Archimedes said:
Hello All
I am just starting out in electronics as a hobby. I learnt Ohms law
i.e. V=I * R
Now just say I had a 1.5 Volt battery (AA cell) and an 1 ohm resistor
So given Ohms law - to find the current
I = V / R
I = 1.5 / 1
I = 1.5 amps
So can a 1.5 V battery kill you will a 1 Ohm resistor ?
No. You are not taking into account the resistance of your body. What kills
you is the current passing through your hard an interfering with the
electrical mechanisms that cause it to pump. It takes around 100mA for this
to happen if I recall correctly.
Your body has atleast 10kOhm resistance and usually much more depending on
many factors.
so if you use ohms law for 10k you'll get 1.5/10k = 0.15mA.
As you can see that is not enough to do much. You'll need atleast 100 more
voltages to get to 15mA and now you can see why the voltage inside your
house is so dangerous. Thats ~ 15mA which might not kill you but could give
you a nasty shock or burn. Ofcourse this is only your wet or sweating and
there is a path for the current to goto ground.
Remember that current must travel in a loop so you cannot get shocked by the
batter if you do not touch both ends together. You can get shocked by your
household power because they use earth ground as ground(it saves extra
wire). So you can stick your finger in an outlet if you are not in any way
grounded to earth(or even close). I wouldn't try this though if I were you.
You can safely play with very high voltage if your sure that the current is
very small. They have HV devices like the van der graaf generators that you
can touch and current can flow from you to the ground and you will get
shocked. But these are not dangerous because they have very little
current(the little current is very strong though).
Current is what kills and long as you remember that you should be fine.
Remember that current must flow in a closed path too. High Voltage though
can make current kill because the higher it is the more current that will
flow(but remember its basically ohms law that decides as it depends on
resistance too). The thing with HV is that if can cause current to jump
across things.
So if you have 1M volts then that can jump several feet across the room and
get you. Your body will take 1M/R in current. if your wet then R might be
10kOhms or smaller and you can see that that is 1kA of current. Ofcourse
things like electrostatic generators cannot produce this much current so its
not that bad. Even though your body will take that much current the general
will loose all its charge very quickly since it contains probably just a few
mA at most.
Air has a breakdown of about 30 kV/cm. That means if you have a voltage
source of 30kV then it can cross that much air to get to ground. 1M volts
means it can jump about 30cm to get to "ground".
Anyways, as you can see you don't have to wory about dieing from such a low
voltage. Just take into account your skins resistance, which you can
measure if you want, and then use ohms law and make sure you are well below
the voltage.
e.g., even 50V is only 5mA at 10kohms.
Note though that the resistance can vary a lot on the body so you should not
go plays around with the idea. Devices like stun guns are made so they
cannot put to much current out so they are usually not lethal.
Heres a site that is more indepth and explains things better:
http://www.physics.ohio-state.edu/~p616/safety/fatal_current.html
Read the bottom about skin resistances as it is what is important.
Jon