Thread: how to determine number of turns needed to generate specific magnetic field?

1. how to determine number of turns needed to generate specific magnetic field?

I want to generate a magnetic field of about 50 micro teslas. so I am going to make my own coil, but I need to know how many turns the coil should have?

Power input is 9 V, the coil diameter is 4 inches, and the desired magnetic field is 50 micro tesla. I am using 20 awg insulated copper wire. Is there any other information needed here?

I was looking at formulas on this site here

ww.vias.org/physics/bk4_07_03

But I get confused about which letters are supposed to stand for which measurements. Please understand I am not some kind of electrician or physics whiz. This may be super simple for somebody else but for me it's not

2. by the way, I can't post links so I dropped one W. from that URL

3. The internal magnetic field of a coil of wire is approximately:

$B=\mu_0 n i$

where $\mu_0 = 4\pi \times 10^{-7}$, i is the current in Amps, and n is the number of turns (loops) per metre.

So, for example, if you want a field of 50 microTelsa with a (large) current of 1 Amp, you'll need a coil with around 40000 turns per metre, or 400 turns per centimeter, or 40 turns per millimetre. That means that the diameter of the (insulated) wire would need to be about 0.025 millimetres or 25 microns, which is very very small.

I'm not sure what 20 awg means, but my guess is that your wire is way too thick.

4. James, you seem to be out by a factor of 1000, unless I'm missing something obvious. He said microtesla, not millitesla?
Also, he could use multiple layers to get more turns per metre (but 40,000 could be a stretch!)

Lapis, what's the context?
Do you need that field strength inside the coil, at an end, or elsewhere?
What power source are you using? (The current it can provide is important).
Where do you want that field strength? Inside the coil, or at the ends? If it's at the ends, you can raise the field strength by using a ferrite (or better) core.
How precise and uniform does the field have to be?

But yeah... if I haven't stuffed up, you can get 50 microtesla in an air coil with 10 turns/cm (which I think about matches the insulated diameter of your wire) with a current of about 40 milliamps (if it's not doing any work). A cheapo AA battery could run it for ten hours. You'll need to add an appropriate resistor (preferably variable) and some kind of meter (eg a simple ammeter) to limit and control the current.

5. Originally Posted by Pete
James, you seem to be out by a factor of 1000, unless I'm missing something obvious. He said microtesla, not millitesla?
Oops.

Well, 40 microTesla looks more manageable than 40 milliTesla...

6. 20AWG is just standard hook up wire - American Wire Gauge.

7. Originally Posted by Pete
James, you seem to be out by a factor of 1000, unless I'm missing something obvious. He said microtesla, not millitesla?
Also, he could use multiple layers to get more turns per metre (but 40,000 could be a stretch!)

Lapis, what's the context?
Do you need that field strength inside the coil, at an end, or elsewhere?
What power source are you using? (The current it can provide is important).
Where do you want that field strength? Inside the coil, or at the ends? If it's at the ends, you can raise the field strength by using a ferrite (or better) core.
How precise and uniform does the field have to be?

But yeah... if I haven't stuffed up, you can get 50 microtesla in an air coil with 10 turns/cm (which I think about matches the insulated diameter of your wire) with a current of about 40 milliamps (if it's not doing any work). A cheapo AA battery could run it for ten hours. You'll need to add an appropriate resistor (preferably variable) and some kind of meter (eg a simple ammeter) to limit and control the current.

The context of this is that I want to conduct an experiment with a pulsed magnetic field, and how it might affect plant growth

I want the strength of the field to be about the same as the Earth's, thus 50 micro tesla. But since the Earth's magnetic field fluctuates it could be anywhere between 20 and 90 micro tesla. It does not have to be absolutely precise

I want to put the coil around the stem of a potted plant, near the soil. So I want the field strength to be inside the coil

Also, I want to pulse the field. So I am using a frequency generator, specifically the atelier robin 165. The output of the generator is variable between zero and 13 V, and puts out a maximum of 125 milliamps. Hopefully that is enough amperage?

Do you still think I would need to put an ammeter in line even though I have using the frequency generator?

8. Originally Posted by Lapis_x
Do you still think I would need to put an ammeter in line even though I have using the frequency generator?
You don't need an ammeter per se, but as others have mentioned, the decisive factor for generating a field in an inductor is the input current. So you will need a stage between the voltage source and the coil. This can be as simple as a resistor, or as fancy as electronics to convert your variable-voltage source into a variable-current source. I.e., an amplifier - this is like an audio circuit where you take an input signal from a playback device (a variable voltage source) and drive the speakers (magnetic coils driven by variable current).

9. Originally Posted by James R
you'll need a coil with around 40000 turns per metre, or 400 turns per centimeter, or 40 turns per millimetre.
How are these turns generated?

