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8B Lab Physics 2 Magnetic Fields rev 4.0 Lab 2 Magnetic Fields Part 1: Magnetic Field of a Bar Magnet 1) Finding the direction of magnetic fields: The term magnetic pole is used to identify a portion of a magnet that has specific magnetic properties. For a bar magnet, the poles are usually located at the ends of the magnet. To find the poles and the direction of the magnetic field of a magnet, you will use a compass. A compass is simply a small magnet on a pivot (shown as an arrow below). When placed in an external magnetic field, it tries to line its magnetic field up with the external field. So the direction of the compass needle shows the direction of the external magnetic field at the location of the compass. N S Bext S N Bext (Before) (After) The compass needles in this lab are not shaped like a little arrow, but instead have a red and a white end to them. So you will need to determine which color corresponds to which end of the compass. Do this by placing the compass in a known magnetic field, namely that of the Earth. a) In Berkeley, the Earth's magnetic field (or rather, the component parallel to the surface of the Earth) points "toward Canada" i.e. toward "geographic north" (see below for a word of caution). Hold the compass in your hand far away from any other magnets on the table, and from the table itself. Which color of the compass needle points in the direction of the Earth's magnetic field? Note: These are inexpensive compasses. The colors will not be uniform from group to group. Furthermore, be careful not to bring the compass in contact with other magnets, as the compass needle can be accidentally re-polarized. Physics 8B Lab 2 Magnetic Fields rev 4.0 Comment about the Earth's magnetic field The Earth's magnetic field looks as if there is a magnetic dipole in its interior. This dipole is not perfectly aligned with the geographic poles but unless you are located in a polar region, this tilt is not very important. But the naming of the poles can be confusing. The Earth's geographic North Pole is a Magnetic south pole, while the geographic South Pole is a Magnetic north pole as shown. Magnetic fields point out of Magnetic north poles and into Magnetic south poles. A compass lines itself up with the Earth's Magnetic Field i.e. it points toward the Magnetic south pole (which happens to also be the geographic North Pole) Earth's geographic North Pole Earth's Magnetic south pole S N S Earth's Magnetic north pole N Earth's geographic South Pole Compass Place the (cylindrical) bar magnet on the table. Use the compass to explore the region surrounding the magnet. The colored end you found in a) will tell you which way the magnetic field points at each location of the compass. Try various locations both close to and far from the magnet. b) Summarize your results by drawing the magnetic field lines for the bar magnet. Be sure to include directions and label the poles of the magnet. Physics 8B Lab 2 Magnetic Fields rev 4.0 c) Does the compass needle always point directly toward the bar magnet? Or directly away? (Analogous to an electric field for a point charge) Explain. d) When the compass is far away from the magnet (as in question a) ), what determined the direction of the needle? Did small changes in the position of the compass affect the direction much? e) When the compass is close the magnet (as in question b) ), what determined the direction of the needle? Did small changes in the position of the compass affect the direction much? f) Are you confident that the field effects you found in b) are mainly due to the magnet? If so what does that mean about the relative strengths of the bar magnet's magnetic field compared to the Earth's magnetic field? Magnetic viewing paper has tiny particles in suspension that are free to move under the influence of a magnetic field. Look at the bar magnet through the magnetic viewing paper in the two orientations shown. i) One end touching the paper. Try both ends to see if there is any difference ii) The paper laid flat on top of the magnet g) Based on your knowledge of the magnetic field for the bar magnet, what color is the viewing paper when the magnetic field passes through the paper? What color is the viewing paper when the magnetic field is parallel to the paper? Physics 8B Lab 2 Magnetic Fields rev 4.0 h) How many poles does a bar magnet have? Explain based on what you saw with the viewing paper for the second arrangement. Place the flat magnetic strip (fridge magnet) on the table. Place the magnetic viewing paper over the strip. i) Based on what you just learned about the magnetic viewing paper, how many poles do you observe for a fridge magnet? j) Sketch the magnetic field lines for a fridge magnet. Show directions and label the poles Top view Edge view Physics 8B Lab 2 Magnetic Fields rev 4.0 2) Finding the magnitude of magnetic fields: A Hall probe is an inexpensive tool for measuring magnetic fields with an accuracy of typically + 2%. The probe will indicate the magnitude and the positive and