14
$\begingroup$

I'm trying one of the most basic physics home experiments: creating an electromagnet by wrapping electrically-conductive wire around a metal screw.

My ingredients:

  1. a metal screw
    I don't know the material, but it is attracted to magnets
  2. a pipe cleaner
    my improvised wire since I don't have a "true" wire
    I don't know the material of the metal core but it is attracted to magnets
    ends are trimmed of chenille (the fuzzy stuff); not 100% clean, but as best as I was able, with scissors
  3. a battery
    I tried 1.5V AA, a 3V CR2450, and a 1.5V D batteries
  4. a paper clip
    a "target object" to try attract with the working electromagnet
    confirmed as attracted to magnets

enter image description here

The process is the familiar one you'd expect: I wrap my "wire" around the screw, in one direction only, then connect the stripped ends of the wire to the positive and negative ends of the battery. But: no electromagnet. As noted above, I tried different battery types.

Question: what could be wrong with my "ingredients" or process?

"Getting the thing to work" is actually of secondary importance: I'd like to learn ideas for how to diagnose (or "debug") what could be wrong.

As you might be able to tell, I have the notion that "attracted to a magnet" also means "able to conduct electricity" (especially with respect to the pipe cleaner-as-a-wire-substitute), which I'm not certain is true.

$\endgroup$
9
  • 1
    $\begingroup$ Your current is too small to actually "feel" repulsion from the magnet, probably. Use a compass. Is your wire also very conductive? $\endgroup$ Jul 30 at 19:33
  • 8
    $\begingroup$ "attracted to magnet" means ferromagnetic (typically only iron, steel, and only a few types of stainless steel). "Electrically conductive" is just about any metal. They are very different. $\endgroup$
    – RC_23
    Jul 30 at 20:07
  • 7
    $\begingroup$ The last three times I've bought a multimeter, I've gotten one from a hardware store for less than twenty dollars. "Every project is an opportunity to invest in at least one new tool," according to an acquaintance with a tool problem. $\endgroup$
    – rob
    Jul 30 at 20:31
  • 2
    $\begingroup$ Without the tools you wont be able to. You could get some iron filings and use this to find the circular rings of the magnetic field of a wire. To test magnetic fields buy a Compus, Iron filings, Multimeter, and Enamaled copper wire[Made for solenoids]. "Enameled" is very important when creating solenoids, otherwise the wire willl short circuit and current will not follow the loops, but go straight. These are cheap on Ebay, it is very nice to be able to put the theory to test in your own home. id also reccommend buying crocodile clips and something to house the battery for easy connection $\endgroup$ Jul 30 at 22:14
  • 4
    $\begingroup$ Your pipe cleaner is only wrapped around the screw around 7 times. This is not going to produce a very strong magnetic field, and I wouldn't be surprised if the field was too weak for you to detect it. On the other hand, I don't know whether the pipe cleaner is long enough to wrap it an adequate number of times. $\endgroup$ Jul 30 at 23:45

5 Answers 5

38
$\begingroup$

The insulation on the pipe cleaner is fine (I tested it) and the only difficulty is getting good electrical contact at the ends.
It is best to burn off the end insulation and then scrape the metal with a knife / emery paper until the metal is seen to be shiny.

Your null result is due to a number of factor the main one being that the magnetic field produced by your electromagnet is very small and only realistically detected with a compass or a sensitive magnetometer.
You can make one by straightening a paper-clip and then stroking along the paper-clip with a magnet to magnetise the paper-clip. I used a large paper-clip as then as a compass it is more sensitive to changes in the magnet field around it.
You can then either float the paper-clip on water but putting in on an upturned bottle top weighted down with some Blu-Tack or suspending it from a fine thread which is what I did. The tread was about $70\,\rm cm$ long anchored on a table top with some Blu-Tack.
You will find that if the paper-clip is suspended from its centre it aligns with the Earth's magnetic field even to the extent that it inclines along the line of the non-horizontal Earth's magnetic field lines.
For ease of use adjust the point of suspension so that the paper-clip was horizontal.

enter image description here

Take a steel screw and tested it by bringing it close to the compass and often you will find it is magnetised because one end of the screw repels one end of compass.
Both ends of an unmagnetised steel screw would attract a compass. If you wished you could demagnetise the steel screw by heating it to red heat whilst it is orientated in a magnetic East-West direction.

