lab notebook. Circuit A Light Bulb 1. In circuit A above we have a single light bulb (the cross) that has a specific resistance of about 6 ohms. If the voltage supplied is 7 volts, what is the expected current flowing in the circuit using Ohm s law? (write answer in your lab notebook) Circuit B 2. In circuit B we have added a 2 nd light bulb (the crosses) with the same resistance in series with the first. Let the voltage still be 7 volts. (a) Do you expect the intensity of the light bulbs in Circuit B to be greater or less then the intensity of the single light bulb in Circuit A why? Discuss this with your group members and come to a consensus and record this in your lab notebook. (b) What is the expected current flowing in Circuit B, using Ohm s law? 3
Circuit C 3. In Circuit C we have created a parallel circuit where the 2 bulbs (two crosses) are parallel to each other in the circuit. Let the voltage still be 7 volts. (a) Compare Circuits B and C, which one do you expect will output the most light why? Discuss and record your consensus in your lab notebook. (b) Calculate the equivalent resistance of the 2 bulbs in parallel. 4. Rank the circuits according to the total resistance of each circuit from least to greatest. Explain your reasoning. B. Activity #2 Circuit Simulation Software You will be using software during your lab experiment to design and test/evaluate circuits. Open the P211 Lab Simulations folder on your lab computer and double click on the Circuit Construction Kit (AC & DC) Virtual Lab. Explore the tools for building a simple circuit using wires, bulbs, batteries, etc. 1. Build simulation Circuit A above, picking a battery voltage of 5-10 volts and a 6-10 ohm resistance value for the bulb. How does the current determine the brightness of the bulb? Experiment with different values of battery voltage and bulb resistance. 2. Build simulation Circuit B above, picking a battery voltage of 5-10 volts and a 6-10 ohm resistance value for the bulb. How does the current determine the brightness of the bulb? Experiment with different values of battery voltage and bulb resistance. Now, use the same voltage and set each bulb s resistance to the same value you used in Circuit A above and compare the brightness of the 2 bulbs in Circuit B to the single bulb in Circuit A. Record your observations in your lab notebook. 3. Build simulation Circuit C below, pick a battery voltage of 5-10 volts and a 6-10 ohm resistance value for the bulb. How does the current determine the brightness of the bulb? Experiment with different values of battery voltage and bulb resistance. 4
Now, use the same voltage and set each bulb s resistance to the same value you used in Circuit B above and compare the brightness of the 2 bulbs in Circuit B (series) to the 2 bulbs in Circuit C (parallel). Develop a theory in your group to explain the differences in brightness of the bulbs in Circuits B & C. Record your observations in your lab notebook and be prepared to present them to the other groups: C. Activity #3 Building Circuits using Lab Components 1. You will now make a simple circuit using lab components. Draw the following circuit in your lab notebook (make certain you use the headings in this lab writeup: Section C: Activity #3, Part 1): Using your digital multimeter, measure the resistance of your rheostat (variable resistor), and set its resistance to 5 ohms. Build the circuit above where the rheostat is in series with the ammeter and the power supply. Have the TA check your circuit before turning on the power supply (set it to 5 volts). What is the total current you measured in the circuit?. Use Ohm s law to calculate the actual resistance of the rheostat based on the voltage reading on the power supply. How does it compare to the value measured with the digital multimeter? NOTE: DO NOT MOVE THE RHEOSTAT KNOB, SINCE YOU WILL USE IT AS A CONSTANT RESISTANCE DEVICE IN THE REST OF THE EXPERIMENT 2. Draw a circuit diagram with a rheostat and an ammeter in series with the single bulb. Let's call this Circuit A1. With your digital multimeter, measure the resistance of one of your two bulbs and record this in your lab notebook. Make sure the power supply is turned off. Now build Circuit A1 using the components in the lab. Have your TA check your circuit prior to turning on the power supply. (a) Set the power supply to 3 volts. (b) Measure the total current of the circuit. 5
(c) Use Ohm s law to calculate the resistance of the circuit. (note that this resistance is due to the bulb and the resistor in the circuit with some small contribution from the wires & power supply) (d) Repeat steps (a)-(c) for power supply settings of 5, 7, 9, 11, 13 volts. Record all this information in your lab notebook and also create a table in Excel as shown below: Voltage (V) 3 5 7 9 11 13 Current (A) Calculated Resistance (Ohm) (e) Using Excel, make a plot of voltage vs. current and fit it with a straight line. What physical information should the slope give? Based on your previous digital multimeter measurements of the resistances of the rheostat and the light bulb, what is the total resistance of your circuit? Does this measurement agree with the slope? 3. Draw a circuit diagram with an additional bulb in series with the bulb in Circuit A1. Let's call this Circuit B1. With your digital multimeter, measure the resistance of your two bulbs and record this in your lab notebook. Make sure the power supply is turned off. Now build Circuit B1 using the components in the lab. Have your TA check your circuit prior to turning on the power supply. (a) Set the power supply to 3 volts. (b) Measure the total current of the circuit. (c) Use Ohm s law to calculate the resistance of the circuit. (d) Repeat steps (a)-(c) for power supply settings of 5, 7, 9, 11, 13 volts. Record all this information in your lab notebook and also create a table in Excel as well. Comment on the relative brightness of the bulbs in Circuit B1 vs. the single bulb in Circuit A1. 6
3. Questions 1. Put all of your answers to the questions underlined in blue in your lab writeup. 2. Write an explanation using your own words that describes how you think current flows in a series circuit with 2 bulbs and a parallel circuit with 1 bulb in each of the parallel branches based on your measurements and observations from Activity #1. 3. In Activity #3, Part 2, (the circuit with a single bulb), comment on how good your fit is using the percent error between your experimental value (from the slope of your line) and the accepted value (from the digital multimeter measurements): % Error Experimental AcceptedValue AcceptedValue 100 noting that percent errors of 0.1% are excellent, 1%-5% are good, 5%-15% are reasonable (expected in this lab course), and greater than 15% are poor measurements. 4. From Activity #3, Part 3, plot the voltage verses current for your data. Does the slope of the line agree with the total resistance of your circuit (rheostat and two bulbs)? Comment on how good your fit is. 5. Are your curves in your plots for Activity #3, Parts 2 & 3 perfectly straight lines? One parameter we have not considered is the temperature of the light bulbs. With more current, the temperature of the light bulb increases. Explain whether the resistance of the light bulb increases or decreases with temperature to account for the nonlinear behavior in your plots. 7