negative polarities of the magnetic field. DC Voltmeter : Plug the wire from the connector box into the V jack and COM jack of the multimeter. Set the multimeter to read DC volts (V with straight line next to it) Hall Probe : Plug the Hall probe (magnetic field sensor) into the small connector box. Make sure the Hall probe switch is in the high 200 amplification position. In this position, a multimeter reading of 1V will indicate a magnetic field of 1.6 Gauss (or 1.610 Tesla). For the purposes of this lab, we will record the voltage reading as being representative of the magnetic field. Hold the Hall probe vertically, far from any permanent magnet. As you rotate the probe, take note of the reading on the multimeter. The average value is the background magnetic field. A reading above this value indicates a positive polarity of magnetic field, i.e. magnetic field pointing away from the white dot. A reading below the value indicates a negative polarity, i.e. magnetic field pointing into the white dot. (NOTE: An easy way to find the average value is to point the probe in two opposite directions and average the readings.) If the probe is held vertically and rotated until the maximum voltage is found, the magnetic field will point away from the sensor's white dot. Explore the direction and magnitude of the Earth's magnetic field with the Hall probe. a) How does the direction of the maximum voltage reading compare to the direction of the Earth's magnetic field found using a compass? -4 b) If the probe doesn't point exactly in the direction of the magnetic field, what does the reading represent? Explain with a sketch of the magnetic field vector and the direction of the probe and label what the probe measures. Physics 8B Lab 2 Magnetic Fields rev 4.0 At most locations on the Earth, the Earth's magnetic field isn't parallel to the surface. We can also use this probe to determine angle of the field into or out of the Earth. This is referred to as the magnetic inclination. Hold the probe vertically so that the white dot is facing the largest voltage you just found, and then tilt the sensor end of the probe up or down until the voltage reaches a maximum. The angle of the probe from the vertical is the magnetic inclination. c) Does the Earth's magnetic field in Berkeley point into the Earth or out of it? d) Based on the magnetic inclination, is Berkeley in the geographic northern or southern hemisphere? Explain. Switch the probe to the low 10 position, where a multimeter reading of 1V will indicate magnetic a field of 32 Gauss (or 3.210-3 Tesla). Again, we will record the voltage reading as being representative of the magnetic field. Take a new average measurement for this setting of the probe. Enter this in the table below. You will subtract this from all other probe readings. (NOTE: An easy way to find the average value is to point the probe in two opposite directions and average the readings.) Position the Hall probe at the end of the bar magnet. The active element inside the plastic tube of the probe is approximately 5mm from the sides. Be sure to measure to the element, not to the outside of the tube. Measure the magnitude of the magnetic field at several positions from the end of the bar magnet and record the results in the table below. Distance to probe (mm) Probe reading (Volts) Average probe reading (Volts) Magnetic Field (Volts) 30 40 50 60 70 80 90 Physics 8B Lab 2 Magnetic Fields rev 4.0 Plot the magnetic field (in volts) versus distance on the axis below. B (Volts) Distance (mm) e) What happens to the magnetic field as you move the probe further away from the magnet? Explain. Physics 8B Lab 2 Magnetic Fields rev 4.0 Part 2: Magnetic Field of Current Carrying Coils In this part of the lab, we are interested in magnetic fields created not by certain materials (i.e. magnets) but in magnetic fields created by currents. The shape we will study is a current coil and then a pair of coils. We use a current coil because the multiple wrappings will increase the magnitude of the magnetic field by superposition. But the shape (i.e. directions) is the same as that of a single loop of wire. We will use an apparatus consists of a pair of coils mounted in the Helmholtz configuration (the separation between the coils is the same as the radius of each coil) and a small disk permanently magnetized along its cylindrical axis. The disk is a model system for an ideal magnetic dipole moment. It is mounted in a plastic gimbal so it is free to rotate about the gimbal axis, and is suspended by a nonmagnetic spring. The disk has an arrow on it to indicate the direction of the magnetic dipole moment. The magnetic dipole will align itself with any external magnetic field created by the coils...i.e. the disk serves as a little compass for this apparatus. The opposite end of the spring connects to a brass rod, which is held in place on top of the plastic tower. You will use this rod to raise and lower the disk to various positions along the coils' axis. DC Power Supply : With the power supply OFF, set it up as follows Readout switch