# Physics Lab -- Don't Understand Procedure...

#### John A

I have a physics lab and the main question is:
'How does the radius of the loops of a solenoid affect the magnetic field produced by the coil?'

For my hypothesis, I wrote:
"The larger the radius of a solenoid is,the weaker the magnetic field will be."

I searched online and found out I was wrong but I'm keeping since it was my initial hypothesis.
I'm stuck on the part where I have to write a procedure. My teacher states my procedure has to clear enough that "someone who knows nothing about it can follow and do it properly." That's the biggest hint I have and I'm lost. How do I write a procedure on how the radius of a selenoid effects the magnetic field. I know it has something to do with ampere's law which calculates the magnetic field, B= μoNI/L

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The "Slinky" toy mentioned in your attachment is not very useful as a solenoid because you need to have the wires insulated if they are in contact with each other. One suggestion for making a solenoid is to wrap a non-magnetic tube, (something like a plastic roll would work ok), with insulated wire. (The insulation doesn't need to be thick. ) You can get the wire from an electronics store like Radio Shack. The strength of the magnetic field inside the solenoid is proportional to the number of loops per unit length. I do suggest you put a resistor in series with the solenoid (Perhaps R=10 ohms) and power it with about a 10 volt power supply to get I=1 ampere or thereabouts. Your 1 ohm resistor needs to be able to handle 1 watt or more of power (You need to get a resistor rated for a couple watts=this component could get a little warm when the power is on). The most difficult part is measuring the magnetic field inside the solenoid. One way I know of is a loop of current carrying wire will experience a torque in a magnetic field. Getting an accurate quantitative measurement of the torque on a loop of wire would be somewhat tricky. Perhaps some other people may have a better idea to get a measure of the magnetic field strength in the solenoid.

#### kq6up

Reading the paper, the procedure you would write is for using the Java app to test a hypothesis. Can you see the Java applet running? Were you able to manipulate it?

Regards,
KQ6UP

#### John A

The "Slinky" toy mentioned in your attachment is not very useful as a solenoid because you need to have the wires insulated if they are in contact with each other. One suggestion for making a solenoid is to wrap a non-magnetic tube, (something like a plastic roll would work ok), with insulated wire. (The insulation doesn't need to be thick. ) You can get the wire from an electronics store like Radio Shack. The strength of the magnetic field inside the solenoid is proportional to the number of loops per unit length. I do suggest you put a resistor in series with the solenoid (Perhaps R=10 ohms) and power it with about a 10 volt power supply to get I=1 ampere or thereabouts. Your 1 ohm resistor needs to be able to handle 1 watt or more of power (You need to get a resistor rated for a couple watts=this component could get a little warm when the power is on). The most difficult part is measuring the magnetic field inside the solenoid. One way I know of is a loop of current carrying wire will experience a torque in a magnetic field. Getting an accurate quantitative measurement of the torque on a loop of wire would be somewhat tricky. Perhaps some other people may have a better idea to get a measure of the magnetic field strength in the solenoid.
Its all theoretical, therefore I'm not actually building one (thank God!)

#### John A

Reading the paper, the procedure you would write is for using the Java app to test a hypothesis. Can you see the Java applet running? Were you able to manipulate it?

Regards,
KQ6UP
I was on the app earlier today, I noticed that the larger the slinky was, the greater the magnetic field expanded but apart from that I wasn't too sure what was going on. Electricity & Magnetism was my worst unit in Physics btw :/

#### kq6up

What bearing on how close together the turns are should effect the magnetic field. I do not see this happening in this Java model. This is the "turn density" or "winding density" and it is equal to $\frac{N}{L}$. The magnetic field is proportional to the turn density and/or the current, not the diameter of the coil. You can change the diameter of the coil, and see that the length of the lines remain the same -- also the spacing. However, I think the Java app is rather limited, and it should be showing a shorter length on the vectors as you move away from the coils of wire. Also, the vectors should be shorter if you increase the spacing between the turns (that is the length of the magnetic field vectors in the center of the solenoid). Is this helpful?

Regards,
KQ6UP

#### John A

What bearing on how close together the turns are should effect the magnetic field. I do not see this happening in this Java model. This is the "turn density" or "winding density" and it is equal to $\frac{N}{L}$. The magnetic field is proportional to the turn density and/or the current, not the diameter of the coil. You can change the diameter of the coil, and see that the length of the lines remain the same -- also the spacing. However, I think the Java app is rather limited, and it should be showing a shorter length on the vectors as you move away from the coils of wire. Also, the vectors should be shorter if you increase the spacing between the turns (that is the length of the magnetic field vectors in the center of the solenoid). Is this helpful?

Regards,
KQ6UP
Thanks for your help, I write what I learned and speak to my teacher tomorrow.

#### kq6up

You know what I just noticed. The vectors are darker or lighter (not longer or shorter) depending on the strength of the $\mathbf{B}$ field. So that is a hint, and from that info -- you should be able to figure it out.

Regards,
KQ6UP

"Physics Lab -- Don't Understand Procedure..."

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