Lab 8 PHSC 1021 Circuit Construction Lab(1) (1)

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Arkansas Tech University *

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1021

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Electrical Engineering

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Apr 3, 2024

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Circuit Construction 1 Objectives • Discuss basic electricity relationships for simple, series and parallel, circuits • Build circuits from schematic drawings • Provide reasoning to explain the measurements and relationships in circuits Introduction The phenomenon we associate with “Static Electricity” is really due to stationary, excess charge that has built up on an object. Walking across carpet and getting a charge or the static “cling” of socks just out of a dryer are examples. We measure amounts of charge in Coulombs . One Coulomb of charge represents some 6,250,000,000,000,000,000 individual electrons (for negative charge) or protons (for positive charge). The flow of "electricity" really means the movement of charges. They move because there is an electric field. This is just like saying that water will flow downhill because there is a gravitational field. When water is stored up high, like in a tank, it has potential energy by virtue of this position. We can let the water flow down pipes and extract this energy to do useful work . The same is true for charges in a conductor. An electric field will make the charges flow “down” the wire. Analogously, a battery is a "storage" device that can store charges (like the water tank) in a way that the charges have potential energy. When the charges flow, we can extract this energy to do useful work. Batteries (or any electric energy source) are characterized by their potential to do work. We define electric potential , or simply "potential difference" as the amount of work each Coulomb of charged delivered. Potential difference is measured in volts . The "12- volts" indication for a battery means that 12 Joules of work can be extracted for every Coulomb of charge that flows. In the water analogy, electric potential is like the height of the water above ground or the water pressure that that creates. The current of a circuit is defined as the amount of charge that flows every second; the units of current are Amperes . So, one Ampere means that one Coulomb of charged has flown past a certain point in a second. This is analogous to the amount of water (mass) that flows through a pipe. f l o w G r a v i t a t i o n a l F i e l d  P E E l e c t r i c F i e l d + + + + + + + W i r e C h a r g e , q C u r r e n t , I  P E

Circuit Construction 2 Suppose we have two water storage tanks that supply water to two different houses. Both tanks are otherwise identical. In the first tank/house we use normal pipe. But for the second tank/house, we use a section of pipe that has gravel in it. Which tank/house situation will the most water flow? Apparently, the pipe with the gravel in it will restrict, or more correctly, resist the flow of water . A similar situation happens in electric circuits. Electrical resistance will oppose the flow of charge. As the charge flows through something that has resistance, some of the charge's energy is deposited. A device that generically offers resistance to current flow is called a resistor . Although real resistors come in many different shapes and sizes, the electrical symbol is shown below. The unit for resistance is the Ohm . R As you might imagine in the water analogy, if our gravel pipe "resistor" has more resistance, then less water "current" will flow. But, if we increase the water pressure, we could make more current flow. In the same manner electric potential, current flow, and resistance are all related. Ohm's law relates them as V = IR (1) The simplest type of circuit will have a voltage source and one resistor (and connecting wires) as shown in figure 1. ? ? ?

Circuit Construction 3 One of the things that you will do in this lab is to vary the electric potential (voltage) applied to the circuit and see how the current flowing through the circuit changes. You will construct a graph of current flowing vs. applied voltage. If Ohm's law is valid, then your graph will be linear. Re- arranging Ohm's law: I = 1 R V (2) This has the form of a straight line: y = m x + b . So, the slope (remember rise/run ?) is related to the resistance of the circuit. Below is a table of parts of a circuit. It also includes how those parts will look in the PhET simulation and how they look in a schematic of a circuit. Object Simulation Appearance Schematic Appearance Battery Resistor Light Bulb Wire R V Figure 1 Simple Circuit Schematic

