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The Dependency of the Magnetic Field on the Diameter of a Solenoid ENS L202 – Scientific Communication Almohaned Aljahdali Eder Bastos John Baycroft Nat Steinsultz Pavel Vasilev Summary An expression for the magnetic field produced by a solenoid can be obtained using Ampere’s Law, which gives a proportional relationship between the current through a solenoid and the magnetic field produced by it. Ampere’s Law also dictates an inversely proportional relationship between the diameter of the solenoid and its magnetic field. By comparing three different solenoids of diameter 2.2 cm, 4.2 cm and 8.9 cm, at 50mV, 100mV and 150 mV of voltage, this experiment verified those relationships to be true with 2% to 36% error. Introduction: In this experiment, the magnetic field within three solenoids of differing diameters is studied. The definition of a magnetic field is a field of force produced by a magnetic object or particle. The field is detected by the force it exerts on other magnetic materials and moving electric charges. At any given point it is specified by both a direction and a magnitude; as such it is a vector field. The term solenoid refers to a loop of wire, often wrapped around a core in the form of a tightly packed helix, which produces a magnetic field when an electric current is passed through it (Figure.1) Solenoids are important because they can create controlled magnetic fields and can be used as electromagnets. The term also refers specifically to a magnet designed to produce a uniform magnetic field in a volume of space. Figure 1 Magnetic Field Lines in a Solenoid In order to measure the magnetic field, we used a Vernier Magnetic Field Sensor (Figure 3) which uses a Hall Effect transducer. The Hall Effect is the production of a voltage difference across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current. Figure 2 - Hall Effect Transducer This sensor is sensitive enough to measure the Earth’s magnetic field and it can also be used to study the field around permanent magnets, coils, and electrical devices. The magnetic field within a solenoid can be predicted using Ampere’s law B dl I 0 enc (1) Where B is the magnetic field, dl is the differential length along the path integral, μ0 is the permeability of free space, and Ienc is the current enclosed by the path integral. Under the idealized condition of an infinitely long solenoid, the magnetic field inside the solenoid is B=μ0nI (2) where n is the number of turns per unit length, sometimes called the "turns density", μ0 is the permeability of free space, and I is the current running through the solenoid. When the length, L, is not significantly greater than the diameter, D, of the solenoid, the equation for the magnetic field becomes B= μ0nI*L/√(L2 + D2) (3) Experimental Setup The experiment consisted of five major components: the multi-meter, power source, solenoids, resistor and magnetometer (Figure 4). The multi-meter was used to measure the voltage across the solenoid, which was generated by the power source. A 50Ω resistor was placed in series with the solenoid and the power supply. This was used to limit the current through the circuit so that it did not overheat... The magnetic field created within the solenoids was recorded through a USB connection to a PC. Using solenoids with three different diameters (2.1cm, 4.2cm, and 8.8cm) we were able to see how the magnetic field varied with respect to voltage as well as diameter. Figure 3 - Vernier magnetic field sensor Figure 4 - Photograph of the experimental set up Procedure First, we connected the power supply to the 50ohm resistor and the first solenoid in series. We placed the multimeter across the terminals of the solenoid, and supplied voltage to the circuit. With the solenoid powered, we placed the magnetometer in the center of the solenoid, and observed the results on the computer attached to the magnetometer. These steps were repeated for each combination of the three solenoids and three voltages: 50mV, 100mV, and 150mV. Results The measured characteristics of each solenoid are summarized in Table 1. # of turns L (cm) D (cm) Resistance (Ohm) 1 88 11.53 2.186 0.409 2 90 11.57 4.235 0.728 3 97 11.9 8.876 1.49 Table. 