Measuring Resistance Using Ohm's Law - Activity 1 | PHYS 102, Lab Reports of Physics

Download Measuring Resistance Using Ohm's Law - Activity 1 | PHYS 102 and more Lab Reports Physics in PDF only on Docsity! 7 DC Electric Circuits A resistor resists electric current. What this means is that a resistor slows down electrons moving through them. We can use a resistor to control the amount of current owing in a given circuit. When a battery is connected across a resistor, current will ow through it. The voltage (V ) and current (I) of a resistor are related by the resistance R according to Ohm's law: V = IR. (7.1) When two resistors are connected in series as shown in g. 7.1(a), the total resistance is increased and given by: Rseries = R1 + R2. (7.2) When two resistors are connected in parallel as shown in g. 7.1(b), the total resistance is decreased and given by: Rparallel = ( 1 R1 + 1 R2 ) −1 . (7.3) In this lab you will build several circuits with dierent resistor congurations. R1 R2 R1 R2 R1 R2 R3 (a) (b) (c) Figure 7.1: Resistor circuits congured in (a) series, (b) parallel and (c) a combination of series and parallel. Meters will be used to measure the current through and voltage across a resistor. An ammeter is a device that measures current. When connected in series with a circuit element, it measures and displays the current passing through it. A voltmeter is a device that measures voltage. When connected in parallel with a circuit element, it measures and displays voltage across it. By nding the current owing through a resistor and the voltage across it, the resistance can be found using equ. 7.1. The principles in this lab should reinforce the following concepts: + Electric current and voltage + Series and parallel circuits + Ohm's law 31 7 DC Electric Circuits 7.1 Activity 1: Measuring Resistance Using Ohm's Law Equipment: resistor, voltmeter, ammeter, variable power supply, solderless breadboard, wire 1. Your instructor will demonstrate how to set-up a circuit using the solderless breadboards. After this demonstration, hook up the resistor, power supply and meters as shown in g. 7.2. The voltmeter has two leads (red and black) with alegator clips on the end. Clamp the red lead at the top of the resistor. Clamp the back lead at the bottom of the resistor. Ammeter Voltmeter V R Figure 7.2: Circuit schematic for variable voltage source showing arrangement of meter. 2. Adjust the power supply until the voltmeter reads 1 V. Record the value of the current measured by the ammeter in tab. 7.1. Make sure your units are correct. Table 7.1: Voltage and current data for a given resistor. Voltage [V ] Current [mA] 1 2 3 4 5 6 7 8 9 10 3. Repeat step 2 for the voltages listed in tab. 7.1. 32 7.6 Data Analysis: Parallel Resistor Circuits R1 R2 R3V Figure 7.4: Circuit schematic for three resistors arranged in simple parallel with a battery. Table 7.5: Table of values for the circuit shown in g. 7.4. R1 R2 R3 Rtotal V [V ] 5 I [mA] R [kΩ] 10 20 12 2. Start by determining the equivalent resistance for the circuit using equ. 7.3. Record this value in column 5, row 4 of tab. 7.5. 3. Using equ. 7.1 determing the current from the voltage in column 5, row 2 and the equivalent resistance calculated in step 2. Record this value in column 5, row 3 of tab. 7.5. 4. Since the circuit is simple parallel, all of the resistors will see the same voltage. Copy the voltage (5 V ) into columns 2, 3 and 4 of row 2 in tab. 7.5. 5. Now, use equ. 7.1 to determine the current through each of the resistors. Record these values in tab. 7.5 and in tab. 7.6 as Icalc. 7.6 Data Analysis: Parallel Resistor Circuits 1. Construct the circuit shown in g. 7.4. 2. Turn on the 5 V power supply. Using the ammeter, measure the current through each resistor and record this data as Imeas in tab. 7.6. Table 7.6: Calculated and measured currents for resistors in parallel. R1 R2 R3 Icalc [mA] Imeas [mA] Error [%] 35 7 DC Electric Circuits 3. Calculate the percent error between your measured value and calculated value of current for each resistor. Record these values in tab. 7.6. 36