Investigations in Magnetism Across Scale

Transcription

1 Computer visualization Magnetism and Size Investigations in Magnetism Across Scale Name Class What this lesson is about: Even though they may not be aware of it, people continually re- organize and revise the private models of the world that exist in their minds as they live and learn. The goal of this lesson is to help you develop an accurate model in your own mind, your mental model, relating to the effects that size has on the magnetic properties of a nanoscale piece of iron: when it is affected by an applied magnetic field, and when the magnetic field applied to it is removed. Vocabulary: Particle a generic term, often referring to atom, group of atoms (domain), piece of iron. Domain group of atoms that are magnetically aligned. Applied field the magnetic field from an external source; for example, the field of a nearby magnet. Magnetic moment magnetic orientation (N or S direction) of any size particle, (including a magnet). Display and variable controls Display 1. A single magnetic domain (piece of iron). 2. Atoms in the domain (iron atoms). 3. Overall magnetic moment of the whole domain. 4. Lines, modeling the intensity of an applied field. 5. Arrows indicating the polarity of the applied field. 6. Field strength indicator. Controls 7. Size of the domain (# of atoms) (keystrokes A/S). 8. Intensity of applied field. (keystrokes G/H). 9. Temperature (thermal energy) (keystrokes K/L). 10. Applied field (On/Off). 11. Polarity (direction of applied field). 12. Pause/ continue. 1 P a g e

2 Modeling atoms up close: Arrows the magnetic orientation (magnetic moment) of an individual atom (can flip either N or S). Shading the lighter the shading of the atom, the greater the probability that the magnetic moment will flip. Variables things you can change Variable Domain size the size of the iron particle. Applied field strength of the field that is applied to the iron particle. Temperature thermal energy (vibrations) of the atoms making up the domain (Kelvins). Can be changed using The slider or by using A / S keystrokes. The slider or by using G / H keystrokes. The slider or by using K / L keystrokesr. Let s take a little tour before we begin: 1. The model of the piece of iron displayed consists of an array of circles with arrows inside of them. What does each circle on the model represent? 2. Explain what the arrow in each of the circles represents. 3. What do the horizontal lines across the screen represent? What do the arrows at each end of the lines represent? 4. What does it mean if most of the arrows inside the atoms making up the iron particle are pointing in the same direction? 5. What does the big red arrow at the top of the screen represent? Explain what it means when the big red arrow gets bigger and smaller. What do you think it means if the big red arrow repeatedly switches direction back and forth? 2 P a g e

3 Computer visualization Magnetism and Size Name Class Your Goal: is to investigate the ways in which magnetic materials are affected by: The size of a magnetic domain (the piece of iron) The strength of a magnetic field surrounding the domain Thermal (vibration) energy of the atoms making up the domain (heat energy). Things you will need: Procedure Computer with internet access Clock with second hand or stopwatch Calculator 1. Open the Google Chrome browser 2. Type in the following web address: http//:web.ics.purdue.edu/~dsederbe/simulation.htm The program will begin with the settings below. Leave them for the time. Domain size = 150 atoms Applied field strength = 50 % Temperature = 290 K Part 1 Being magnetized is a matter of alignment 1. Start with the settings: 150 atoms; 50 % applied field strength; and temperature of 290 K. 2. The green Polarity button reverses the direction of the magnetic field that is affecting the iron particle. Try clicking the button a few times and look for changes in the appearance of the iron particle and its surroundings. List three things that change. What effect does changing the polarity have on the magnetic moment indicator arrow at the top of the screen? 3. Now Pause the program. Pick a block of 25 atoms (5 atoms x 5 atoms) anywhere in the domain. Count the atoms whose magnetic moments point in each direction. How many atoms are aligned with the applied field? How many atoms are aligned against (opposing) the applied field? 3 P a g e

4 4. Press Continue to re-start the program and let it run for a few seconds, then Pause again. Count the atoms again whose magnetic moments face each direction. Repeat once more and record the results of your three trials in the table. Number of atoms aligning with the applied field. Number of atoms aligning against the applied field. Trial 1 (from #3) Trial 2 Trial 3 a. What trend do you observe in the counts among your trials? b. Explain what you think this means. 5. Describe how the direction of the magnetic moment indicator (the big red arrow) compares with the overall alignment of the 25 atoms you counted. Part 2 Becoming magnetized depends on the strength of the field used to magnetize it 1. With the program running, adjust the settings to: Domain size 1200 atoms; Applied Field Strength 10 % field; and Temperature 290 K 2. Let the program run for a few seconds and click Pause. a. Would you say that the iron particle in the model is magnetized? b. Describe two observations you see that you could use to support your decision? 4 P a g e

