NORTHERN ILLINOIS UNIVERSITY PHYSICS DEPARTMENT Physics 211 E&M and Quantum Physics Spring 2018 Lab #6: Magnetic Fields Lab Writeup Due: Mon/Wed/Thu/Fri, March 5/7/8/9, 2018 Background Magnetic fields are something we have seen and heard about since we were children. The magnetic force of even small magnets has been observed for 1000 s of years and the source of these fields has been intensely studied. This lab will explore and measure magnetic fields produced by bar magnets and electromagnets. 1. Overview A magnet can be modeled as a dipole with the North Pole and south poles always being found together there is no monopole magnet, unlike electrical charges. The magnetic field lines (outside the actual magnet itself) always go from the North Pole towards the south pole of the magnet and a compass needle will always point in the direction of the B-field vector. The number and concentration of magnetic field lines is roughly proportional to the magnitude of the B field. Magnetic flux, is a measure of the number of magnetic field lines through a perpendicular surface area and has units of Wb (Weber). The magnitude of the B-field is defined as the magnetic flux per unit area and has units of Tesla, which is equal to 1 Wb/m 2. B B A BAcos The formulas for determining the magnetic field at various distances from a bar magnet are complex, but the magnitude is generally inversely proportional to distance. Materials are magnetized when a large % of the domains have a magnetic moment that points in the same direction. Magnetic fields and current carrying wire loops Current flowing in a conductor will generate a magnetic field that encircles the wire, which can be seen by placing compasses around the wire.
A loop of wire will concentrate the B-field lines at the center of the loop. So, stacking multiple wire loops carrying a current is used to produce a relatively large B-field inside the loops. This device is a solenoid or more commonly referred to as an electromagnet because it produces mag field lines very similar to a bar magnet. Since the magnetic field lines are concentrated along the central axis of the solenoid, the equation for determining the B-field is relatively simple, but only applies to the B-field inside the solenoid. The B-field outside the solenoid is relatively weak as the picture indicates. NI B 0 0nI L where n is the number of wire loops or turns of wiren per unit length L ; I is the current in Amps. Hence, B is directly proportional to the number of turns of wire and the magnitude of the wire s current. 2
2. Procedure Apparatus includes:. Cylindrical & thin rectangular bar magnets. Plastic iron filing filled viewer. Ruler (plastic). Graduated compass. Magnetic Field sensor probe attached to LabPro and LoggerPro software. Power supply (black). On/off switch. Variable rheostat (large, low resistance ~5-11 ohms). Ammeter. Wires, alligator clips. Solenoids A. Activity #1 Visualizing Magnetic Field Lines 1. Shake up the iron filings in the mineral oil filled plastic viewer until they are evenly distributed. Position 1 of the cylindrical bar magnets in the middle of the plastic viewer with the North Pole facing to your left. Observe how the filings align along the magnetic field lines in a 3-dimensional pattern. Sketch the field lines you observe both from the top and end of the plastic viewer in your lab notebook. 2. Remove the magnet and shake up the plastic viewer again. Now place 2 cylindrical magnets in the plastic viewer with the north poles facing each other. Sketch the side view of the magnetic field lines in your lab notebook. 3. Place 1 of the rectangular bar magnets on a piece of graph paper, then arrange 8 small compasses around the bar magnet as follows: 1 at each end (poles) and 3 evenly spaced on each side. The north end of the compass will point along the direction of the magnetic field lines at these locations. Sketch out this pattern, this time putting arrowheads on your magnetic field lines to indicate the direction of the field. How do the magnetic field lines compare/differ to what you have learned about electric field lines? B. Activity #2 Measuring B (magnetic field) of a Bar Magnet 4. Use the larger compass calibrated in degrees to find earth s north. Note: you should stay away from the computers, power supplies and metal frames of the tables to get an accurate reading. Check with the other student groups in the lab everyone should find the same north direction! 3