10. A permanent magnet is not good?

11. Originally Posted by Emil

A permanent magnet is not good?
I would like to pulse the magnetic field, so it permanent magnet will not work

You don't need an ammeter per se, but as others have mentioned, the decisive factor for generating a field in an inductor is the input current. So you will need a stage between the voltage source and the coil. This can be as simple as a resistor, or as fancy as electronics to convert your variable-voltage source into a variable-current source. I.e., an amplifier - this is like an audio circuit where you take an input signal from a playback device (a variable voltage source) and drive the speakers (magnetic coils driven by variable current).
okay thanks. the output plug on my frequency generator is a BNC plug. How do I go from a BNC to a resistor, and then connect that resistor to the two ends of the magnetic coil?

Also, can you give me an idea of what kind of resistor I need? For instance can you link to a product somewhere? I honestly have no clue what type of resistor I am looking at here. Or show me what amplifier I should get.

Thanks for any help

as others have mentioned, the decisive factor for generating a field in an inductor is the input current. So you will need a stage between the voltage source and the coil. .
also, why exactly do I need a stage between the voltage source and the coil?

14. any input on my last two questions?

15. How do I go from a BNC to a resistor?

Maybe coaxial cable or plug something into the BNC.

16. Originally Posted by Lapis_x
okay thanks. the output plug on my frequency generator is a BNC plug. How do I go from a BNC to a resistor, and then connect that resistor to the two ends of the magnetic coil?

Also, can you give me an idea of what kind of resistor I need? For instance can you link to a product somewhere? I honestly have no clue what type of resistor I am looking at here. Or show me what amplifier I should get.

Thanks for any help
If you can set the frequency generator to produce the required current, then i think you won't need anything else.
If you can only set the voltage, then you might be able to get the required current by just matching it to the resistance of you coil. However, the coil resistance is likely to be so low (unless you make a wde coil with heaps of turns, ie use a lot of wire) that an resistor on the end of the coil wire might be necessary.

Do you understand the relationship between voltage, current, and resistance?
Have you seen a resistor like this before?

You might also consider a variable resistor (potentiometer, or pot):

If you need a resistor, you need to be able to figure out the right one (ie how many ohms), buy it at an electronics shop (very cheap), and attach it to the end of your coil (Soldering is best. Not too tricky to learn, and a basic soldering iron doesn't cost much.)

The BNC connection to your frequency generator means you'll need a short length of cable with a BNC connector:

Ask at a computer shop. They might have some cables lying around from when it was a common network cable years ago. Or try the electronics shop. You can also ask at the electronics shop about stripping back the insulation and attaching it to your coil. Again, soldering is probably the best way.

Be careful of my advice, by the way - check everything I say with an electronics shop geek. You don't want to blow up your frequency generator because some random guy on the Web gave you bad advice.

17. Originally Posted by Pete
If you can set the frequency generator to produce the required current, then i think you won't need anything else.
If you can only set the voltage, then you might be able to get the required current by just matching it to the resistance of you coil. However, the coil resistance is likely to be so low (unless you make a wde coil with heaps of turns, ie use a lot of wire) that an resistor on the end of the coil wire might be necessary.

Do you understand the relationship between voltage, current, and resistance?
Have you seen a resistor like this before?

You might also consider a variable resistor (potentiometer, or pot):

If you need a resistor, you need to be able to figure out the right one (ie how many ohms), buy it at an electronics shop (very cheap), and attach it to the end of your coil (Soldering is best. Not too tricky to learn, and a basic soldering iron doesn't cost much.)

The BNC connection to your frequency generator means you'll need a short length of cable with a BNC connector:

Ask at a computer shop. They might have some cables lying around from when it was a common network cable years ago. Or try the electronics shop. You can also ask at the electronics shop about stripping back the insulation and attaching it to your coil. Again, soldering is probably the best way.

Be careful of my advice, by the way - check everything I say with an electronics shop geek. You don't want to blow up your frequency generator because some random guy on the Web gave you bad advice.
okay thanks

By the way, no, I do not understand the difference between voltage, current, and resistance

Also, the frequency generator only allows you to adjust voltage, the amperage is not adjustable

18. The resistor Pete has shown is call an axial lead resistor.
If the power level if 1/4 watt, it will plug directly in to a female
BNC connector. The freq generator will no doubt have an
output BNC, so a BNC adaptor will connect the
resistor directly to the generator and no soldering
will be needed. We EE's often do this with scopes,
connecting a resistor directly to the scope for some given test.

============================

Is it that magnetic field on the order of the Earth's
magnetic field can affect plant growth ?
Or inhibit plant growth ?

What will be your control, a plant against which you can compare ?

Long ago when I was in High School I took 4 plants and
varied their environment using sun and water; one got both,
one got neither, and the other two got one of each.
The results were as you might expect, but I can not recall if
I concluded water or sun the more important factor.

Cheers

19. Lapis,
What frequency range are you going to use, because odds are your coil is going to ring like a bell and your peak B field is going to be off by say an order of magnitude or so.

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