OhmsLaw LAB IVC FL 2018

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Irvine Valley College *

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Electrical Engineering

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Feb 20, 2024

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+ - Resistor Black Red Digital METER (Current) Red Black Red Black Digital METER (Voltage) Physics 2B/4B Ohm’s Law Ohm’s Law The fundamental relationship among the three important electrical quantities current , voltage , and resistance was discovered by Georg Simon Ohm (1789-1854). The relationship and the unit of electrical resistance were both named for him to commemorate this contribution to physics. One statement of Ohm’s law is that the current through a resistor is proportional to the voltage across the resistor. In this experiment you will test the correctness of this law using a voltmeter and ammeter. V = IR These electrical quantities can be difficult to understand, because they cannot be observed directly. To clarify these terms, some people make the comparison between electrical circuits and water flowing in pipes. Here is a chart of the three electrical units we will study in this experiment. Electrical Quantity Description Unit Water Analogy Voltage or Potential Difference A measure of the Energy difference per unit charge between two points in a circuit. Volt (V) Water Pressure Current A measure of the flow of charge in a circuit. Ampere (A) Amount of water flowing Resistance A measure of how difficult it is for current to flow in a circuit. Ohm ( WW ) A measure of how difficult it is for water to flow through a pipe. Figure 1

Physics 2B/4B Ohm’s Law OBJECTIVES  Determine the mathematical relationship between current, potential difference, and resistance in a simple circuit. MATERIALS 2 Voltmeters (digital) Wires Adjustable 5-volt DC power supply One resistor 100 Ω Light Bulb assembly PRELIMINARY QUESTIONS 1. Given a resistor with a value of 1000 Ohms, what current is drawn from a power supply with an EMF of 100V? Show all calculations. PROCEDURE 1. In this lab you will be using one digital multimeter to measure the current, and a second one to measure the voltage. However, first use one of the multimeters as an “ohmmeter” and measure the value of the resistor. Record this value in the provided Excel Sheet and your notebook. 2. You will measure two devices (One resistor 100 Ohms and one light bulb. From this data you will make two plots one for each device (Voltage vs. Current). By analyzing the Voltage vs. Current data you will determine if Ohm’s law holds in each case. You must have two data tables and two graphs in your lab book in addition to the procedure and diagram of the apparatus. 3. With the power supply turned off, and all the dials turned counter clockwise, connect the power supply too one of the resistors with wires as shown in Figure 1 .  Be careful about the red and black wires from the multimeters, and the red and black terminals on the power supply, connect everything as shown in Figure 1 using Figure 2 as a reference . If you are not sure of your connections please call the instructor over.  Note that when used to measure amperage (Current I), the multimeter must be placed “in” the circuit (see figure 2 below) . Think of the multimeter as a flow measurement system. The wires are like pipes that the current (water) must flow through to make a measurement. The multimeter must also be set to measure amps [ A ] . There are several settings and we will need to choose a specific one to complete the lab.  When used to measure the voltage across the resistor, the multimeter is connected so that one lead is at high potential and the other at low potential . Like measuring the height of a waterfall. One measurement must be at the top and the other at the bottom.  One thing that is often not clear is how to plug the wires into the multimeters for each of the measurement modes (Voltage and Current). For the voltage mode you will need to set the multimeter to 1V, 5V, or 20 V depending on which type or multimeter you have and connect the leads to the ground line (usually black) and the voltage line (usually has a V or D.C. next to it and is red) make sure you don’t select the A.C. mode. For the current mode you will need to plug the red wire into the terminal marked ( A ) or Amps, there is also usually a number by the symbol to represent the amount of current that terminal can handle. Make sure this number reads greater than or equal two 1A!

Voltage or resistance Negative Positive Ground or (-) Set to middle position Use to set voltage Amps Ohms Figure 2.0 Multimeter and Power supply: Yellow instrument is the multimeter (yours may look different) and the grey and black instrument is the power supply. Physics 2B/4B Ohm’s Law 4. Make sure the power supply is set to 0.0 V, and turn on the power supply. 5. Now increase the voltage on the power supply to approximately 0.5 V. Check the current meter to see that you are getting a reading. Second: IMPORTANT: check to see that V/I is roughly the value of the resistance you measured in part 1. When the readings are stable, record the value of the voltage and current (from the multimeter) a table. 6. Increase the voltage by about 0.5 V so that you get 1.0 V. When the readings are stable record the values in a data table provided. Repeat this process until you reach a voltage of 5.0 V. Make sure you record the error in the measurement for both voltage and current. This will be the least significant digit. So if the meter reads 5.00V you report 5.00V +/- .01 V assuming the hundredths place is stable. 7. You will need to repeat this procedure for the light bulb. ANALYSIS 1. Once you have taken all the data from part (6) create a graph of Voltage vs. Current (Voltage on the vertical axis current on the horizontal). Once you have entered your data insert a best fit line to fit your data.

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Physics 2B/4B Ohm’s Law 2. Linear regression was used to fit your data with a line in step 1 and the equation of this line is displayed on the graph. Do the points seem to lie along a line, i.e., is there a proportional relationship between voltage and current? i. If so, write the equation for each run in the following form: potential = constant ´ current. (Use a numerical value for the constant.). ii. Does this seem consistent with y = mx+b? Explain the physical nature of each term in the equation. 3. You will need to calculate the error in your slope by modifying the excel spread sheet and using the min-max method . 4. Calculate a percent difference between the slope and the expected value of the resistance. Discuss the result. 5. Resistance, R , is defined using R = V/I where V is the potential across a resistor, and I is the current. R is measured in ohms ( W ), where 1 W = 1 V/A. Does the slope in your best fit line equation fall within the appropriate range of values? Use a number line to show agreement or disagreement. 6. Discuss the accuracy and precision of the measurement. 7. Does your resistor follow Ohm’s law? Explain why you know this is true! 8. For the light bulb does the V vs. I curve follow Ohms law? Can you provide an