5 3. Now Continue the program and let it run for a few seconds. Then, increase Applied Field Strength to 25 % field. Do not pause the program, let it keep running. Describe two changes in the appearance of the domain. Feel free to switch back and forth to the previous setting (10 % field) for a comparison. At least two other things change in the workspace when you changed the field. What do you think they are? 4. Now increase the Applied Field Strength to 90 % field. Is the domain more or less magnetized than it was before? Describe two observations you can use to support your conclusion. Part 3 The size of the piece affects its tendency to stay magnetized In this experiment you will need a clock with a second hand to time how long it takes for the magnet moment indicator arrow to reverse direction. 1. With the program running, adjust the settings to: Domain size 300 atoms; Applied Field Strength 50 % field; and Temperature 240 K Let the program run for about 10 seconds. In the next step you will be turning off the magnetic field that is affecting the iron domain and measuring the number of seconds it takes for the red magnetic moment indicator arrow to reverse directions at least once 2. When you are ready to start timing, click Applied Field On/Off to turn off the magnetic field. Record your time below. Turn the field on and repeat this one more time. Enter your times in the table and calculate the average. seconds seconds seconds Time 1 Time 2 Average time 5 P a g e

6 3. Repeat the process again, this time with a domain size of 25 atoms. seconds seconds seconds Time 1 Time 2 Average time 4. Now let s go for the max! Turn the field on again and increase the Domain Size to the maximum size of 4800 atoms. Repeat what you did in your other trials and record your times below. seconds seconds seconds Time 1 Time 2 Average time 5. What conclusions could you make about the size of the iron particle and the likelihood that it would stay magnetized when the field affecting it is removed? 6. What do you think might be the reason for this trend? Write an explanation to support your rationale. Part 4 What changes with particle size besides size? Adjust the settings to: Domain size 25 atoms; Applied Field Strength 50 % field; and Temperature 160 K 1. Let the program run during the rest of the investigation. Count (or calculate) the number of atoms on the outside edges (surface) of the domain. If you take away the outer layer of atoms, what is the number of atoms are on the inside of the domain? Outside atoms Inside atoms Percent of atoms on the surface of the domain = 6 P a g e

7 2. Increase the domain size to 100 atoms. Count (or calculate) the number of atoms on the outside edges (surface) of the domain. If you take away the outer layer of atoms, what is the number of atoms are on the inside of the domain? Outside atoms Inside atoms Percent of atoms on the surface of the domain = 3. Lastly, increase the size of the domain to 1000 atoms. Count (or calculate) the number of atoms on the outside edges (surface) of the domain. If you take away the outer layer of atoms, what is the number of atoms are on the inside of the domain? Outside atoms Inside atoms Percent of atoms on the surface of the domain = 4. Write a statement that could summarize the general relationship between the size of an iron particle and the number of atoms on the surface of the particle. 5. Based on what you concluded in Part 3, and what you stated above, write a general statement that summarizes how the size of an iron particle is related to the tendency that will spontaneously de-magnetize when a magnetic field affecting it is removed. 6. Write an explanation for why you think this might happen. 7 P a g e

8 Part 5 The effect of thermal energy on magnetic materials (like nanoscale iron particles) 1. With the program running, adjust the settings to: Domain size 300 atoms; Applied Field Strength 25 % field; and Temperature 300 K 2. Let the program run for a few seconds and then Pause. Estimate what percent of the atoms in the domain have magnetic moments that are flipped (opposite the polarity of the magnetic field affecting the domain). What two pieces of evidence do you have that the domain is still magnetized? % 3. Continue running the program. Use the letter K keystroke to decrease the temperature 10 K at a time. Find the temperature at which the magnetic moments of the atoms in the domain first stop flipping for at least 10 seconds? Explain what you think might cause certain atoms might become unaligned with the rest. Think of at least two factors that you think might contribute to keeping atoms from reversing their polarity (flipping). 4. Continue running the program. Using the letter L keystroke, start slowly increasing the temperature 10 K at a time. Find and record the temperature at which the magnetic moment indicator arrow at the top of the screen first starts flipping from one direction to the other. K What do you think it means when the magnetic moment indicator arrow reverses directions back and forth? Compare the temperature you found with two other groups. First group Second group What do you think might be special about this temperature? 5. Leaving the temperature and field settings where they are, increase the size of the particle to its maximum size. Describe the effect that increasing the particle size has on the magnetism of the particle. 8 P a g e