Faraday cage

[imull]

I was watching a a repeat of top gear earlier when they used a car as a faraday cage and richard hammond didnt get fried.

I understand basically how a faraday cage works. SO, why do you get electrocuted if you drop an electrical appliance in the bath. Surely as the water is all around you and you are not directly earthed then it should act similar to a faraday cage?

Im not bored at all

[TheNewBob]

I'm guessing if you dropped an applience in your bath then electricity wud be the last of your worries - wouldn't like a fridge landing on me in the bath! Never mind how it got upstairs (in our house) in the first place lol

Can't work out either your queries tho - I'll wait until I'm awake or drunk before pondering technical stuff like that!

[imull]

haha. it was the drink that brought the question up as i fell into hte bath at 5am

good night down the pub though

[1200Datto27]

Originally Posted by imull

I was watching a a repeat of top gear earlier when they used a car as a faraday cage and richard hammond didnt get fried.

I understand basically how a faraday cage works. SO, why do you get electrocuted if you drop an electrical appliance in the bath. Surely as the water is all around you and you are not directly earthed then it should act similar to a faraday cage?

Im not bored at all

The cars metal body acts as a conductor of the energy, isolating you from it, as long as you do not touch any metal part of the bodywork, you will be Ok, where as in the bath, you are in contact with the conducting material, therefore the current can get you.

[R59]

The theory of a faraday cage is that the electrical charge migrates to the surface of the object, and has no effect on whatever is within the object.

It relates to electric fields, rather than electric current. Therefore, if your car becomes connected to a mains supply, it will all be live, and the tyres will insulate it from ground, so while you're in it, you won't get a shock, but if you become grounded, then you will be the path for the current, and you get a belt, or you fry!

Faraday cages are used to shield things from electromagnetic fields, such as lightening strikes, EMP weapons, Radio Frequency fields, etc..

With the bath, the issue is AC current. For a moment, until it all frys, the water (and you) becomes the conductor for current to ground. The bath is at a ground potential, the live conductors are placed in the water, you are in the water, you are in contact with the bath, you carry a portion of the current. Goodnight vienna! Frying tonight!

Rob.

[imull]

ok think i follow all of that.

told you i was bored yesterday.

[dtype38]

Alternatively.....

If you wire a car to the mains, the current can't go anywhere because the voltage isn't high enough to drive the current through the tyres, so nothing happens (unless you step out, or have any of those silly grounding strips on you car). That's completely different to why Hammond didn't get fried. In the case of a lightning strike, the voltage is high enough to force the current through the tyres to ground, and something definitely happens! Even if it didn't go through the tyres it would just jump out of the bottom of the car straight to ground. What's happening here is that the lightning strike is looking for the least resistance down to ground, and the car shell conducts much better than the hapless Hammond, so he gets ignored. Tend to agree with Racing59 that its not much to do with what Faraday was working one

Moving on though, if you drop a small electric appliance in the bath so that it doesn't squash you, the mains should ground to the earth terminal and blow the fuse. If you're unlucky enough that your wife has not only cut the earth wire, but put a six inch nail across the terminals of your consumer unit.... then the electricity will try and find some other way to earth. As the water in your body is full of impuritites and salts, that makes you a much better conductor than the water, so that's the route the electricity will take to ground.

My advice would be to put car tyres on the feet of your bath.... and don't splash

[imull]

have now taken the 3 bar heater and toaster off the shelf next to the bath

[dtype38]

I'm thinking of taking the ariel off the roof of my car.

[R59]

What you have to remember is that lightning is typically 30,000,000v. It has enough potential difference create a spark that can breach the gap between ground and sky (a number of thousand feet). Therefore, a few inches from car to ground = smokin'! Tyres or no tyres.

However, you are just as likely to win the lottery.

So good luck.

And for the budding trainspotters out there, the typical impedance of the human body is circa 300ohms. I say impedance, because it is not pure resistance, it has an inductive and a capacity element, where is shall be regarded as Z and not R. We are a tuned circuit!!! Somebody stop me before I need taking outside and shooting.

Rob.

[R59]

If you haven't worked it out, I work in the industry that is known for it's wizardry, and cunning science such is called the "black art" by many. Wireless waves are our friends, and to make wireless waves work in nasty noisy sparky environments, that racing cars are, is sometimes clever clever stuff.

Mr Faraday's cages cause us great enjoyment / and / or / hair pulling at times!

Rob.

[phoenix]

Originally Posted by racing59

However, you are just as likely to win the lottery.
Rob.

If my information is correct, you are 14 times MORE likley to be struck by lightning than you are to win the lottery!

Martin

[dtype38]

I think the lottery thing is about the same likelyhood as getting hit by a meteor or something that fell off an aeroplane!

Racing59... For us poor mechanical souls, impedance, inductance, and capacitance are all just so much mumbo-jumbo. I never understood small signal equivalence or how to calculate curent flow in a field saturated inductance coil. But I will happily continue my life in the knowledge that somebody does :-)

[R59]

All you need to know is that a 85mph in your Delorean, with 1.21GW of power, you CAN travel in time, provided that your flux capacitor is in fully working order.

In some ways, after 20 years of practice, I'm almost glad that I've forgotten 50% of the calc crap that I had to endure at college, since I've never needed it, and I'm not likely too either.

Working with aerials, and that sort of stuff, I've had a few belts from electromagnetic pulses courtesy of local strikes, I'll have to count them up, and see if I've got to 13 yet, then I should expect a lottery win, or do I need a direct hit to get 6 numbers?

Rob

[TEAM78]

Water isnt a very good conductor plus were made up of 97% of it. humans can take upto 50 volts before were starting to geting into trouble with it (hence 42v systems on cars). as for the Faraday cage, electric follows the easiest path so when ur in a bath that easy path im guess is you!

[richard_sykes]

Originally Posted by racing59

All you need to know is that a 85mph in your Delorean, with 1.21GW of power, you CAN travel in time, provided that your flux capacitor is in fully working order.

In some ways, after 20 years of practice, I'm almost glad that I've forgotten 50% of the calc crap that I had to endure at college, since I've never needed it, and I'm not likely too either.

Working with aerials, and that sort of stuff, I've had a few belts from electromagnetic pulses courtesy of local strikes, I'll have to count them up, and see if I've got to 13 yet, then I should expect a lottery win, or do I need a direct hit to get 6 numbers?

Rob

What about the flux capacitor