Lab 37: Magnetic Field ; Magnets - Drawing magnetic fields - Magnetic poles - Forces between magnets

Transcription

1 Lab 37: Magnetic Field ; Magnets - Drawing magnetic fields - Magnetic poles - Forces between magnets 1) The following simple magnet configurations were shown to you in class - draw the magnetic field lines for each case (at least 8 lines per diagram), marking the directions of the field lines with arrows. a) bar magnet b) horseshoe magnet c) opposite poles of 2 magnets (other ends not shown)

2 d) like poles of 2 magnets (other ends not shown) e) Why do you think b) is a popular shape for a magnet? A horseshoe magnet is just a bar magnet bent around so that the N- and S- poles are near the same location. The strongest parts of the field are near the poles, so this puts the strongest part of the magnet all in one location (so that itʼs easier to pick up magnetic objects). 2) Here are some more complex configurations of magnets - using the same basic rules as you did in 1), label the magnetic field lines. (Hint: check the online lectures for a set of rules in drawing these field lines.) a) 2 bar magnets side by side, opposite-aligned b) 2 bar magnets side by side, like-aligned

3 c) 2 perpendicular bar magnets d) bar and horseshoe magnets e) 3 bar magnets f) Earth Recall that the North Pole is actually an S-pole, and the the South Pole is actually an N-pole. Also, recall that the magnetic poles are slightly offset from the actual poles.

4 Lab 38: Magnets and Magnetic Materials - Interaction of magnets with other matter - Magnetic poles of the earth - Broken magnets 3) When an object is attracted to a magnet, does it actually become a magnet? List some observations that support your answer. Yes - a magnetized object actually becomes a magnet. When you magnetized paper clips, and removed the original magnet, the paper clips retained their magnetic fields (and attracted other paper clips). Also, when you hung two paper clips from an N-pole, their opposite ends repelled, because they were also both N-poles. 4) If you break a magnet with an N and S pole, is it possible to get an N pole or an S pole by itself? Is it ever possible to break a magnet into small enough pieces to do this? What would a magnetic field diagram for a lone N-pole look like? Would this diagram break any of our rules for drawing magnetic field diagrams? If so, which rule(s)? No! If you break magnets into smaller and smaller pieces, it just becomes smaller and smaller magnets, all with both N and S poles. A magnetic field diagram for a lone N-pole (shown here) would break the rule requiring that all magnetic fields lines connect from an N-pole to an S-pole. Note that the shown field isnʼt really physically possible, because N-poles are ALWAYS paired with an S-pole. 5) For diagrams c) and d) in #1, explain for both cases how the force of the left magnet on the right magnet compares to the force of the right magnet on the left magnet. How would your answer change if you pushed or pulled the left magnet?

5 c) According to the Law of Interaction, the force on the left magnet is ALWAYS exactly equal to the force on the right magnet. d) According to the Law of Interaction, the force on the left magnet is ALWAYS exactly equal to the force on the right magnet (again). Lab 39: Electromagnets - Relation between electricity and magnetism - Magnetic field produced by a coiled wire - Building a simple motor 6) In this lab activity, you learned that an electric current produces a magnetic field. a) Why was the wire coiled around the straw or nail? Do you think any wire with a current running through it will produce a magnetic field? Yes - as we saw with the aluminum foil, ANY electric current produces a magnetic field; coiling the wire places small segments of the wire carrying current parallel to one another, so it increases the strength of the field. b) Part of this activity involved you scraping the insulation off the end of the wires with sandpaper. Why not just use non-insulated wire to save you the trouble? If non-insulated wire was wound into a tight coil, the current would flow between every point the wires touched, instead along the path of the coiled wire. c) Does the electromagnet you produced with the coiled wire have an N and S pole, or just one of these? The electromagnet has an N and S pole, just like a bar magnet. 7) When you built your motor, you blackened part of the exposed wire with a magic marker. a) Explain what purpose this served, and what would have happened if you didnʼt do this step.

6 Blackening added insulation to the back half of the wire, so that the motor wouldnʼt turn backward halfway through, and end up stuck in one place (or bouncing back and forth...) b) Give a 1-2 sentence summary on how your motor was able to work. Running a current through your small loop turned it into a magnet - this magnet interacted with the magnetic field of the permanent magnet to give it a push. When the ring turned over to the blackened side of the end wires, inertia kept it spinning until it rotated back to where the wire conducted again, giving it another push. c) Many people doing this activity noticed their setup became quite hot. Why did this happen? The setup is actually a short circuit - in this case, the short circuit is actually wanted, because short circuits have very large currents, and a large current creates a large magnetic field. d) Gary suggests that adding a light bulb to the circuit in series might solve the heat problem. Is he correct? Why? What problems might this cause in this experiment? This would solve the heat problem, since adding a load (or resistance) reduces the electric current. However, the problem is that there now might not be enough current to generate a strong magnetic field.

7 13. Billy claims that all magnets are electromagnets, with the wires and the battery hidden inside. How would the wires have to run, to produce a bar magnet like the ones we have been using? (It might be useful to make a drawing) The coil would have to be along the length of the magnet, as shown, to create an N- and S- pole. (Reversing the current would reverse the N and S poles.) 14. If we wrapped two coils the same way around a single straw and had currents going around them in the same direction, would they attract or repel? Explain your answer. As long as the electric currents run in the same direction, as shown, the S pole of the left magnet would meet up with the N pole of the 2nd magnet, and they would attract. (If one of the currents was reversed, they would repel, since two like poles would be together. If BOTH currents were reversed, both pairs of poles would reverse, and they would again attract.)