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Circuit Construction 4 Voltmeter Ammeter 2 resistors connected in series 2 resistors connected in parallel Activity 1- Observing Voltage Relationships 1. Open the PhET Circuit Construction Kit: DC -Virtual Lab 1 on Blackboard or you can go to the simulation on the PhET website ( https://phet.colorado.edu/en/simulation/circuit- construction-kit-dc-virtual-lab ) 2. Drag out three batteries. Measure the voltage of each using the voltmeter and record the voltage in a table like the one shown. To use the voltmeter, you should place one lead of the meter on one side of the battery and the other lead on the other side of the battery or batteries. Then move the batteries end to end as below to measure combined voltage. Figure 2 can be used as a reference of the different battery combinations. 1 2 3 1+2 1+2+3 Figure 2 Battery Combinations

Circuit Construction 5 3. Describe the relationship between the number of batteries and the voltage and explain what you think might be happening. 4. What could you vary to test your description about the relationship? (To change the battery characteristics, click on the center of the battery and a slide bar will appear at the bottom of the screen.) Run several tests recording your data in an organized table. Activity 2 Ohm’s Law 1. In the simulation, create the circuit shown in the schematic to the right. Note that it is made of a battery, resistor, ammeter, and wires. Click on the resistor and change its resistance to 100.0 ohms (you can use the slider or the arrows at the end of the slider to change the values). Then click on the battery, so that the slider bar appears that allows you to change the voltage of the battery. 2. Set the voltage to 10 V and observe the new current reading on the ammeter. Record the voltage and current in the table below. 3. Increase the voltage by 10 V and again record the voltage and current in the table. 4. Complete the table by incrementing the voltage by 10 V each time up to 120 V. Battery Voltage (V) 1 2 3 1+2 1+2+3

Circuit Construction 6 V and I Data for an Ideal Resistor Potential (V) Current (A) 5. Using Excel and the data from table 1, make a graph of current vs. voltage. Make sure to label the axes. Current should be on the y-axis and voltage on the x-axis. Find the equation of the best-fit line and write it here. Then save a copy of your graph to include in the lab worksheet. y = (3) 6. The following questions refer to equation 2 from the Introduction. I = 1 R V Remember that this is the equation for Ohm’s Law rearranged into y=mx + b, where b=0. a) What is your independent variable in equation 2? b) What is your dependent variable in equation 2? c) What constants from equation 2 represent the slope? d) What is the numerical value of your slope from equation 3? e) Use your answers to c and d above to find the R you measured. f) How does the R you measured compare to the R value you chose in step 1?

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Circuit Construction 7 Activity 3 - Using Voltage in Series Circuits 1. Use the simulation to build the 3 circuits below with a battery at about 12 volts and light bulbs. Use the voltmeter and ammeter to measure the voltage of the battery and the current through the circuit. As shown in figure 3 below, to measure the current the ammeter must be connected in the circuit, and the voltage is measured across the component using the voltmeter. Record bulb brightness with descriptive language such as either remains unlit , dimly lit , s omewhat bright , or very bright. Table of Voltages for Series Circuits # of bulbs Battery voltage (V) Current through the circuit (A) Brightness of bulbs 1 2 3 1. Summarize the relationships you observed and explain what you think is happening. 2. Test to see if changing the battery voltage causes you to modify any of your conclusions. Explain what you measured and any conclusions you draw from your tests. Series Circuit 1 Series Circuit 2 Series Circuit 3 Figure 3: Simulation of a Light Bulb Connected to a Battery. The current through the circuit is measured with an ammeter, and the voltage across the battery is measured with a voltmeter.

Circuit Construction 8 3. What happens when you take a wire out of a circuit? Explain what you think is happening 4. Test using the voltmeter or ammeter in different ways. For example: Does it matter if you take the reading on the left or right of the battery? Switch the meter ends? Describe your tests and results. Activity 4 - Using Voltage in Parallel Circuits 1. Repeat Activity 3 using the 3 parallel circuits shown below. 2. Make a new table and answer the questions. References 1. This lab was adapted from an activity provided by: University of Colorado. (). PhET Interactive Simulations - Physics , PhET: Interactive simulations. Retrieved at May 8, 2020, from the website https://phet.colorado.edu/en/contributions/view/3121 Parallel Circuit 1 Parallel Circuit 2 Parallel Circuit 3