Magnetic Field Solenoid Quick Question

n is the number of turns per unit length of the solenoid, so 1/1000, I believe.

I've yet to revise this particular module :p:

Reply 2

OP

ashy

n is the number of turns per unit length of the solenoid, so 1/1000, I believe.

I've yet to revise this particular module :p:

I suppose you mean 1000/1? :p:

That would have to be the fattest wire ever. 1 turn in 1000metres! Sheesh! Haha thanks for the help though :^_^:

Reply 3

No I mean 1/1000. You have one metre of solenoid, and 1000 turns in it. That means the length of one turn on the solenoid is 1/1000.

EDIT: hang on, let me think about this a minute :p:

No you're right, it is 1000/1. My brain is being boiled by revision, it seems.

\displaystyle\int_o^L B.dl = \mu_{o}\displaystyle\int j.dS = \mu_o I_{encl}

BL = \mu_{o} N I

B= \mu_{o}\frac{N}{L}I

B=\mu_{o}nI

If in doubt, derive! :biggrin:

Reply 4

OP

\displaystyle \frac{N_t} {L_s} = n

Now if we have 1000 turns (

N_t

) and a 1 metre (

L_s

) long solenoid... :redface:

Yayyy! Haha. Yeah don't worry, its getting to all of us. I had to use my calculator to divide 7 by 3.5 the other day... I came out with 2.5 in my head. Seriously what were Imperial thinking giving me an offer?

Reply 5

eventhorizon

\displaystyle \frac{N_t} {L_s} = n

Now if we have 1000 turns (

N_t

) and a 1 metre (

L_s

) long solenoid... :redface:

Yayyy! Haha. Yeah don't worry, its getting to all of us. I had to use my calculator to divide 7 by 3.5 the other day... I came out with 2.5 in my head. Seriously what were Imperial thinking giving me an offer?

I've spent the last 2 days cramming differential equations into my head - seems the maths briefly displaced the Physics.

Reply 6

Theorist

10

eventhorizon: Don't sweat it. Everyone has moments like these.

Best of luck with your exams. :smile:

Reply 7

OP

ashy

I've spent the last 2 days cramming differential equations into my head - seems the maths briefly displaced the Physics.

I've spent the last 14 days cramming 80 pages-worth of latin into my head. Bloody OCR...

Reply 8

OP

Theorist

eventhorizon: Don't sweat it. Everyone has moments like these.

Best of luck with your exams. :smile:

Cheers

Reply 9

eventhorizon

I've spent the last 14 days cramming 80 pages-worth of latin into my head. Bloody OCR...

Just remember that Caecilius est in horto and you'll be fine :biggrin:

Reply 10

OP

ashy

Just remember that Caecilius est in horto and you'll be fine :biggrin:

Hahaha! High five to that! :rofl:

Canis est in via?

Reply 11

F1 fanatic

16

You're not the only one it confuses, I was constantly confusing myself with this kind of thing for my MPhys 4th year project last term, as to whether I was dealing with turns total or turns per unit length.

Reply 12

eventhorizon

Hahaha! High five to that! :rofl:

Canis est in via?

Yep, and if I recall correctly, Matella is always sedensing somewhere.

Reply 13

F1 fanatic

You're not the only one it confuses, I was constantly confusing myself with this kind of thing for my MPhys 4th year project last term, as to whether I was dealing with turns total or turns per unit length.

It's so easy to get confused with the semantics of it - hence we're taught to derive everything from scratch if we get confused. Some lecturers even tell us not to bother learning final equations, but instead understand where they come from so we can always derive them.

Reply 14

OP

ashy

It's so easy to get confused with the semantics of it - hence we're taught to derive everything from scratch if we get confused. Some lecturers even tell us not to bother learning final equations, but instead understand where they come from so we can always derive them.

My physics teacher has been doing that, but I've tended to skip over some of it - I'm really not able to learn the ins and outs of the Biot-Savart law, thanks :rolleyes:

Reply 15

eventhorizon

My physics teacher has been doing that, but I've tended to skip over some of it - I'm really not able to learn the ins and outs of the Biot-Savart law, thanks :rolleyes:

You do Biot-Savart at A-level? :s-smilie:

Either he's been deriving everything from scratch properly, you're learning too much, or you're on a funky exam board that goes into more detail than mine did :p:

Reply 16

OP

ashy

You do Biot-Savart at A-level? :s-smilie:

Either he's been deriving everything from scratch properly, you're learning too much, or you're on a funky exam board that goes into more detail than mine did :p:

Well, the Solenoid formula I talked about is derived from it (somehow, right?) and so he decided to take the whole class talking french and discussing where it came from. It took us about 10 minutes to work out that Biot and Savart were actually names of people :p:

, and he wasn't having a joke about the initials being BS...

We also had a great laugh in the fact that the units for permeability of free space is measured in henries per metre (my name), and I foolishly made a bet with him that that wasn't the unit of it (he likes to joke with us sometimes so it was a fair chance he was pulling my leg), and subsequently lost a tenner.

We aren't supposed to do Biot Savart or most of this other stuff we've been learning on the side, but we do have to fill up the time doing something, and it's kinda interesting, if not stupidly difficult. Edexcel, if you were wondering :wink:

I'm gonna miss him...

Reply 17

eventhorizon

Well, the Solenoid formula I talked about is derived from it (somehow, right?) and so he decided to take the whole class talking french and discussing where it came from. It took us about 10 minutes to work out that Biot and Savart were actually names of people :p:

, and he wasn't having a joke about the initials being BS...

We also had a great laugh in the fact that the units for permeability of free space is measured in henries per metre (my name), and I foolishly made a bet with him that that wasn't the unit of it (he likes to joke with us sometimes so it was a fair chance he was pulling my leg), and subsequently lost a tenner.

We aren't supposed to do Biot Savart or most of this other stuff we've been learning on the side, but we do have to fill up the time doing something, and it's kinda interesting, if not stupidly difficult. Edexcel, if you were wondering :wink:

I'm gonna miss him...

Well Biot-Savart gives the magnetic field at a point near a current. I suppose you could use that to derive the solenoid equation you used. I did it using Ampere's Law earlier, tis easy!

Ampere's Law in a time invariant electric field is:

\displaystyle\int B.dl = \mu_{o}\displaystyle\int j.dS

where j is the current density, dl is the unit length in the direction of the magnetic field and dS is the unit area through which the magnetic field passes.

Since

\displaystyle\int j.dS = \mu_{o}I_{enclosed}

, this becomes

\mu_{o}NI

for the solenoid as the current I passes through N turns.

The integral on the left hand side between 0 and L (the length of the solenoid) is

BL

so the whole thing becomes

B = \mu_{o}\frac{N}{L}I

which becomes

B = \mu_{o}nI

Wow I must really be putting off revising to write all that out :redface:

To be honest, if I'd had Biot-Savart explained to me at A level I would have actually run away screaming.

Reply 18

F1 fanatic

16

Ampere's law is just an easier form of the Biot-Savart law anyway. You're right incidentally with deriving stuff, but when doing practical stuff as I was it's a faff and in my case it wasn't a solenoid, it was a cylinder. Does a cylinder have 1 turn per unit length or 1 turn? :p:

Reply 19