- Computers are NOT needed for lab this week
- The best advice I can give you: drink a cup of chamomile tea before lab. This one will test you nerves!
- The lab benches are set to around 6 volts, powered by the big DC power supply in the stockroom. Jeff should have turned it on before lab, but occasionally forgets, so ask him to do so nicely so that he doesn't tear you a new asshole
- A model of a bar magnet and loop are available for the pre-lab discussion. The Styrofoam magnet is labeled N and S, with paint brushes representing the B vectors at each end
- Move the N end of the magnet towards the loop, and ask students if the magnetic flux is increasing or decreasing; they will all correctly answer 'increasing'.
- Flip the magnet around with the S end moving toward the loop, and again ask if the flux is increasing or decreasing. The majority (all?) will incorrectly answer 'decreasing'. A teachable moment
- I also have a couple of busted galvanometers that I'll leave on the front bench. Students can examine the inner workings to see how it works.
- Students will write their observations on this worksheet. Jeff will print them out ahead of time, and leave them in the lab (if he has his shit together).
- Here are the solutions for the worksheet. They are from 2012, but still valid.
- It's important that students connect the galvanometer in series with a 100 kΩ resistor before connecting to DC power. If not, the meter gets fried.
- All galvanometers point against the flow of current (they point in the direction that a positive test charge would follow). They figure this out after connecting the galvanometer to the DC power supply
- The important point here is to tell them to pay attention to the relationship between the change in magnetic flux (is it increasing or decreasing?) and the directions of B and induced B (Opposite directions if flux increased, same direction if flux decreased). This is the info they'll need for the exit quiz!
- A horseshoe magnet and compass are provided so that students can check the polarity of their bar magnet. All compasses should be oriented correctly, but they can switch when brought close to strong magnets. The white dot (or bump) on the bar magnet is North. Make sure they're not using a magnet with three poles!
- After completing the exercise, all students have to pass the 'exit quiz'. Another galvanometer - the same as the one they used - is near the blackboard, along with a bunch of generic bar magnets whose ends are covered with tape.
- A different coil is used, and the wires are twisted so that the coil is connected to the galvanometer backwards from the orientation used in the experiment (be sure to tell them that this will be the case during the pre-lab intro so they don't try to memorize the pictures, or you'll get "if the needle moves left then you've inserted the north pole…")
- Students do the exit quiz individually, without notes or compass
- I let them choose which magnet they want to use (as long as it's not the same as a magnet just used), which side of the coil they'll use, and whether they'll insert or pull out the magnet. Most will insert from the right side. Feel free to give them a magnet and tell them which way they'll do it
- Only one motion is required! Have them move the magnet and put it down in place (so you remember which side and which motion they used!)
- Now, ask them leading questions, as follows:
- Which way did the needle move? (hold your finger right or left after they answer; it helps both of you figure out what's going on)
- Which way does the current go through the galvanometer? (Again, use your finger after they answer)
- Tell them to follow the direction of current through the wires; which direction is it going around the coil?
- Tell them to use the right hand rule to figure out the direction of the induced B from this current.
- Did you increase or decrease the magnetic flux?
- Knowing that you increased/decreased magnetic flux, what is the relationship between the direction of B and induced B?
- So, having figured out the correct direction of B (hopefully!), which side of the bar magnet is north?
- Finally, ask them why this is happening (or what is the coil trying to do?) I try to get them to say something along the lines of "the coil is trying to resist the change in magnetic flux". It's like pulling teeth.
- If they answer successfully, write your initials on the top page of their report. If you think they need to do it again, grab another magnet, cover the galvanometer with your hand as you place the magnet inside the coil, and have them try again by pulling it out from the left side of the coil.
- If they're totally clueless, have them sit down and review their notes. Hopefully you've had plenty of herbal tea to keep you from popping an aneurysm
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- Update Spring 2023: I've decided to remove the section about inserting an iron bolt into the coil, so our meters will be safe. They have a difficult enough time wrapping their head around the basic concept. The following is left for historical significance:
- Students will insert an iron bolt into the core of the electromagnet to see the enhanced effect on the galvanometer. It's important that they use the old galvanometers for this part, or they'll break one of the good meters!
- Jeff can fix the needle of an old galvanometer if it gets pinned to the side, or you can try following these repair instructions.
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