MAGNETISM & ELECTROMAGNETIC INDUCTION

Name:Partner(s):Investigation #12MAGNETISM & ELECTROMAGNETIC INDUCTIONIn these activities, you will become familiar with the behavior of magnets and magneticforces as you watch a video about the Earth’s magnetic field. The concept ofelectromagnetic induction is investigated as you apply these ideas to examine generatorsand motors.You may notice there is very little background information provided in the activities thatfollow. This is by design. For this investigation, you are essentially going to berecording observations and then looking for patterns in the observed phenomena in orderto explain what causes electromagnetic induction.Part IMagnetic Storm VideoYou will need to go to the following URL for this part:https://www.youtube.com/watch?v NJUTUFAWfEYWatch the video (about 55 min) and answer the following questions:Question: What generates the Earth’s magnetic field?Question: How long will the Earth’s magnetic field last given its current rate of decay?Question: At the present time, where do the magnetic field lines emerge from the Earthand where do they reenter?Question: Why is the Earth’s magnetic field important in order for Earth to remain a“living” planet?205

Question: Describe how the auroras (the northern and southern light) are produced?Question: What did the Mars Global Surveyor discover with regard the current magneticconditions on Mars? (Choose one.)a) Mars has global magnetism from its core.b) Mars no magnetism whatsoever.c) Some areas on the surface were magnetic.d) No conclusions could be made.Question: Describe in detail, how ancient pottery provides information about thebehavior of Earth’s magnetic field over time?Questions: How much has the Earth’s magnetic filed declined over the past 300 years?Is the rate of this change increasing or decreasing?Question: For what two reasons did was sodium metal used in experiments that modeledthe behavior of the Earth’s magnetic field?1)2)Question: Based on “sodium dynamo” experiments, what do some scientists think wasthe cause of Mars’ magnetic situation today?Question: What information is contained in the frozen lava layers of the Hawaiianarchipelago? What starling discovery was made?206

Question: According to geologic records, when did the last magnetic reversal occur?Question: On average, how often does the Earth’s magnetic field undergo a reversal?Question: In the computer model of the Earth’s magnetic field, what were someindications that the simulation was about to undergo a reversal?Question: Why were 18th and 19th century sailors obsessed with the Earth’s magneticfield?Question: What is the “South Atlantic anomaly” and what has it been doing over the past100 years?Question: At the height up the magnetic upheaval that took place on Steen’s mountain 16million years ago, by how much did the direction of the Earth’s magnetic field changeover the 10 days or so it took the flow to cool according to the geologic records?Question: Roughly, how long was the Earth without its magnetic field during thereversal of 16 millions years ago?207

Question: What is the predicted increase in annual deaths due to skin cancer as a resultof the increased radiation that reaches the Earth’s surface if the field vanishes?Question: (Open-ended) Is there anything we can do to prevent the collapse of the Earth’smagnetic field? If so, what?Part IIMagnetic InductionYou will need to go to the following URL for this law/latest/faradays-law en.htmlWhen you do, you should see the following:208

In this activity, you will use a light bulb, a bar magnet and a wire coil to qualitatively toexplore how electromagnetic induction occurs. This will be done by inserting andremoving a magnet from a wire coil in various ways.The main features of the simulation are:1. The magnet can be dragged just about anywhere on the screen, in and around thecoil.2. You can click the “Field lines” box to toggle the display of the magnetic fieldcreated by the bar magnet.3. You can click of the “2 or 4” coil box at the bottom to compare and contrast thebehavior of system when different numbers of coil loops are used.4. The green box with the bar magnet at the bottom will toggle the polarity of themagnet by flipping the poles.5. The orange circular button reset the simulation to its default settings.Procedure1. Start with the default set-up. Explore the simulation by dragging the bar magnetaround the screen, near the coil, and through coil. Note the behavior of the light bulband the voltmeter. Then follow the steps below to systematically record yourobservations for the various situations described2. In a very slow and steady motion move the lowly move the “North” pole of a barmagnet into the coil from the right side until the center of the magnet is in the centerof the coil. For each question below, record you observations for the light bulb andthe voltmeter.Question: While the N-pole of magnet was slowly entering from the right side.The light bulb:lights up.does not light up .The voltmeter needle:deflects negative (to the left).does not deflect at all.deflects positive (to the right).Question: When the magnet is stationary inside the coil.The light bulb:lights up.does not light up .The voltmeter needle:deflects negative (to the left).does not deflect at all.deflects positive (to the right).209

