MINISTRY OF EDUCATION SECRETARY OF MEDICAL AND TECHNOLOGICAL EDUCATION FEDERAL INSTITUTE OF EDUCATION, SCIENCE AND TECHNOLOGY - AM. GRADUATION TEACHING BOARD ANA LÍVIA FERREIRA PONTES JOSEH EMMANUEL DE SOUSA GOMES LUCAS DA SILVA COSTA PAULO HENRIQUE L COUTO VICTOR HUGO TAVARES NEVES PHYSICS III MANAUS 2018 ANA LÍVIA FERREIRA PONTES JOSEH EMMANUEL DE SOUSA GOMES LUCAS DA SILVA COSTA PAULO HENRIQUE L COUTO VICTOR HUGO TAVARES NEVES EXPERIMENTAL PRACTICE OHM LAWS Report presented as a partial requirement for approval in the discipline PHYSICS III, of the course Mechanical Engineering, taught by Prof.MsC J. Anglada Rivera. MANAUS 2018 3 1. INTRODUCTION Electrodynamics aims to study situations where electrically charged particles, electrons, lose their electrostatic equilibrium and begin to move in an ordered direction.Every time there is a displacement of electrons towards a specific direction has called electrical current. In order for this movement to occur it is necessary to have a Potential Difference (ddp) between two points known as electric voltage, it will be responsible for allowing the locomotion of these electrons and thus flowing the current.As electrons move, they encounter natural oppositions presented by matter that characterize electrical resistance. The laboratory practice reported in this paper was directed to the analysis and understanding of such phenomena of electrodynamics as expressed by the Ohm Laws.Being divided in two practical parts, the first one has the purpose of the analysis of the First Law of Ohm and the following of the Second Law of Ohm. 4 2. EXPERIMENTAL PROCEDURES 2.1. PRIZE PRACTICE OF PHYSICS LABORATORY 3 Objective: To verify experimentally the law R = U / i Materials: Ohm's Law Kit, Voltmeter, DC Power Supplies, Ammeter, Ruler and Wiring Wires. For the following experiment, we set up the circuit using the Ohm Law Kit, selecting the Nickel-Chromium material to determine the intensity of the current, so that through the formula R = U / i we obtain the value of the resistance D(cm) L (cm) V(V) I (A) R (Ω) 0,072 20 3,3 0,30 11,0 0,072 40 5,0 0,45 11,1 0,072 0,072 60 80 6,6 8,3 0,60 0,75 11,0 11,067 We obtained the following plot of V = f (I), and through its angular coefficient R = V1-V2 / (i1-i2) we obtain the same average value of the nickel-chromium wire resistance: R = 11Ω. 5 Calculating the error in relation to the Nominal Value of the Resistance through the resistivity of the nickel chromium material.We adopted the resistivity value 1.1x10 ^ -6 Ω .cm, for this we used the formula for home measured value: E = (| R-Rn |) / RN) * 100 Error 1: 0.035% Error 2: 0.018% Error 3: 0.0011% Error 4: 0.0012% Choosing now another Resistance, the Iron, we obtained the following values: D(cm) L (cm) V(V) I (A) R (Ω) 0,0051 20 2 0,30 6,67 0,0051 40 3 0,45 6,67 0,0051 60 80 4 0,60 6,67 5 0,75 6,67 0,0051 We obtained the following graph of V = f (I), and through its angular coefficient R = V1-V2 / (i1-i2) we obtain the same average value of the resistance of the iron wire: R = 6Ω. Calculating the error in relation to the Nominal Value of the Resistance through the resistivity of the nickel chromium material.We adopted the resistivity value 1x10 ^ -7 Ω .cm, for this we used the formula for home measured value: E = (| R-Rn |) / RN) * 100 Error 1: 0.01708% Error 2: 0.00854% Error 3: 0.00566% Error 4: 0.00425% 6 By means of these experimental data it is possible to prove the relationship R = U / i, confronting with the nominal values and to perceive a very small error index, corroborating the