Gauge of wire to use in an induction experiment

[nmsurobert]

I'm setting up a Faraday/Lenz Law lab and was wondering if anyone had a suggestion on what gauge of wire I should use to get the best results. We don't have any wire here so I can't test it myself.

Thanks.

 

[kuruman]

What's your setup?

 

[gleem]

How do you intend to measure the effect? Are you going to connect a galvanometer to the coil and measure the current? The more turns in the coil the higher the emf. The gauge will only affect the current that will be generated. Using enamel coated wire is the best because the insulation is thin and you can get more turn per inch of winding. 20 ga. copper has a resistance of about .01 ohm/ft. and a diameter of about 32 mils. compared to a 14 ga. wire with twice the diameter and .0025 ohms/ft. The 20 ga wire will produce about twice the emf per unit length of the coil but has 4 times the resistance so it will produce only half the current.

OTOH if you use a voltmeter the current is basically irrelevant.

 

[nmsurobert]

How do you intend to measure the effect? Are you going to connect a galvanometer to the coil and measure the current? The more turns in the coil the higher the emf. The gauge will only affect the current that will be generated. Using enamel coated wire is the best because the insulation is thin and you can get more turn per inch of winding. 20 ga. copper has a resistance of about .01 ohm/ft. and a diameter of about 32 mils. compared to a 14 ga. wire with twice the diameter and 2.5 ohms/ft. The 20 ga wire will produce about twice the emf per unit length of the coil but has 4 times the resistance so it will produce only half the current.

OTOH if you use a voltmeter the current is basically irrelevant.

Do you h

ahh ok. perfect.

we are using a voltmeter. we are simply going to wrap some wire around pvc of varying diameter and move some magnets into and out of the coils.

 

[kuruman]

For that you will need as many turns as possible in your pick up coil. Enameled wire is the best for reasons already explained. The thinner the wire, the more turns per unit length, the better the sensitivity. Why is the diameter of the pvc varying? The effect of the area on the induced emf will make sense if you have a precise way to control the speed of the magnet going in and out.

 

[nmsurobert]

They do not have a precise way to control the speed of the magnet. However, it doesn't hurt to let them play around with it. I think only going to provide two different diameters, .5 inch and 1 inch. I find something larger laying around I might use that too.

I don't remember doing a lab like this in college so I'm a little curious myself on what can be measured without expensive equipment. I'm also just following the lab manual provided by the college board.

 

[kuruman]

An old fashioned needle galvanometer works much better than a digital voltmeter to show the change in polarity. If you don't have access to that sort of thing, you can put two low-current LED's of different color in parallel but wired in opposite directions. Put the combination in series with the coil. Then the direction of the current will be indicated by the color of the LED that lights up.

 

[tech99]

I do this to demonstrate Faraday's Law of Electromagnetic Induction. I think the coil former is about 2.5cm diameter and I have about 200 turns pile wound of about 26 SWG (AWG is similar) enamelled copper wire. Ideally you need an analogue meter of some sort, like a centre zero meter with 1 milliamp or less full scale deflection. I took the shunt out of an old meter to do this. Using an ordinary bar magnet it gives a good kick both ways.

 

[nmsurobert]

I might have some led lights floating around. ill try that out once I get the wire. thanks for all the suggestions!

I could really use some physics people on campus lol

 

[sophiecentaur]

They do not have a precise way to control the speed of the magnet.

It would not be too hard to drop the magnet from a given height. That would produce a consistent known speed through the coil. Or, by varying the dropping height, you can produce a range of different (measured / known) speeds. The plastic tube could, perhaps, have a light coat of silicone furniture spray polish to deal with irregularities inside the tube. Check by finding the range of output volts for a number of drops from the same height. That is a good intro into experimental uncertainty.
The optimum coil design will actually depend on the load you drive (implied further up the thread) and leds would be an excellent qualitative test. But the brightest illumination would be obtained from fewer, thicker turns that what gives you the highest induced voltage. All this would need to be sorted out before the students actually get involved i.e. initial lesson preparation should always involve preparation time that's at least as long as the intended lesson.

 

[tech99]

I might have some led lights floating around. ill try that out once I get the wire. thanks for all the suggestions!

I could really use some physics people on campus lol

I tried an LED on my coil today and the voltage is not high enough to light it. You could display the voltage using an oscilloscope.

 

[nmsurobert]

It would not be too hard to drop the magnet from a given height. That would produce a consistent known speed through the coil. Or, by varying the dropping height, you can produce a range of different (measured / known) speeds. The plastic tube could, perhaps, have a light coat of silicone furniture spray polish to deal with irregularities inside the tube. Check by finding the range of output volts for a number of drops from the same height. That is a good intro into experimental uncertainty.
The optimum coil design will actually depend on the load you drive (implied further up the thread) and leds would be an excellent qualitative test. But the brightest illumination would be obtained from fewer, thicker turns that what gives you the highest induced voltage. All this would need to be sorted out before the students actually get involved i.e. initial lesson preparation should always involve preparation time that's at least as long as the intended lesson.

dropping it is a good idea. that's for the students to figure out though . We have been discussing uncertainty all year, so that is definitely something I expect them to address in their reports.