3. Now, slowly remove the magnet by backing it out toward the same side that youinserted it. Try to pull the magnet out with the same speed that you inserted it.Question: While the N-pole of magnet was slowly being pulled out the right side.The light bulb:lights up.does not light up .The voltmeter needle:deflects negative (to the left).does not deflect at all.deflects positive (to the right).Question: When the magnet is stationary outside the coil.The light bulb:lights up.does not light up .The voltmeter needle:deflects negative (to the left).does not deflect at all.deflects positive (to the right).4. Repeat Steps 2 and 3 moving the magnet faster while noting the behavior of the lightbulb and the voltmeter. Record your observations.Question: While the N-pole of magnet was quickly entering from the right side.The light bulb:lights up.does not light up .The voltmeter needle:deflects negative (to the left).does not deflect at all.deflects positive (to the right).Question: While the N-pole of magnet was slowly being pulled out the right side.The light bulb:lights up.does not light up .The voltmeter needle:deflects negative (to the left).does not deflect at all.deflects positive (to the right).210

Question: In what way(s) was the behavior of the system similar for the faster motioncompared to the slower motion? In what way(s) was it different?Similarities:Differences:5. Flip the magnet and repeat Steps 2-4 so that the N-pole enters and recedes from thecoil from the left side.Questions: In what ways (if any) was the behavior of the system in Step 5 (insertion andremoval of the N-pole on the left side) the same as in Steps 2 and 3? In what ways wasthe behavior of the system different?Similarities:Differences:6. Repeat Steps 2-5, with the “South” pole of the magnet moving into the coil slowly,quickly, from the other side, etc.) Note how the light bulb and voltmeter behaves ineach case.Questions: When using the S-pole, in what ways (if any) was the behavior of thegalvanometer the same as in Steps 2-5? In what ways was the behavior different?Similarities:Differences:Questions: Do the light bulb and voltmeter show any activity when the magnet isstationary? What must be happening in order to get the light to light or the voltmeter todeflect?211

Part IIIMagnetism from ElectricityWhile most of this material is in your textbook, here is the punch line:“Moving electric charges (in other words, electric currents) produce magnetic fields.”You will need to go to the following URL for this icfieldwire en.htmRead the description of the activity. Move the compass around the wire. If necessary,consult your textbook to answer the following questions:Question: What is the shape of the magnetic field the surrounds a long straight currentcarrying wire?Question: By definition, current flows from high voltage to lower voltage (as depictedthe simulation). So, why do the electrons move in a direction opposite the electroncurrent in the simulation? (Hint: What is the sign of the charge on an electron?)Question: What happens to the magnetic field (and the compass needle) when the currentis reversed?If you turn the straight wire in coil, you can strengthen the magnetic field in the regioninside the coil to produce a magnetic field that behaves in a similar way as a bar magnet.Such a device is called an electromagnet. (That is, a magnet that runs on electricity.)Question: What is at least one advantage of an electromagnet over a permanent magnet?Question: What is at least one advantage of a permanent magnet over an electromagnet?212

Part IVThe Magnetic ForceA current-carrying wire will experience a force when placed in an external magneticfield. (An external magnetic field is one created by some other outside system. That is,we are not considering the magnetic field created by the electric current moving in thewire itself.)The magnetic force is unusual in that it act perpendicular to both the direction of thecurrent in the wire and the direction of the field magnetic field. To observe this, go to thefollowing force en.htmIn the simulation, the U-shaped (or “horseshoe”) magnet creates the external magneticfield. Read the description activity. Explore the simulation by closing the switch“On/Off” button, turning the magnet, reversing the current and noting the direction of themagnetic force in comparison to the direction of the magnetic field and the direction ofthe electric current in the external field. Then answer the following questions:Question: By convention, magnetic field lines outside of a magnet emanate from the Npole and converge on the S-pole. In the simulation, which pole (red or green) is the Npole and which pole is the S-pole for the horseshoe magnet?Question: When the external magnetic field is down and the electric current in themagnetic field is “left and toward you,” what is the direction of the magnetic force?Question: Turning the battery does what to the magnetic force?Question: Reversing the current does what to the magnetic force.213

Part VMotors and GeneratorsIn this last part, you will need to explore the simulations at the following ctricmotor ator en.htmExplore the simulation and answer the following questions:Question: What happens to the spin direction if you reverse the direction of the current?Try it and see.Question: What would you expect to happen to the spin direction if you could flip theexternal magnet? Why do you think that?Question: What would you expect to happen to the spin direction if you could flip theexternal magnet AND reverse the direction of current simultaneously? Why do you thinkthat?Finally, go to the following URL and watch the 5-min rkAnswer the following questions based on all of your observation from these activities.214

1. From your own life experiences, do magnetic forces appear to penetrate throughmaterials that are not magnetic themselves? How do you know?2. What test(s) could you perform in order to tell the difference between an actualmagnet (like an iron bar magnet) and an object that is merely made of a magneticmaterial (like an ordinary iron bar)?3. Suppose you have two iron bars that appear to be identical, except that one is a barmagnet and the other is not. How would you be able to tell them apart if you had noother materials to help you? A sketch might be helpful here. (Hint: Where are thepoles of a typical bar magnet?)4. What is the basic difference between a generator and a motor?5. List at least three changes that you could make to the system in Part II that wouldcause a greater deflection of the galvanometer needle.1)2)3)215