farmer1

I am looking for the resistance of brick and a material with a much higher resistance so I am guessing glass?

If anybody can point me in the direction of a website with such information.

Regards

Ian

OllyK

Are you talking friction here or electrical resistence / conductivity?

farmer1

Electrical resistance.

Steve vRS

Brick will vary as it's porus and so it's resistance would depend on it's moisture content. Mineral oil is a good insulator as is glass and ceramic. Air is also quite good.

Steve

OllyK

Well they are both going to be very high. Consider that a brick is fired clay which has a high proprtion of silica and guess what glass is?! You may be able to find out the conductivity of glass (although specialised plastics may be even higher), but a house brick is likely to be pretty variable

http://www.google.com/search?hl=en&l...uctivity+brick

Comper100

Have you tried http://www.matweb.com ?

Hanslow

Glass has an electrical resistance of 5x10^11 Ohm/m I believe, at least it has in the software we write

farmer1

I have tried MatWeb

The reason I ask is that I have to measure the resistance of a house brick for a Physics exam and I want to find a material to place it on with a much higher resistance, so that the current flowing through it is negligable.

I have an approximate value for the resistance of glass, and was just looking for a rough resitivety of brick so I can say that the current lost through the surface is negligable, because bladdy bladdy blah.

Hanslow

Don't have resistance of bricks to hand

speedking

Suspend the brick from the conductors you're using to apply the voltage?

matt.bowey

Porcelain or glass is OK.....we the national grid seem to think so , just look at the towers!! Air is also good!!

A real good insulator is only required to hold off Very High Voltage! If you are talking about 240V, then a yellow pages will do....!

AndyC_772

iTrader: (2)

Lol Resistance of a house brick! Good to see that examiners still have a sense of humour...

So, I presume you're planning to clamp the brick between a pair of flat plate electrodes, apply a ludicrously high voltage and measure the current with a microammeter - right?

It might be a fun experiment; I suspect that what you'd actually find is you'd get dielectric breakdown of the air in the pores of the brick, giving you a corresponding sharp increase in current. Even if that didn't happen due to the air spaces not all joining up, you might find you'd get dielectric breakdown of the air surrounding the brick if you did the experiment in air. It all depends on just how high a voltage you need, and how sensitive your current meter is.

Suggestion: measure the resistance not of one brick, but of a few hundred bricks in parallel. Maybe you could attach tinfoil to both sides of a brick wall with conductive adhesive, then apply your voltage across that? If it's a big enough wall, and you pick (say) a couple of square metres of area for your experiment, then that solves your insulator problem too. The insulator is the rest of the wall!

Make the measurement area large compared with the thickness of the wall, and you can reasonably neglect edge effects. (Note that you've also built a parallel plate capacitor - apply an alternating voltage and you could determine the dielectric constant of a brick wall with the same setup )

Failing that, your best bet might be to place the brick on a flat conducting surface, attach your other electrode to the other side of the brick, cover the brick with a bell jar and attach a vacuum pump. Vacuums have very high resistance - though you then have thermionic emission to worry about instead!

I'm sooooo glad that I don't actually need to use this stuff any more (Professional electronic engineer btw, in case you didn't know!)

farmer1

That was the idea, well I was hoping to find an electrically conducting adhesive.

Fortunately we don't have to conduct the experiment, just plan it.

Because there was also the matter of testing the resistance at a range of temperatures 20-800C for which I am planning to use a thermocouple to measure the temperature.

So no the examiners have not lost there sense of humour, well apart from when it comes to marking my plan were a sense of humour will disappear into thin air.

Ohh and just to throw a spanner in the works they gave us a non uniform brick.

All this effort for 2.5% of an A Level hardly seems worth it.

Iain Young

Surely the resistance of a brick will depend entirely on what it is made out of. All bricks vary slightly depending on where they were made (slightly different materials / proportions of ingredients etc), so I doubt you'll find a uniform resistence value anywhere...

OllyK

Be careful the glass doesn't start to melt, it is a good conductor when it starts to melt!

AndyC_772

iTrader: (2)

Electrically conductive adhesive certainly exists; you can buy reels of copper tape with a conductive adhesive backing, which is used during radio interference testing of electronic products. Unwanted radio emissions are generated by all equipment that generates and uses radio frequency switching (and that's pretty much everything), and these can leak out of any gaps or slots in the metal enclosure. Copper tape is used to seal the gaps and thereby reduce the amount of rf noise that escapes.

It's not good at high temperatures though, so you'd need to find somethng else, like a clamp.

800 deg C is rather hot for a lot of thermocouples, you don't want them to melt! At that level you might be better off measuring the temperature with a pyrometer, which is a non-contact device that determines temperature by measuring the amount of infrared radiation emitted by it. You'll need to look up the emissivity of the brick. Failing that, look up the working range for your chosen thermocouple and make sure it's not going to be exceeded.

800 deg C is also above the melting point of aluminium, so you'll need to use something else as your electrode material. I'd suggest carbon, which is usually classed as rather resistive, but its resistance will still be tiny (read negligible) compared to the brick. Copper is OK too.

farmer1

I was going to use a chromel/alumel thermocouple which has a wider temeperature reading range then I will be heating to.

I assumed glass would be ok as a surface as its softening point is around 1700C (1683 according to my reasearch)

I'd already decided to use copper for the above reasons you point out.

All I have to do is figure a way of getting the electrodes in place and actually heating it to 800C I am thinking along the lines of blow torch (although you will have problems with patch heat) because I am not quite sure if you could lift a brick out of a kiln at 800C