Author Topic: What type of hob do I have?  (Read 1281 times)

ian

  • fatuously disingenuous
    • The Suburban Survival Guide
Re: What type of hob do I have?
« Reply #25 on: May 20, 2020, 09:45:38 am »
A big hunk of iron will heat up, just more slowly and take more power. Aluminium is a good conductor of heat, so the thin ferrous base in an induction hob pan will get hot quickly with minimal power (induction is very efficient) and the aluminium body will distribute that heat into the pan. A big iron pan will heat up slowly and take more power, so is far less efficient. The counter, and benefit of such cookware, is that they don't get hot and cold quickly and that thermal inertia makes a better cassoulet.

I hated my physics A level with a passion. It's like a group of people had sat down with the curriculum and the mission: how do we make this so dull it will actually hurt? It turned out to be very Mission Possible.
!nataS pihsroW

Morat

  • I tried to HTFU but something went ping :(
Re: What type of hob do I have?
« Reply #26 on: May 20, 2020, 09:53:27 am »
Pans generally need a flat bottom because of the inverse square law (you were paying attention in physics, weren't you? admittedly I switched off soon after) so you can't just plonk a wok on them.

I was, sadly, paying attention in physics.  It's an inverse cube law for magnetic fields (unless you use single-ended magnets, I suppose).

This is why your laptop doesn't get its had drive reformatted when you take it on the tube, and why your wireless bike computer doesn't work properly.

Too many dimensions for me, I never did good with maths. Anyway, field strength falls off rapidly, which is also why 5G transmitters are very unlikely to bake our heads.

Unlikely? so it's Possible!! and if you're the one then it's 100% OMG OMG think of the CHILDREN!!!!
Tandem Stoker, CX bike abuser (slicks and tarmac) and owner of a sadly neglected MTB.

Mr Larrington

  • A bit ov a lyv wyr by slof standirds
  • Custard Wallah
    • Mr Larrington's Automatic Diary
Re: What type of hob do I have?
« Reply #27 on: May 20, 2020, 12:16:30 pm »
Pans generally need a flat bottom because of the inverse square law (you were paying attention in physics, weren't you? admittedly I switched off soon after) so you can't just plonk a wok on them.

I was, sadly, paying attention in physics.  It's an inverse cube law for magnetic fields (unless you use single-ended magnets, I suppose).

This is why your laptop doesn't get its had drive reformatted when you take it on the tube, and why your wireless bike computer doesn't work properly.

No, no, the reason your wireless bike computer doesn't work is because The Devil's Radio.
External Transparent Wall Inspection Operative & Mayor of Mortagne-au-Perche
Satisfying the Bloodlust of the Masses in Peacetime

slope

  • Ride Fettle Ride
    • Current pedalable joys
Re: What type of hob do I have?
« Reply #28 on: May 24, 2020, 07:48:25 pm »
Will my 1960s very expensive at the time Swiss made Silcro stainless steel saucepans with thick copper bottoms work ok on an induction hob?

ETA: the interior base of the pan is stainless steel, so one assumes the external copper is 'bonded' to the SS.


Re: What type of hob do I have?
« Reply #29 on: May 25, 2020, 12:13:31 am »
Sorry no, what steel is there is too far from the hob electromagnet due to the thick copper base. Inverse cube law again. Also I expect it's autensic (non ferrous) stainless steel. Nice socks BTW.
Just been reading up about induction hobs.Good technical description here. https://www.onsemi.com/pub/Collateral/AND9166-D.PDF

Re: What type of hob do I have?
« Reply #30 on: May 25, 2020, 08:35:00 am »
Sorry no, what steel is there is too far from the hob electromagnet due to the thick copper base. Inverse cube law again. Also I expect it's autensic (non ferrous) stainless steel. Nice socks BTW.
Just been reading up about induction hobs.Good technical description here. https://www.onsemi.com/pub/Collateral/AND9166-D.PDF
I think you need to experiment. It will depend on the frequency used by the hob and the thickness of the copper. You want high electrical conductivity (copper beats iron) and high permitivity (iron beats copper mahoosively). I am not convinced about this “inverse cube law” at close distances.


Sent from my iPad using Tapatalk

slope

  • Ride Fettle Ride
    • Current pedalable joys
Re: What type of hob do I have?
« Reply #31 on: May 25, 2020, 08:45:20 am »
^ Thanks aidan.f and for the tech link too  :)

That's a bummer. They were a wedding present from a rich aunt for my late wife's first marriage in 1964 and are still wonderful despite almost daily use. 1 x 15cm and 1 x 17cm saucepans + 1 x 19cm and 1 x 21 cm saucepans with stacking steamer tops. Lovely 'Bakelite-ish' knobs and handles :D

A quick search also reveals they won't be any good on a ceramic hob either. So it looks like my much needed replacement hob is going to be crappy old skool electric, again  :(

(if one is going to wear socks with sandals - make them ridiculous I say)


I think you need to experiment. It will depend on the frequency used by the hob and the thickness of the copper. You want high electrical conductivity (copper beats iron) and high permitivity (iron beats copper mahoosively). I am not convinced about this “inverse cube law” at close distances.