1 In order to calculate the hypothetical magnetic field, the current running through the solenoid must be used. Since voltage and resistance across the solenoids were measured, current can be calculated using Ohm’s Law. The measured voltage, the calculated current and the measured magnetic field inside the solenoid are summarized in Table 2. 2.2cm 4.2cm 8.9cm V (mV) I (mA) B (mT) measured 0.05088 0.124400978 0.160 0.10016 0.244889976 0.315 0.15056 0.368117359 0.475 0.04954 0.068049451 0.072 0.100838 0.138513736 0.134 0.15074 0.20706044 0.228 0.0503678 0.033803893 0.027 0.1009 0.067718121 0.060 0.15045 0.100973154 0.090 Table. 2 The theoretical field inside the solenoid can be calculated using the equation 0 NI L2 + D 2 (4) . The ideal magnetic field was calculated and used to determine the percent error for each trial. The results are summarized in Table 3. B (mT) calculated 2.2cm 4.2cm 8.9cm Error %Error 0.117224562 -0.042775438 -36.4901667 0.230762817 -0.084237183 -36.50379392 0.346881487 -0.128118513 -36.9343762 0.062465602 -0.009534398 -15.26343676 0.127147889 -0.006852111 -5.389087788 0.190069941 -0.037930059 -19.95584283 0.027755499 0.000755499 2.721980267 0.055601592 -0.004398408 -7.910579062 -0.007093563 -8.556105955 0.082906437 Table. 3 As illustrated by Figures 5, 6, and 7, each solenoid demonstrates a linear relationship with current, which is the expected behavior. The errors for the two largest solenoids were close to the instrument error associated with the magnetometer. However, the measurements for the smallest solenoid were consistently imprecise. The fact that the three errors for this solenoid were so close to each other suggests that it is the result of a systematic error. It is possible that this error comes from an inaccurate measurement of the solenoid characteristics, or a fault in the construction of the solenoid Figure 5 – The calculated and measured magnetic field vs. current for a 2.2 cm diameter solenoid Figure 6 – The calculated and measured magnetic field vs. current for a 4.2 cm diameter solenoid Figure 7 – The calculated and measured magnetic field vs. current for a 2.2 cm diameter solenoid Figure 8 - The measured magnetic field vs. the current for all three solenoids and a linear trend line for each separate data set By using Equation 4, the slope of the linear relationship between current and magnetic field for each solenoid can be determined. The predicted slope is compared with the slope from a least squares fit of the data is summarized in Table 4. Predicted Slope 2.2cm 4.2cm 8.9cm Measured Slope 0.940775948 0.915357709 0.817769286 Error in Measured Slope 1.292508 0.00346 1.121084 0.141833 0.938045 0.020471 Table. 4 The measured slope does not exactly agree with the calculated slope, but the inverse variation of slope with diameter does hold consistent. The difference between the calculated and measured slopes is probably the result of not having enough data points to get a truly accurate measurement of the slope. Repeating this experiment with more data points should yield a more accurate slope. Conclusion The objective of our experiment was to attempt to observe the behavior predicted by Ampere's law, and we did accomplish this. As expected, the smallest solenoid produced the most magnetic field, and the largest produced the least. What did not match up as precisely were the exact results, which had varying amounts of error. It is impossible to pinpoint what caused all the error, because the environment in which the experiment was performed was riddled with potential disruptions. These included computers with their spinning hard drives, the lack of a way to clamp the magnetometer in place, the lack of a core in the solenoid, and many other omissions that reflect the lack of resources we had in performing the experiment. An improved version of the experiment might use solenoids with metallic cores, which would ideally be wound by machines for a greater number of turns; it would also likely take place with a purpose-built apparatus and environment. References http://myweb.msoe.edu/~schenstr/Course_Web_Pages/2009-10-2-Winter_PH-2020/Lab%20Instructions/PH2020%20Exp%2007%20-%20The%20Magnetic%20Field%20Produced%20by%20a%20Solenoid.pdf http://www.vernier.com/probes/mg-bta.html http://en.wikipedia.org/wiki/Magnetic_field http://www.netdenizen.com/emagnet/solenoids/thinsolenoid.htm