Physics Help Forum Magnetic field inside

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 Mar 19th 2018, 05:22 AM #1 Junior Member   Join Date: Mar 2018 Posts: 2 Magnetic field inside What happens to the magnetic field inside this solenoid, if you double the number of revolutions, simultaneously stretching the solenoid twice as much as the original length? Suppose that the applied voltage was kept constant, and the additional wire length for additional turns of the coil did not noticeably increase the resistance of the solenoid? Who can give a detailed answer and explain why this works this way? Last edited by topsquark; Mar 19th 2018 at 03:19 PM.
 Mar 19th 2018, 05:24 AM #2 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 995 Note Advertisement topsquark likes this.
Mar 19th 2018, 09:09 AM   #3
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 Originally Posted by AKM17 What happens to the magnetic field inside this solenoid, if you double the number of revolutions, simultaneously stretching the solenoid twice as much as the original length? Suppose that the applied voltage was kept constant, and the additional wire length for additional turns of the coil did not noticeably increase the resistance of the solenoid? Who can give a detailed answer and explain why this works this way?
If you just want a general description of Ampere's law applied to a solenoid, you can try these sites:

There's hyperphysics:

HyperPhysics

Wikipedia:

https://en.wikipedia.org/wiki/Solenoid

There's probably a lot more websites too, but I usually use those two. They explain the formula for the magnetic flux density of the solenoid as a function of the number of turns, N, current, i, and length, l.

$\displaystyle B = \mu_0 \frac{N i}{l}$

where $\displaystyle \mu_0$ is the permeability of free space. so comparing two solenoids with the following turns and lengths:

1: $\displaystyle N = N_1, l = L_1$
2: $\displaystyle N = N_2 = 2 N_1, l = L_2 = 2 L_1$

then

$\displaystyle B_1 = \mu_0 \frac{N_1 i}{L_1}$

$\displaystyle B_2 = \mu_0 \frac{N_2 i}{L_2} = \mu_0 \frac{\cancel{2} N_1 i}{\cancel{2} L_1}= \mu_0 \frac{N_1 i}{L_1} = B_1$

so the magnetic flux density is the same.

Last edited by topsquark; Mar 19th 2018 at 03:19 PM.

Mar 19th 2018, 09:03 PM   #4
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With respect to benit I think the last thing a "junior member" wants is lines of equations .

I know it makes me switch off

 Originally Posted by AKM17 What happens to the magnetic field inside this solenoid, if you double the number of revolutions, simultaneously stretching the solenoid twice as much as the original length?
Turns ....If you double the number of turns (of wire) and the current is the same , the magnetic effect will double ....If the path of the magnetic field is the same

But you're stretching the solenoid to twice it's original length , this will reduce the magnetic strength somewhat ... but not too much , you may get an increase of perhaps 50% ... and not 100% if the length was the same.

 Originally Posted by AKM17 Suppose that the applied voltage was kept constant, and the additional wire length for additional turns of the coil did not noticeably increase the resistance of the solenoid? Who can give a detailed answer and explain why this works this way?
If there is no increase in resistance then the current remains the same (DC only), so with the extra turns the magnetic field increases as outlined above .....

When making a coil the turns close to the core are much more effective ,a small amount (hence low resistance) of wire has made one turn ... the turns on the outside use more wire to make a turn but only generate the same amount of magnetism per turn...

You can stretch the solenoid to help things , but a compact shape is often more convenient .

Mar 22nd 2018, 10:56 AM   #5
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 Originally Posted by AKM17 Many thanks for the detailed answers among other things.
That website studydaddy is a site where people get paid to help, i.e. professional tutors. Here we tutor for free, some of us are as educated than those tutors and some are more educated. In fact my website was supposed to the front piece of my private tutoring job I was going to create. Health problems got in the way though.

Last edited by topsquark; Mar 29th 2018 at 07:57 AM.

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