affirmation of the first Ohm Law. 2.2. SECOND PHYSICS LABORATORY PRACTICE 3 I - Dependence of R of length L For the following experiment, the Ohm Law Kit was used, selecting the IRON material to perform the variations of length L (m) according to the table below: S(cm2) V(V) I (A) L (cm) p (Ohm*cm) R(Ohm) 0,000000081 0,4 0,15 0,4 0,000000540 2,67 0,000000081 0,4 0,1 0,6 0,000000540 4 0,000000081 0,4 0,07 0,8 0,000000579 5,71 0,000000081 0,4 0,06 1 0,000000540 6,67 For the obtained diameter, shown in the table above and with the variation of the length for the same material, we obtained the resistance values through the formula: R = V / i: V(V) I (A) L (cm) R(Ohm) 0,4 0,15 0,4 2,67 0,4 0,1 0,6 4 0,4 0,07 0,8 5,71 0,4 0,06 1 6,67 We obtained the values of resistivity through the formula: ρ = R * S / L S(cm2) L (cm) p (Ohm*cm) R(Ohm) 0,000000081 0,4 0,000000540 2,67 0,000000081 0,6 0,000000540 4 0,000000081 0,8 0,000000579 5,71 0,000000081 1 0,000000540 6,67 Calculating the error in relation to the Nominal Value of the Resistance through the resistivity of the iron material.We used the resistivity value 0.5 ^ -6 Ω .cm, for this we used the formula for home measured value: E = (| ρ - ρ n |) / ρ N) * 100 Error 1: 0.4% Error 2: 0.592% Error 3: 0.866% Error 4: 0.894% 7 II - Dependence of R of cross section S For the following experiment, the Ohm Law Kit was used, selecting the material Nickel-Chrome to perform the variations of cross-sections S (cm2) according to the table below: I L p D(cm) S(cm2) V(V) (A) (cm) (Ohm*m) Rn(Ohm) R (Ohm) 0,036 0,00102 3,7 0,35 10 0,00108 13,4 10,57142857 0,051 0,00204 3,7 0,68 10 0,00111 6,7 5,441176471 0,072 0,00407 3,7 1,64 10 0,00092 3,36 2,256097561 For the circuit below, we applied the voltage V according to the table, and we obtained the following current measurements for the cross section change: S(cm2) V(V) I (A) 0,00102 3,7 0,35 0,00204 3,7 0,68 0,00407 3,7 1,64 With the current values obtained it was possible to calculate the resistivity values ρ through the formula ρ = V * S / I * L, we obey the following values: S(cm2) (V) 0 ,00102 ,7 0 ,00204 ,7 0 ,00407 ,7 V (A) 3 ,35 3 ,68 3 ,64 I (cm) 0 0 0 0 1 0 L ρ (Ohm*m) 1 0,00 108 1 0,00 111 1 0,00 092 The resistivity values for the different cross sections of NickelCromo allowed us to calculate the resistance values of each wire through the formula: R = ρ * L / S 8 (cm2) ,00102 ,00204 ,00407 S (cm) 0 0 0 0 0 0 L (Ohm*m) 1 ,00108 1 ,00111 1 ,00092 ρ R (Ohm) 0 1 0,57142857 0 5, 441176471 0 2, 256097561 Once the values are plotted, the following graph shows the inversely proportional relationship between R and S, in addition that it does not follow a linear pattern and that as the section increases the resistance decreases and consequently increases the intensity of the current by the conductor.This is due to a larger area of electron passage and a decrease in the collision between free electrons. 9 3. CONCLUSION In practical terms, it is not yet possible to same results obtained through the theory, and this will occur by a series of factors. Among the main ones: due to lack of instrument calibration, human error and constant approximation of values.These final results will always be accuracy, which will set the error variation for more or paramenes In the case of this experiment, after a series of steps, it is concluded that the experimental accuracy is relatively good considering the low nominal error. It was possible to verify the approximations of the formulas of the first and second laws of Ohm from nominal to experimental.