should be set to read AMPS All four knobs (fine & coarse current and voltage) should be turned all the way CLOCKWISE The High/Low button should be PUSHED IN to the LOW setting. This will help make sure you never exceed the maximum current for this lab. Do NOT exceed 3 Amps in this lab. If you need to adjust the current, use the Coarse Current knob. 1) Magnetic field due to a single current coil: Connect only the Upper Coil to the DC power supply, such that current flows into the red jack. With current flowing into the red jack, current will flow around the coil in the counterclockwise direction (when viewed from above). Adjust the brass rod to hang the magnetized disk in the center of the upper coil. a) Predict what will happen to the disk when the power supply is turned on. Physics 8B Lab 2 Magnetic Fields rev 4.0 b) Check your prediction by turning on the power supply. How does your prediction compare to your observations? c) Raise and lower the brass rod to move the disk along the axis of the coil. How does the direction of the magnetic field vary along the axis of the coil? Take the regular compass and hold it so the case is vertical. Now use it to investigate the direction of the coil's magnetic field at locations other than the central axis. What happens as you move away from the center axis toward the coil? What happens above the coil? Outside the coil? Below the coil? d) Summarize your results by drawing the magnetic field lines for the current carrying coil. Show the results both on the axis (from the disk) and off axis (from the compass). Be sure to include directions. I Physics 8B Lab 2 Magnetic Fields rev 4.0 e) Do the magnetic field lines begin and end on the coil? If so, where? If not, explain what they look like. f) Predict what will happen to the magnetic field and the hanging magnetic disk if you reversed the current in the coil. g) Turn the supply off. Reverse the current through the coil. Turn the supply on. Raise and lower the hanging magnetic disk. How does your prediction compare to your observations? 2) Magnetic field due to a pair of current coils: Turn the power supply off. Connect the both coils in series to the DC power supply, such that current flows in the same direction in each coil (i.e. current flowing into red jack for the lower coil and then into the red jack for the upper coil). Adjust the brass rod to hang the magnetized disk in the center of the two coils. h) Predict what will happen to the magnetic field and the hanging magnetic disk at points along the axis of the coils. i) Check your prediction by turning on the power supply. Raise and lower the disk along the axis. How does your prediction compare to your observations? j) Are your results for the magnetic field along the axis of the two coils consistent with the principle of superposition? Explain. Physics 8B Lab 2 Magnetic Fields rev 4.0 Turn off the power supply. Connect the coils in series to the DC power supply, such that current flows in the opposite direction in each coil (i.e. current flowing into red jack for the lower coil and then into the black jack for the upper coil). Adjust the brass rod to hang the magnetized disk in the center of the two coils. k) Predict what will happen to the magnetic field and the hanging magnetic disk at points along the axis of the coils. l) Check your prediction by turning on the power supply. Raise and lower the disk along the axis. How does your prediction compare to your observations? m) Are your results for the magnetic field along the axis of the two coils consistent with the principle of superposition? Explain. Physics 8B Lab 2 Magnetic Fields rev 4.0 (Time Permitting) 3) Magnetic force on a magnetic dipole: With the power supply off, raise the magnetized disk to the top of the coils. The coils should still be connected in series with the current flowing opposite directions in each coil (i.e. current flowing into red jack for the lower coil and then into the black jack for the upper coil). Turn the power supply on. Slowly lower the disk, by lowering the brass rod, through the central region to below the bottom coil. Then raise the disk up to the top of the coils. Repeat several times. n) As the disk is lowered from above and approaches the center of the two coils, would you say the amount the disk lowers is the same as, greater than or less than the amount you lower the rod. You may need to do this slowly and several times to get a feel for it. o) What does this say about the magnetic force on the disk above the center of the coil? p) As the disk is raised from below and approaches the center of the two coils, would you say the amount the disk raises is the same as, greater than or less than the amount you lower the rod. You may need to do this slowly and several times to get a feel for it. q) What does this say about the magnetic force on the disk above the center of the coil? r) What happens to the magnetized disk when it passes through the central region between the coils? Explain. ... View Full Document

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