Experiment 15: Determining Resistances using Ohm's Law, Exercises of Law

Download Experiment 15: Determining Resistances using Ohm's Law and more Exercises Law in PDF only on Docsity! Experiment 15: Ohm’s Law Figure 15.1: Simple Series Circuit EQUIPMENT Universal Circuit Board Power Supply (2) DMM’s 150 Ω Resistor (R1) 330 Ω Resistor (R2) 560 Ω Resistor (R3) Miniature Light Bulb and Socket (R4) (1) Jumper (6) Wire Leads Figure 15.2: Schematic: Simple Series Circuit 1 2 Experiment 15: Ohm’s Law Advance Reading Text: Ohm’s Law, voltage, resistance, current. Objective The objective of this lab is to determine the resistance of several resistors by applying Ohm’s Law. Students will also be introduced to the resistor color code and refresh their graphing skills. Theory Ohm’s Law states that the current, I, that flows in a circuit is directly proportional to the voltage, V , across the circuit and inversely proportional to the resistance, R, of the circuit: I = V R (15.1) In this experiment, the current flowing through a resis- tor will be measured as the voltage across the resistor is varied. From the graph of this data, the resistance is determined for Ohmic resistors (Ri, i = 1, 2, 3). Non- Ohmic resistors (R4, light bulb) do not obey Ohm’s Law. Ammeters are connected in series so that the cur- rent flows through them. The ideal ammeter has a re- sistance of zero so that it has no effect on the circuit. Real ammeters have some internal resistance. Voltmeters are connected in parallel to resistive elements in the circuit so that they measure the poten- tial difference across (on each side of) the element. The ideal voltmeter has infinite internal resistance. Our voltmeters have approximately 10 MΩ (10×106 Ω) internal resistance so that only a minuscule amount of current can flow through the voltmeter. This keeps the voltmeter from becoming a significant path for current around the element being measured. Resistors are labeled with color-coded bands that indi- cate resistance and tolerance. The first two color bands give the first two digits of the value (Fig. 15.3). The third band gives the multiplier for the first two, in pow- ers of 10. The last band is the tolerance (Fig. 15.3), meaning the true value should be ±x% of the color code value. Refer to Table 15.1 for standard color val- ues. There is no need to memorize the color codes for lab. For example, a resistor that has two red bands and a black multiplier band has a resistance of 22 Ω. Figure 15.3: Color Code Schematic Color Number Multiplier Black 0 100 Brown 1 101 Red 2 102 Orange 3 103 Yellow 4 104 Green 5 105 Blue 6 106 Violet 7 107 Grey 8 108 White 9 109 Tolerance Gold 5% Silver 10% (No Band) 20% Table 15.1: Resistor Color Code Values 5 Procedure Part 1: Measures of Resistance 4. Determine the nominal resistance for the three resis- tors: interpret the color codes according to the color code chart provided at the bottom of the page. 5. Measure the actual resistance of the three resistors using the ohmmeter and record them in the Table. 6. Do the measured resistances fall within the tolerance of the nominal resistance for each resistor? If not, what might cause their measured values to differ from those listed? (3 pts) 7. An ideal ammeter has no resistance; this ammeter does have a small resistance. Measure the resistance of the ammeter. (2 pts) RA = Color Number Multiplier Black 0 100 Brown 1 101 Red 2 102 Orange 3 103 Yellow 4 104 Green 5 105 Blue 6 106 Violet 7 107 Grey 8 108 White 9 109 Tolerance Gold 5% Silver 10% (No Band) 20% Table 1 R1 Color Code Code Value 10∧ ± % Nominal Resistance ± Ω Measured Resistance (5 pts) R2 Color Code Code Value 10∧ ± % Nominal Resistance ± Ω Measured Resistance (5 pts) R3 Color Code Code Value 10∧ ± % Nominal Resistance ± Ω Measured Resistance (5 pts) 6 Part 2: Ohm’s Law Applied 8. Build a simple series circuit using R1, an ohmmeter, an ammeter, and a jumper. 9. Measure the equivalent resistance of the circuit us- ing the ohmmeter and record this value in the table. Include units and uncertainty. 10. Is the equivalent resistance what we expect it to be – is it equal to the resistance of R1 plus the resis- tance of the ammeter? What would cause Req to be different than R1 +RA? (4 pts) 11. Remove the ohmmeter and connect the unplugged power supply to the circuit. Connect a voltmeter to the circuit, across the power supply leads (in paral- lel). 12. Have your TA check your circuit. Plug in the power supply and turn it on. 13. Test Ohm’s Law (V = IR) by verifying that current increases linearly with applied voltage. Apply 1V, 2V, 3V, and 4V to the circuit. Measure current and voltage and record them in the table. Include units and uncertainty. 14. Repeat the Ohm’s Law Applied procedure for R2 and R3. Table 2 R1 circuit Equivalent Resistance: Req= Voltage Current R2 circuit Equivalent Resistance: Req= Voltage Current R3 circuit Equivalent Resistance: Req= Voltage Current (18 pts) 7 Part 3: Non-Ohmic Device 15. Build a series circuit using R4, the light bulb. 16. Measure the current and voltage as you increase the applied voltage in 0.2V increments up to 2.0V, the continue in 1.0V increments up to 4.0V. Adjust the voltmeter scale to obtain the most significant figures possible. 17. Turn off and unplug the power supply; turn off the DMM’s. 18. If necessary, Tables 2 and 3 may be copied to the back of the extended worksheet. Table 3 R4 circuit Voltage Current (12 pts)