Wind a few turns of pipe cleaner around the screw connected it to a $1.5 \,\rm V$ C-type battery (or one that is close at hand) using finger and thumb and you might notice that the ends got warm.

Bring the electromagnet close to the compass, note the effect on the compass and then reverse the battery and again note the effect on the compass.
Hopefully that you will get attraction with the battery connected one way and repulsion with the battery connected the other way around.

In some ways all this is "old school" and you can use an iPhone to detect magnetic field.
The magnetometer is at the top right of the iPhone and I used the app Sensor Kinetic Pro which I downloaded from the Apple app store.

Here is a screen shot of the recorded magnetic field before and after I switched on the electromagnet which shows how small the field due the electromagnet is.

enter image description here

As a final point think of what you had at hand, a battery, a pipe cleaner etc and what you did not have, eg a compass, a reel of insulated copper wire, etc and then remember that Faraday and other also lacked basic "off the self" apparatus.
They had to make the apparatus as they went along and to me it makes their discoveries all the more remarkable.

$\endgroup$
7
  • 8
    $\begingroup$ Have an upvote for addressing the querent as they come to you and showing them what they could do with their materials, unlike so many commenters'/answerers' admonitions to buy different materials and tools. $\endgroup$
    – nitsua60
    Jul 31 at 15:12
  • 1
    $\begingroup$ One AA battery and a dozen turns around an iron core makes an electromagnet that readily attracts a paper clip, in my experience. $\endgroup$
    – hobbs
    Jul 31 at 18:47
  • 1
    $\begingroup$ @hobbs That may be so but the OP used a steel screw, a pipe cleaner and but a few turns. The question was to explain why that arrangement seemed not to work. $\endgroup$
    – Farcher
    Jul 31 at 21:27
  • $\begingroup$ Where did you measure the magnetic field (at what point with respect to the coil)? Just curious. $\endgroup$
    – akhmeteli
    Aug 1 at 6:59
  • $\begingroup$ @akhmeteli I did not spend too much setting it up and placed the magnetometer probe as close as I could to one end of the screw. $\endgroup$
    – Farcher
    Aug 1 at 14:49
5
$\begingroup$

I think the problem why your electromagnet did not work is:
The pipe cleaner (which you used as the wire) has a conducting surface. Therefore the current did not flow around the screw, but just along from one end of the screw to the other.
To make an electromagnet you want the electric current to flow around the central core many times.

  • The wire needs to be conductor. So you can use any metal (e.g. copper). It doesn't need to be a magnetic metal (like iron).
  • You want the windings of the wire to be isolated from each other and from the central screw. By doing so you are guiding the current to flow around the core, instead of just flowing from one turn to the next turn witout flowing around. So I recommend you replace your iron pipe cleaner by a wire with a plastic isolation.
    enter image description here
    (image from Wikipedia - Electromagnet)
    Alternatively you may also use a wire without an isolation. But then you need an isolating sheet (like a paper) between screw and wire. And you need to wind the wire carefully so that neighboring turns do not touch each other.
  • Currently your coil has only 7 turns. To get a reasonably strong electromagnet you need many more turns (may be a few hundreds).
  • It is a good idea to use an iron core (like your screw) to make a strong electromagnet. But this is not strictly necessary. You would get a weaker electromagnet already without an iron core.
$\endgroup$
2
  • 5
    $\begingroup$ The pipe cleaner (which you used as the wire) has a conducting surface. This is probably not the case. $\endgroup$
    – Farcher
    Jul 31 at 10:43
  • $\begingroup$ The pipe cleaner is not well insulated and there could be metal-to-metal contact. $\endgroup$ Aug 1 at 13:53
4
$\begingroup$

There is a popular joke: "electronics is a science about contacts".

First, you need to check that there is current in your circuit, because there can be breaks in your "wire", as others told you, or the ends of the "wire" can be covered with oxides (in this case you need to dress the ends with a knife or a file). As you don't have a multimeter, you can use your tongue for that (don't try that if the voltage is greater than, say, 10 V).

Another thing: where do you measure the magnetic field? If it is at the sides of your "coil", then the field can be very low there, so it might be better to measure the field at the ends of the coil.

How to measure the magnetic field? So you use a compass app in your phone, but maybe you should use a more specialized app in your smartphone (see, e.g., https://arxiv.org/abs/1901.00857, but I believe there are quite a few such apps).