I'm limited on supplies. we will be using 22 gauge wire. next year ill buy lower gauge wire and let them mess around with different gauges.

 

[nmsurobert]

I tried an LED on my coil today and the voltage is not high enough to light it. You could display the voltage using an oscilloscope.

we have one oscilloscope somewhere in the building. I'm not sure if it works. but I would like to make use of it if I knew where it was.

 

[sophiecentaur]

I think 22WG is thicker than optimal. Plus, it could cost more than necessary. You want many turns for such a low power system. are you planning to supply them with ready made coils? Winding coils could be a demanding exercise for some students using thin wire. A lathe can be a handy coil winder, too. You can look up coil winding tables to find turns per cm of coil layers for different WG.

 

[sophiecentaur]

A scope will certainly show you what you want. It's definitely worth while locating it and learning what all the buttons do before any lesson you try to present with it. A steep learning curve so be prepared for a bit of learning time if you want to emerge with no egg on face.

 

[alan123hk]

we are using a voltmeter. we are simply going to wrap some wire around pvc of varying diameter and move some magnets into and out of the coils.

I am curious to know how the induced voltage changes relative to the diameter of the PVC pipe. I believe that for the same number of turns and same magnet, since the magnetic flux is roughly the same, the induced voltage should not change significantly.

In addition, if the induced voltage will be used to drive the LEDs, it is probably better to first estimate whether there is enough voltage and current to light up several different brightness levels on thoes LEDs. In the initial evaluation of the system, all factors should be considered, including the total magnetic flux generated by the magnet, the dropping speed of the magnet, the number of turns, the coil resistance, the LEDs voltage and current specifications..etc.

However, if an active amplifier circuit can be used, then there is no need to worry too much about it.

 

[sophiecentaur]

I agree. If you want to teach Physics in a practical way, it is essential to provide experiments that can be relied on to work. It is not good for students to conclude that Physics doesn’t always work. A teacher must be one step ahead in practicals. I have seen that happen with some teachers.

 

[tech99]

I am curious to know how the induced voltage changes relative to the diameter of the PVC pipe. I believe that for the same number of turns and same magnet, since the magnetic flux is roughly the same, the induced voltage should not change significantly.

In addition, if the induced voltage will be used to drive the LEDs, it is probably better to first estimate whether there is enough voltage and current to light up several different brightness levels on thoes LEDs. In the initial evaluation of the system, all factors should be considered, including the total magnetic flux generated by the magnet, the dropping speed of the magnet, the number of turns, the coil resistance, the LEDs voltage and current specifications..etc.

However, if an active amplifier circuit can be used, then there is no need to worry too much about it.

You need an analogue display to show fleeting events like this. An LED did not flash in my test.
If you use a large diameter coil, there is more leakage flux.
Coil winding just using a scamble winding is the easiest thing in the world. Wire gauge is not critical. Remember to scrape off the enamel insulation.
If you want more voltage, you could try a stack of Neodymium button magnets.

 

[sophiecentaur]

A search should reveal a lesson plan with details. It may not be on the first page of hits but no

 

[alan123hk]

I tried an LED on my coil today and the voltage is not high enough to light it. You could display the voltage using an oscilloscope

I think it is a good idea to drop the magnet from a given height, and using an oscilloscope to show the precise curve of the induced voltage over time is another good idea.

It is worth studying and interpreting the curve of the induced voltage captured by the scope. When the falling magnet passes through the coil, the magnetic flux passing through the coil will increase and then decrease, so I think the curve will be sinusoidal to some extent. But this single cycle sinusoidal curve should exhibit certain asymmetry since the falling speed is being increased by gravity.

 

[nmsurobert]

I played around with it today. Making a coil out of 22 gauge wire with 40 turns yielded results that were measurable with the multimeters I have. The coil is about 1.5 cm in diameter. I didn't mess around with a second diameter but I did vary the amount of magnets I was dropping through the coil and getting different voltages. I may have the students create coils of varying turns (something low vs something high) and varying amounts of magnets instead of varying diameters.

 

[nmsurobert]

I agree. If you want to teach Physics in a practical way, it is essential to provide experiments that can be relied on to work. It is not good for students to conclude that Physics doesn’t always work. A teacher must be one step ahead in practicals. I have seen that happen with some teachers.

haha that's why I'm here. I wish I could use one of my students as a "lab assistant" sometimes. Trying some of these labs out alone has not been easy.

 

[tech99]

I played around with it today. Making a coil out of 22 gauge wire with 40 turns yielded results that were measurable with the multimeters I have. The coil is about 1.5 cm in diameter. I didn't mess around with a second diameter but I did vary the amount of magnets I was dropping through the coil and getting different voltages. I may have the students create coils of varying turns (something low vs something high) and varying amounts of magnets instead of varying diameters.

Well done.
Michael Faraday used an astatic galvanometer as his indicator by the way.

 

[sophiecentaur]

Trying some of these labs out alone has not been easy.

I've been there. By 16.00 everyone had gone from my Sciecne Department. Teaching a practical can be a one man band. To be fair, out Lab Tech was very helpful but he wasn't paid to hang around after school.