5. Tape a couple of pieces of paper to the center of the table with the short sides parallel with north. Place a magnet on the left side of the paper and point the north pole of the magnet parallel with the red arrow on the magnet. Trace the shape of the bar magnet on the paper. Draw a line perpendicular to the magnet that bisects the center of the magnet. 6. Move the compass along the line you drew close to (but NOT touching) the magnet. The compass will flip 180 and now point to the south pole of the magnet. Rotate the compass until the 0 degrees mark is aligned with north. 7. Slowly move the compass to the right keeping it centered on the line you drew until the compass needle moves 90 degrees (pointing along the east-west direction). Move the compass back and forth along the line until you find the exact point at which the needle switches to pointing east-west. Measure the distance of this point from the bar magnet and mark it with an X. This point is where the earth s magnetic field (horizontal component) is equal in magnitude, but opposite in direction to the magnetic field produced by the bar magnet. Draw a magnetic field line at this point and connect it to the bar magnet using what you learned from seeing the magnet in the plastic viewer. 8. Draw additional magnetic field lines coming from the bar magnet to represent the magnetic field produced by the bar magnet these lines should be at different distances along the line you drew perpendicular to the center of the magnet. Make a rough sketch of the bar magnet and associated magnetic field lines in your lab notebook. C. Activity #3 Magnetic Field Sensor The magnetic field sensor is composed of the wand, the amplifier, and the LabPro data acquisition device. These parts are sketched below. 4
Wand Amplifier Box Adapter Plug Analog Input Port 1 LabPro The Wand is a hollow plastic tube with a Hall effect transducer chip at one end (shown above as the circle on the right hand end of the wand). The chip produces a voltage that is linear with the magnetic field. The maximum output of the chip occurs when the area vector of the white dot on the sensor points directly toward a magnetic south pole, as shown below: 9. Make sure the magnetic field probe sensor is attached to the LabPro and the computer. Make sure the switch box is set to Low. Start the Logger Pro software on the lab computer (or close and reopen), which should bring up a magnetic field experiment program, if not have your TA set it up. 10. Position the probe wand so that the sensor (white dot) is perpendicular to the earth s magnetic field, i.e. the wand would be perpendicular to the direction of the earth s magnetic field. Also, make sure the wand is not near the bar magnet, computer, power supply, etc. it is best if you raise the probe in the air to reduce the extraneous magnetic interference. Zero out the probe using the Logger Pro software (double click on the sensor icon near upper left of screen, then the down arrow). The probe should read 0.0, or a very small value like 0.003. 5
11. Slowly move the probe along the bisector line to the X you marked on your paper (or the desk) making sure the white dot is pointing perpendicular to the line and down towards the south end of the magnet. You should get a near zero reading at this mark if not, repeat Step 10 and/or Step 7. 12. Slowly move the probe along the line you drew towards the magnet, stopping every 2-3 cm to record the magnetic field reading and distance from the magnet in your lab notebook (make a table of magnetic field and distance). Note: You will get very small readings until you get within about 10 cm of the bar magnet. Note: Although your magnetic field reading at point X was zero, you should record the magnitude of the bar magnets B-field, which you indirectly found in your PreLab activity. D. Activity #4 Electromagnet (Solenoid) You have a plastic pipe with 2 areas of insulated copper wire wrapped in varying # s of turns (loops). You will measure the B-field in each of the solenoids, one at a time using different current settings to see the relationship between the magnitude of the B-field, current flowing in the wire loops and the # of wire loops. **BE CAREFUL: YOU WILL BE USING A RELATIVELY LARGE CURRENT SUPPLY, WHICH COULD PRODUCE A SIGNIFICANT SHOCK** MAKE SURE THE SWITCH IS OPEN AND ONLY CLOSE IT DURING THE TIME YOU ARE TAKING READINGS ONLY ONE STUDENT CONTROLS THE SWITCH & POWER SUPPLY You will be using the magnetic sensor probe again to measure the magnetic field inside the solenoid. 13. Close the LoggerPro program. Set the switch box to the High setting. Start Logger Pro, go to File, then Open. Find the Physics folder, then open Experiment #25 titled Magnetic Field in a Coil. 14. Have your TA check your circuit setup which includes the following components all connected in series: a. Black power supply with switch off and voltage dial all the way to the left b. Large variable resistor c. Ammeter set to 20 Amp d. On/off switch in the open position e. Electromagnets (solenoids) on a plastic pipe alligator clips connected to the leads from one of solenoids 6