$\endgroup$
2
  • 2
    $\begingroup$ If you try doing the experiment the heating of of the contacts between pipe-cleaner and battery terminal will show that a current is flowing. $\endgroup$
    – Farcher
    Jul 31 at 13:24
  • $\begingroup$ @Farcher : I guess it depends on the contacts and the "wire"'s resistance. Anyway, the tongue is typically readily available :-) and quite sensitive. $\endgroup$
    – akhmeteli
    Aug 1 at 7:08
3
$\begingroup$

As others have noted, your field is too small to be noticeable. The 1.5V D battery is your best power source. You can get about 2 amps out of it, which will pull it down to about 1V over most of its life. It'll last about 2hrs at this draw.

The field you generate is proportional to the ampere-turns around the screw. To get 2 amps out of the battery, you need a resistance of about 0.5 ohms. To maximize ampere-turns, you need a wire with 0.5 ohm resistance that is a long as possible while being thin enough to be wound pretty close to the core.

Your core looks to be about an inch long and a little less than 1/4 inch in diameter. In that case, your approximately best results will be achieved by winding about 4m of AWG 26 magnet wire tightly around your core. It's important to use magnet wire, because the insulation is very thin and that allows you to get as much copper close to the core as possible. If your winding is tight and orderly, you should get over 200 turns in 4 layers.

That's 400 ampere-turns, which is probably 50 to 100 times more than you're getting with your 5 turns of pipe cleaner.

The strength of the magnet you make this way will be "noticeable", but not "strong". You'll be able to attract your paper clip from 1-2cm away. Making "strong" electromagnetic forces requires optimizing the magnetic circuit in addition to the electric circuit.

$\endgroup$
1
$\begingroup$

A few off the cuff thoughts: your pipe cleaner might have breaks in it (check with a multimeter), your electrical contact between the batteries and the pipe cleaner may not be great.

But the bigger issues are probably:

  • the pipe cleaner wire seems like actually two wires that are twisted together. This will not create the correct solenoidal magnetic field you are looking for like a single wire winding around a core would.
  • the magnetic field $B$ produced by an electromagnet is given by the Biot Savart Law:

$$ B = \frac {\mu N I}{L} = \frac {\mu N V}{LR} $$ ($\mu =$ permeability (let's say your nail is steel)), $L=$ length of nail that's wrapped in coil, $N=$ number of turns in the coil, $I =$ current, $V =$ voltage, $R =$ resistance.

Let's estimate some values for this. You have about 5 turns with 1.5 V battery, over about 2 cm of length. The permeability of iron is on the order of $1000*10^{-7 }= 10^{-4}$. The resistance of your pipe cleaner is probably quite high, but let's generously assume 1 Ohm. That gives us:

$$B = 0.04 \, \mathrm {Tesla}$$

This is actually roughly on the order of the field of a refrigerator magnet, but I made some very generous assumptions about your setup, and like I said the wire should not be twisted. Typically to do this you would take a long copper wire and your number of windings would be in the hundreds or at least 10s, not single digits.

$\endgroup$
9
  • 7
    $\begingroup$ Why would a twisted pair not work in this case (if the contacts are OK)? The current will go in the same direction in both wires? $\endgroup$
    – akhmeteli
    Jul 30 at 22:27
  • 5
    $\begingroup$ It depends on how you connect the wires. If both wires at one end of the solenoid are connected to the same pole of the battery, and both wires at the other end are connected to the other pole, the fields will not cancel. $\endgroup$
    – akhmeteli
    Jul 30 at 23:50
  • 4
    $\begingroup$ I lack motivation for that. Typically, a twisted pair is connected in such a way that the currents in the two conductors have opposite directions, and then the fields of the two conductors do cancel. If, however, you connect the twisted pair in such a way that the currents in the two conductors have the same direction, the fields will not cancel. $\endgroup$
    – akhmeteli
    Jul 31 at 0:36
  • 2
    $\begingroup$ The magnetic field due to arrangement described by is OP is much, much smaller than that estimated here and the fact that the metal core of the pipe-cleaner is multi-stranded is no different to using insulated multi stranded wire. $\endgroup$
    – Farcher
    Jul 31 at 13:22
  • 7
    $\begingroup$ "If the wires are parallel and twisted, the fields from the two cancel out and you end up with nothing." – Only if the current is going in opposite directions in the two wires. If the current is going the same way in both wires, then both wires produce very nearly the same magnetic field, so the fields add constructively. The only place where the fields add destructively (which is to say, partially cancel out) is in the tiny space between the two wires. $\endgroup$ Jul 31 at 13:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.