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LLG Paris–Abu Dhabi Advanced Math and Science Pilot Class Academic year 2014-2015 Physics – Experiment Ohmic devices Objective: Study the variation with voltage of the current for some resistors or their series (parallel) combination. I. Relation between current:I and potential difference (voltage): ΔV for a resistor 1. Experiment. (1) generator with variable voltage + (2) generator with variable voltage + G G R R Q1. Reproduce the above circuit (1), adding an ammeter and a voltmeter in order to measure the current passing through the resistor and the potential difference between its terminals. E2. Set up the physical circuit which is described by the circuit diagram using resistor 1. Use the least sensitive range of the meters, you will increase sensitivity during the measurements if needed. 1st CALL: Call for the teacher BEFORE SWITCHING ON THE GENERATOR. E3. For different values of the voltage supplied by the generator (different values of the potentiometer), measure V and I. Record the measurement results in the table. Note: The data points should be spread out uniformly over the range. Resistor 1 V (V) 0 I (A) 0 E4. Replace resistor 1 by resistor 2 and repeat the previous operation. Resistor 2 V (V) 0 I (A) 0 2. Analysis of measurements. a. Current–voltage characteristic. Definition: A current–voltage characteristic is a relationship, typically represented as a graph, between an electric current and a corresponding potential difference. 2nd CALL: Call for the teacher before entering the data in the table. E5. Using the computer, represent on the same graph, the current–voltage characteristic for resistor 1 and the current–voltage characteristic for resistor 2. b. Conclusion. Q6. What can you say about the current–voltage characteristic of a resistor? Q7. Give the characteristic ( the mathematical equation) of the graphs for each resistor. LLG Paris–Abu Dhabi Advanced Math and Science Pilot Class Academic year 2014-2015 Physics – Experiment 3. Electrical resistance: R of a conductor. Electrical resistance (R) of a conductor is expressed in ohms (Ω). The instrument to measure the electrical resistance of a conductor is called an ohmmeter. The ohmmeter is connected directly between the terminals of the conductor to measure the value of its resistance. 3rd CALL: Call for the teacher before using the ohmmeter. E8. With the ohmmeter, measure the resistance of resistors 1 and 2. R1 = ………….. R2 = ………….. Q9. Compare your measurements with the characteristics obtained at question I.2.b. What do you notice? Q10. Write a general relation that links the resistance R of a resistor, the electric current I passing through it and the potential difference V between its terminals. This relation is called Ohm’s law. 4. Resistor combination. E11. Use the circuit from E2, replacing R1 with a combination of R1 and R2 in series. Collect data points and plot them as they are recorded using regressi software. Q12. Deduce the total resistance of a series combination. E13. Now replace R1 with a combination of R1 and R2 in parallel. Collect data points and plot them as they are recorded using regressi software. Q14. Deduce the total resistance of a parallel combination. II. Using the resistor color code? Four-band identification is the most commonly used color-coding scheme on resistors. It consists of four colored bands that are painted around the body of the resistor. The first two bands encode the first two significant digits of the resistance value, the third is a power-of-ten multiplier or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. The first three bands are equally spaced along the resistor; the spacing to the fourth band is wider. Each color corresponds to a certain digit, progressing from darker to lighter colors, as shown in the chart below. Color 1st band 2nd band 3rd band (multiplier) 4th band (tolerance) Black 0 0 ×100 Brown 1 1 ×101 ±1% 2 ×10 2 ±2% 3 Red 2 Orange 3 3 ×10 Yellow 4 4 ×104 Green 5 5 ×105 6 ×10 6 ±0.25% ×10 7 ±0.1% ×10 8 ±0.05% ×10 9 Blue Violet Gray White 6 7 8 9 7 8 9 Gold ×10−1 Silver −2 None ×10 ±0.5% ±5% ±10% ±20% Ex: green-blue-yellow-red is 56×104 Ω ± 2% = 560 kΩ ± 2%. The first band, green, has a value of 5 and the second band, blue, has a value of 6, and is counted as 56. The third band, yellow, has a value of 104, creating 56.104 Ω at ±2% tolerance accuracy (tolerance given by the fourth band, red). Q15. Give the value of the resistance of resistor 1 and 2 with the four-band identification. Q16. Compare those values with the measurements obtained in part I. Conclude. Order your table like it was when you came.