15. You will take one set of data with each solenoid at different current settings. Each solenoid has a large hole in the middle of the wire loops where you will place the magnetic sensor probe with the white dot aligned parallel with the length of the solenoid and in the middle of the pipes diameter. Record, in your lab notebook, the # of turns of wire and the length of the portion of the plastic tube that the wire turns cover for each solenoid #1 and #2. 16. One student should control the LoggerPro software collecting and recording data. Another student should operate the on/off switch and power supply to control the current magnitude, and one student should place the magnetic sensor in the solenoid. 17. Zero the magnetic sensor probe when holding it near the solenoid before closing the switch and collecting your first set of data. 18. The MAXIMUM current you should use is 2.0 A only closing the switch when everyone is ready to gather the data. 19. Make the following Table in your lab notebook: Solenoid # 1 Number of turns of wire: Length I (A) B (mt) B ni 0 and there should be a similar table for Solenoid #2. 20. For each current reading hit the green Collect button, you will get a continuous reading of the magnetic field strength in mt you just need to take data for a few seconds then hit STOP in LoggerPro. Record the maximum reading you obtain. Make sure the sensor white dot is as close to the middle of the plastic tube and facing towards the open end of the tube. Zero the sensor probe between each set of data. 21. After you are done taking data for both solenoids, set the current to 1.0 A, close the switch and note the sign of the B-field (record the sign of the B-field in your lab notebook) Open the switch, turn OFF the power supply. 22. Reverse the direction of the current through the solenoid by switching the wires attached to the power supply now see what the sign of the B-field is (record the sign of the B-field in your lab notebook) 7
Your data section should contain 4 sketches of magnetic field lines from the plastic viewer, 1 sketch of the magnetic field lines of the bar magnet; 1 data table of B-field values and distances (bar magnet) and 1 data table of current and B-field value for each solenoid. Your Results section should contain graphs of B-field magnitude vs. 2 3 (1/, 1/, 1/ ) d d d from the edge of the bar magnet use a distance scatter plot with line fits. Which of the plots best fits the data? Your results section should also contain graphs of B-field magnitude vs. current for each solenoid. Note: graph both your measured values and the theoretical values you calculated using the solenoid formula. Do line fits in Excel. 4. Questions 1. In your lab writeup, take good cell phone pictures of your lab notebook sketches and insert them into your Word document. Also answer all questions underlined in blue associated with your sketches. Use Excel to make tables of data and insert these tables into your Word document. 2. How do the magnetic field lines you observed compare to electric field lines? Consider single electrical charges vs. 2 charges. What is similar? What is different? What is the electrical equivalent of the north poles of 2 bar magnets facing each other? 3. In Step #7, explain why the compass switches from a north-south direction to an eastwest direction. What is the bar magnet's field strength at this point? 4. What does the graph of the bar magnet's B-field vs. 1/distance tell you? Which plot 2 3 (1/, 1/, 1/ ) d d d is the best fit to the data? Note: you may need to eliminate some of the farthest distance data points if you got extraneous readings due to the weakness of the bar magnet's field there. You should keep the last point ( X ) since you know this value. 5. Regarding the solenoid, what do the graphs of B-field vs. current tell you is there a linear relationship and if not, what is the relationship? When you changed the direction of the current through the solenoid, what changed and why do you think it changed? 6. How did the number of coils of wire in the second solenoid change your data? Was there a linear relationship? Comment on the comparison of your measured and calculated B-field values what could be the sources of any differences? Were the differences a constant offset or did they vary for each current setting? 8