You will be making magnetic field measurements using a special device called a Hall Effect probe, so-named because it uses the "Hall Effect" to make its magnetic field measurements. All that you need to know about the Hall Effect probe is that it contains a thin, flat wafer of a semiconductor material and is sensitive to the DIRECTION of the magnetic field as well as its MAGNITUDE. The wafer is mounted about 1 cm back from the tip of the probe with its SURFACE-NORMAL parallel to the long axis of the probe. This results in the probe reading the greatest magnetic field strength (magnitude) when the magnetic field is parallel (or anti-parallel) with the long axis of the probe. This further means that the probe measures zero field strength when the magnetic field direction is PERPENDICULAR to the long axis of the probe.
You will use Vernier equipment to measure values of actual magnetic fields, such as those created by permanent bar magnets. Open the Logger Pro File for Part 1. Set the range switch on the magnetic field probe to the 6.4 mT setting. With the magnetic field probe held in place on the surface of the lab bench, zero the reading, and then begin collecting data.
Keeping the magnetic field probe motionless on the bench top, slide the bar magnet along the table top toward the probe along an extension of the long axis of the probe. Watch the probe meter reading, and be careful NOT to exceed a magnetic field magnitude of 5 mT (which is just below the upper-reading limit of the magnetic field probe).
Move the bar magnet some distance away, turn it end-for-end, and slide it in directly toward the probe again. When at a reading of approximately 5 mT magnitude, don't bring the magnet any closer but rotate it around its center slowly while watching the magnetic field reading carefully. Rotate through at least 360°.
Now, rotate the magnet so its long axis is perpendicular to the long axis of the probe. Then translate the position of the magnet so it is pointed at the probe about 1 cm behind the tip of the probe and far away from the probe. Now, move the magnet in a line toward the probe, and, as you do so, adjust the distance of its line of travel either closer or further from the tip in order to make the probe reading close to zero. Because the magnetic field lines diverge from one another after leaving the tip of the bar magnet, the magnetic field in the vicinity of the Hall Effect wafer (inside the probe) will have a small component along the axis of the probe unless the center of the bar magnet is aimed exactly at the wafer, and by slight adjustment of the bar magnet's line of travel, you can aim the center of the bar magnet end directly at the (hidden) wafer, so that the measured field strength is zero.
At this point you have established that the probe is most sensitive to fields parallel (or antiparallel) to the probe's long axis and oblivious to fields that are perpendicular to the long axis of the probe. (Actually, this field sensitivity varies as the cosine of the angular difference between the probe axis and the direction of the magnetic field at the wafer.)
Take a paper clip and bring it up to the tip of the probe. Turn the paper clip end-for-end, and determine from this test that the paper clip is not magnetized. If it does cause a small reading as you turn it end-for-end, throw the paper clip vigorously to the floor or against the wall a few times, and that should remove most of any residual magnetism. (Related admonition: Do NOT drop the permanent magnets on the floor or on the lab bench. With abuse, even a permanent magnet can lose a bit of its magnetization.)
Move the magnet so it is a distance away from the probe and facing the probe, with its long axis aligned with the long axis of the probe. Slide the bar magnet back to a point a large distance away from the probe, and place a paper clip near one end of the bar magnet (near enough that the clip leaps to that near end of the magnet). Because the paper clip is made of a magnetic material (soft iron), the paper clip is attracted to the magnet and can be made to cling to one end. Holding the paper clip so that it touches the magnet and sticks straight out along the long axis of the magnet, slide this arrangement toward the probe along the probe's long axis with the paper clip between the magnet and probe. As the clip+magnet arrangement approaches the probe, a nonzero field value should begin to register, and, as long as the reading never exceeds 5 mT, you can bring the free end of the paper clip right up the flat end of the probe.
With the paper clip held firmly in place against the probe, take the magnet out of contact with the paper clip, bring the magnet back again, and repeat this cycle several times while watching the probe reading.
Reverse the direction of the bar magnet, and repeat the previous steps.
With the clip + magnet touching the end of the probe, leave the magnet in place, and this time, remove the paper clip. Reinsert and remove the paper clip through a few cycles, noting the behavior of the probe reading as you do that.
Then, reverse the direction of the clip + magnet arrangement, and again remove and reinsert the paper clip from between the magnet and the probe through a few cycles.
Now, slide the magnet back until it is at least three times as far away from the probe as it was in the previous steps. Place the paperclip in between the magnet and the probe, with its long axis aligned with both the long axis of the probe and the magnet. Now, slide the paperclip in a direction perpendicular to the direction of the probe until it is offset by a few inches. This will be the starting position for the paperclip. Start collecting data. Slide the paperclip along a line perpendicular to the direction the probe is facing. Keep sliding it along this path until it has passed between the magnet and the probe and is a few inches offset on the opposite side. Continue to collect data through the next two steps.
Rotate the paperclip 90 degrees so it is facing a direction perpendicular to the direction the probe is facing. Again, slide it past the magnet, to its original starting position.
Now rotate the paperclip 45 degrees, so that it is facing a diagonal direction compared to the direction the probe is facing. Again, pass the paperclip between the magnet and the probe until it is again on the opposite side. You can stop collecting data. Record in your worksheet what the peaks were for each of the three orientations of the paperclip: in line with the probe, perpendicular to the probe, and diagonal to the probe.
Take away the magnet and place it at some distance. Place the paper clip against the probe tip and observe the probe-reading change as you rotate the clip end-for-end. You should observe that the paper clip retains some residual magnetization, and you will be asked to describe your observations on your Lab Report. Now throw the paper clip down on the floor at least two or three times and repeat the end-for-end measurements as above. What do you observe now?