I suggested thinner wire because you could get a couple of hundred turns on your former and that would give much more oomph.

 

[vanhees71]

I played around with it today. Making a coil out of 22 gauge wire with 40 turns yielded results that were measurable with the multimeters I have. The coil is about 1.5 cm in diameter. I didn't mess around with a second diameter but I did vary the amount of magnets I was dropping through the coil and getting different voltages. I may have the students create coils of varying turns (something low vs something high) and varying amounts of magnets instead of varying diameters.

I hope you use a good old galvanometer and not some digital device. With the galvanometer you can make those measurements quantitative, providing a great exercise about the the theory of the galvanometer as a damped harmonic oscillator subject to time-dependent external forces/torques and the electromagnetic Lorentz force providing them.

 

[sophiecentaur]

I hope you use a good old galvanometer and not some digital device. With the galvanometer you can make those measurements quantitative, providing a great exercise about the the theory of the galvanometer as a damped harmonic oscillator subject to time-dependent external forces/torques and the electromagnetic Lorentz force providing them.

A good place to go after they have got a grasp of the basics - like getting the equipment optimised to produce a reasonable power output. Slinging things together to see what happens is never the best way to present Science / Engineering to students. Designing credible demo equipment requires either a lot of experience or at least three passes through the process.

 

[ZapperZ]

ahh ok. perfect.

we are using a voltmeter. we are simply going to wrap some wire around pvc of varying diameter and move some magnets into and out of the coils.

A voltmeter may not be the best thing to use.

As as been stated, you need a sensitive galvanometer with zero in the center. This is because you need to be able to show both positive and negative deflections, corresponding to the two direction of the current going through the coil.

Here's the setup that my students use as part of their Lenz's law lesson.

I didn't teach them about Lenz's law, but rather let them discover the "rules of the game" by letting them find out the direction of the current themselves as they move the magnet in and out of the coil.

Zz.

 

[sophiecentaur]

I didn't teach them about Lenz's law, but rather let them discover the "rules of the game" by letting them find out the direction of the current themselves as they move the magnet in and out of the coil.

How did they find out about the direction of the expected force? Was it by using a battery to give a known current direction? (I think that information could help the thread.)

 

[ZapperZ]

How did they find out about the direction of the expected force? Was it by using a battery to give a known current direction? (I think that information could help the thread.)

Force?

This is all about the magnitude and direction of the induced current in relation to the magnet's
movement, I.e. Lenz's Law.

Zz.

 

[sophiecentaur]

Without a force measurement / direction, how do they determine direction?
Ahh, I think I see where you are coming from but don't they still need some way to determine the sense of the current so that they can conclude it is in the 'opposite direction'? Opposite to what??

 

[ZapperZ]

The galvanometer has + and - terminals. If current goes in the + terminal, it has a positive deflection.

It is this experiment, but in real life.

https://phet.colorado.edu/sims/html/faradays-law/latest/faradays-law_en.html

Zz.

 

[sophiecentaur]

The galvanometer has + and - terminals. If current goes in the + terminal, it has a positive deflection.

It is this experiment, but in real life.

https://phet.colorado.edu/sims/html/faradays-law/latest/faradays-law_en.html

Zz.

My problem is what direction (from what they know) would the current be, to be 'opposing'? You would need to know the 'other' current, to be sure it's in the other direction. That's why I suggested using a battery. If you don't actually know both the induced current direction and the current it 'would be' opposing then I don't see how they can conclude that Lenz applies - without just believing what they're told.

 

[ZapperZ]

My problem is what direction (from what they know) would the current be, to be 'opposing'? You would need to know the 'other' current, to be sure it's in the other direction. That's why I suggested using a battery. If you don't actually know both the induced current direction and the current it 'would be' opposing then I don't see how they can conclude that Lenz applies - without just believing what they're told.

I don't understand your problem here.

If you look at the simulation, I don't need a battery. I already told you that for the galvanometer, if a current goes into the positive terminal, it will deflect positively. If it goes into the negative terminal, it will deflect negatively. One doesn't need to believe in this. One can easily check it.

Once we know that, and the way the coil is connected, one can see if cw or ccw current IN THE COIL produces a positive or negative deflection.

Then one brings a bar magnet close to one of the opening of the coil. Say it deflects positively. It means that we know that the induced current in the coil is, say, ccw. The direction of the induced current tells you also the direction of the induced magnetic field!

Doing this with several difference scenarios tells you a "pattern" or rules on the nature of the induced current and induced magnetic field.

Zz.

 

[sophiecentaur]

@ZapperZ
You have two options to present /demonstrate Lenz's law to them. You either tell them the field direction in a current carrying coil or they can measure / discover it by using a DC source and the same magnet. Either way, it needs to be known so that it becomes obvious that Lenz's law applies. If you have decided that telling them is enough then that's fair enough. Personally, I would think it more in keeping with a 'learning by doing' approach, not to rely on just telling them.

 

[tech99]

haha that's why I'm here. I wish I could use one of my students as a "lab assistant" sometimes. Trying some of these labs out alone has not been easy.

If you want help with future practicals please ask us here.