Mmmmmm, shall start enquiring of neighbours (when acceptable to do so) to see if any have an induction hob to experiment on, perhaps.


What type of hob do I have?
« Reply #32 on: May 25, 2020, 10:06:26 am »
Pans generally need a flat bottom because of the inverse square law (you were paying attention in physics, weren't you? admittedly I switched off soon after) so you can't just plonk a wok on them.

I was, sadly, paying attention in physics.  It's an inverse cube law for magnetic fields (unless you use single-ended magnets, I suppose).

This is why your laptop doesn't get its had drive reformatted when you take it on the tube, and why your wireless bike computer doesn't work properly.

Too many dimensions for me, I never did good with maths. Anyway, field strength falls off rapidly, which is also why 5G transmitters are very unlikely to bake our heads.
A 3D world gives an inverse square law. If I have a bbq in my garden, with distance the attractive smell is diffused over a sphere and the area is proportional to square of distance. If I also have a stinky compost heap the same applies. If I am close to the bbq the inverse square rule applies and I can pretty much ignore the compost heap. As I move away the bbq attractiveness decreases with the square of the distance and when this distance becomes significant compared to the separation to the compost heap I will need to subtract the repulsive smell of the compost to get net attractiveness. As distance increase further the cancelling effect increases in proportion to distance relative to separation and so the net attractiveness of the bbq decreases with distance cubed.


Sent from my iPad using Tapatalk

citoyen

  • Cat 6 Racer
Re: What type of hob do I have?
« Reply #33 on: May 25, 2020, 10:15:22 am »

Kim

  • Timelord
Re: What type of hob do I have?
« Reply #34 on: May 25, 2020, 12:37:49 pm »
I am not convinced about this “inverse cube law” at close distances.

Indeed not: When the field is being generated by a coil that has some non-zero diameter, the inverse cube law doesn't apply inside (which presumably can be read as 'less than a coil diameter's distance from') the coil.  I forget the maths, but this is particularly relevant to room-sized induction coils used to feed unadulterated audio to hearing aid users: The field is nearly constant within the enclosed area, but falls away rapidly (though not always rapidly enough[1]) outside.


[1] Eg. for avoiding crosstalk in adjacent cinema screens, classrooms or lecture theatres.  Hence those tend to use IR or radio systems instead.
Careful, Kim. Your sarcasm's showing...

Re: What type of hob do I have?
« Reply #35 on: May 25, 2020, 01:10:15 pm »
To uneducated plebs like me, Kim's reply above reminds me of Mornington Crescent rules - the inverse cube law doesn't apply.

Where's my coat? Oh, I am wearing it. It must be time to go.

fuaran

  • rothair gasta
Re: What type of hob do I have?
« Reply #36 on: May 25, 2020, 01:50:21 pm »
An induction hob can still work if you hold the pan a centimetre above the hob.

Re: What type of hob do I have?
« Reply #37 on: May 25, 2020, 01:54:50 pm »
An induction hob can still work if you hold the pan a centimetre above the hob.

A trivet?
Sic transit and all that..

Re: What type of hob do I have?
« Reply #38 on: May 25, 2020, 02:45:05 pm »
An induction hob can still work if you hold the pan a centimetre above the hob.
It might work (on my hob the pan detector requires the pan to be within a few mm) but it will be less effective. I experimented yesterday comparing a pan directly on the hob and a pan raised by 2-3 mm using three bits of corrugated cardboard. Directly on the hob a mug of water came to the boil in about 85s, when raised it took about 105s.

Re: What type of hob do I have?
« Reply #39 on: May 25, 2020, 02:46:27 pm »
An induction hob can still work if you hold the pan a centimetre above the hob.
It might work (on my hob the pan detector requires the pan to be within a few mm) but it will be less effective. I experimented yesterday comparing a pan directly on the hob and a pan raised by 2-3 mm using three bits of corrugated cardboard. Directly on the hob a mug of water came to the boil in about 85s, when raised it took about 105s.
That would be the difference in altitude


Sent from my iPad using Tapatalk

Re: What type of hob do I have?
« Reply #40 on: May 25, 2020, 03:06:43 pm »
The effect is smaller if one uses a large pan on one of the smaller rings, i.e. there is still an increase in the time to boil but the increase is a smaller fraction of the total time.

I think this can be explaind by how much of the magnetic field is "captured" by the pan. The magnetic field has some sort of toroidal shape and a large pan on a small ring will still capture most of the field when raised. When the pan is smaller then raising the pan allows more of the field to miss the pan around the edge.

Wowbagger

  • Dez's butler
    • Musings of a Gentleman Cyclist
Re: What type of hob do I have?
« Reply #41 on: May 25, 2020, 10:17:07 pm »
At risk of repeating stuff stated above, if a fridge magnet sticks to your pan-base, the the pan will work on an induction hob.
Eating's a serious business. Don't bollocks around wagging your tail.