Here's the point. In all magnetic materials there are lots of little magnetic domains where all atoms in a given domain have their microscopic magnetic moments—their tiny bar magnet equivalents—all lined up in the same direction. In a permanent magnet, all these little domains can be more or less lined up, giving a large resulting magnetic field, AND these domains tend to stay lined up for a LONG time unless the temperature of the magnet is raised to a high-enough level to cause the domains to disalign OR unless the "permanent" magnet suffers enough dropped-to-the-floor type abuse. We call such material a "hard" magnetic material (a "hard" magnetic material requires a LOT of abuse to lose any of its magnetism, but we don't want our magnets to lose even 1%, so treat them CAREFULLY). A paper clip, of the other hand, is "soft". Although it has domains containing atoms that tend to be all magnetized in the same direction, these domains only line up cooperatively when an external magnetic field is applied, and this alignment largely disappears when the field is removed. But "largely" does not mean "completely". You'll no doubt be interested to learn that any residual alignment of domains in soft iron (like a paper clip) can usually be completely disrupted by a few sharp impacts (like throwing the paper clip to the floor a few times).
As you know, the Earth has a magnetic field, which is the subject of your magnetic field measurement for today.
Set the probe sensitivity switch on the magnetic field probe to 0.3 mT, open the Logger Pro File for Part 2, begin data collection, and move the probe around in space so that it points in all possible directions during one recording interval. Once the data collection process stops after 60 s, you will probably want to scale the graph. During data collection, note the probe orientation when the probe reading is at a maximum, and similarly note the orientation for the minimum reading. (The reason for the difference in readings is the Earth's magnetic field is directional, and the max and min readings should be found at probe orientations 180° apart.)
You may be surprised to find that the probe does not generally register the largest field strength in a horizontal orientation. Because the Earth itself creates a magnetic field similar to the field that would be produced by a large bar magnet buried way down at the center of the Earth, the field lines this far north of the equator have substantial vertical components (whereas field lines are pretty much parallel to the Earth's surface at the equator). ALSO, the building itself (because of a large amount of structural iron) can distort field lines within the building, causing their directions to be noticeably different from textbook specification. Bottom line: the probe angle you observe for maximum magnetic field readings will have a vertical component, AND the angle may be somewhat different from one lab station to the next.
Orient the probe at 90° to the orientations of those extreme probe readings, zero the reading, and then start collecting data. During this recording, first sweep the probe's position around in the vicinity of the maximum reading (in order to get a good maximum value), and then reverse the probe direction by 180° and sweep through all possible nearby orientations (in order to get a good minimum value). The difference between max and min is twice the value of the Earth's field, and if you zeroed the probe at a good point, the max and min should be reasonably centered about zero. (It would be a good idea to determine whether your measured field strength is somewhere in the vicinity of 0.5 gauss. The Vernier probe is calibrated in tesla, the accepted unit standard for our customary system of units, BUT the Earth's field strength is such a tiny fraction of a tesla, you will usually find the Earth's field strength expressed in gauss, a unit of field strength that is 104 times smaller in size than the tesla.)
You are now going to use the Vernier equipment to measure the magnetic field created by a 0.40 A current flowing in a pair of 400-turn wire coils. Hook up the coils to the power supply and current meter as instructed by your Lab Instructor. Before turning on your power supply, be sure to turn the "Current Adjust" fully CCW and the "Voltage Adjust" a couple of turns in the CW direction. (If you are using a Pasco supply, it's called the "Current Limit Adjust," and you should also put the meter switch in the "Amps" position at the start. If you are using an H-P supply, depress the "Range" button to the "2 A" position.)
Make sure that the magnetic field probe is set to the 6.4 mT sensitivity, and open the Logger Pro File for Part 3. Then turn on the power supply, zero the Vernier probes, and then turn up the supply current until the Vernier current probe reads 0.40 A. If the current meter on the supply does not read approximately 0.40 A, please have your Lab Instructor look at your setup.
The two coils should be located beside one another so that the bore of each aligns with the other. Begin a 60 sec data collection, and slowly insert the magnetic field probe in one end of the bore and slowly slide the probe through until it emerges from the second coil while watching the magnetic field graph that is being recorded in real time. (Be sure that the probe reading does not exceed the 6.4 mT level at any time.) Separate the two coils a few millimeters so that you can see when the field-sensitive region of the probe is at the halfway point. As you slide the probe through the first coil, you should see the magnetic field rise to a maximum value and then decrease a bit as it approaches the gap between the two coils. When the field-sensitive region of the probe is right in between the two coils, the reading should be at a relative low point (if the coils are creating fields in the same direction), and then the field strength increases again as the probe is slid through the middle of the second coil. If the coils are creating oppositely directed fields, the field strength at the halfway point should go through zero and then reverse polarity as you slide the probe through the second coil.
Given whichever arrangement you start with, slide the probe through the pair of coils at least a couple of times while recording the field and current readings. Then turn the current down to zero, reverse the wire connections to one of the coils, thus reversing its polarity, turn the current back up to 0.40 A, and slide the magnetic field through both magnets another couple of times, again while recording the field and current readings.
Now slide the second coil away at least a couple of centimeters from the first coil (so that it doesn't contribute much to the reading of the probe when the probe is in Coil 1). Zero the probes again, then slide the probe slowly through the center of Coil 1 while recording data so that you can obtain a good reading of the maximum field and corresponding current, which you then should record on your data sheet Then turn down the current to 0.30 A and repeat the above procedure, again recording results on your data sheet. Repeat a third and fourth time for currents of 0.20 A and 0.10 A.