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Author Topic: P=VI  (Read 6240 times)
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leeyiren
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on: September 24, 2010, 12:28:21 am »

Since P=VI, high voltage means low current. Does it mean you can touch a wire with voltage of 10^10 V?
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Fu-Kwun Hwang
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Reply #1 on: September 24, 2010, 04:52:57 am » posted from:,,Taiwan

If the current (or power) is small enough or for AC single: the frequency is high enough.

Our heart beat is also driven by small electric signal.
It could be dangurous if the current happen to influence your heart beat (stop) .
So even small current (1-10mA) could be dangurous.

It is much safer if the current is less than nA or even smaller.

Actually, if the voltage is too larger, it will break down even across air. (e.g. 20kV/cm at 1atm)
So you would not be able to find a constant voltage 1010V wire.
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leeyiren
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Reply #2 on: September 25, 2010, 10:37:51 am »

is it safe to touch bare power station transmission cables since they have such high voltage and low current?


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Fu-Kwun Hwang
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Reply #3 on: September 25, 2010, 08:54:18 pm » posted from:,,Taiwan

The power transimission cables deliver the power for the whole city far away.
Why you thought it is low current?

Touch 110/220 power line could cause people die.
High power tramission line is even much more dangerous.
Do not even think about trying it! NEVER!
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leeyiren
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Reply #4 on: September 25, 2010, 09:10:34 pm »

this is because my textbook says that step-up transformers changes low voltage from power station to extremely high voltage to minimize the current in the power cables to reduce the loss of power due to joule heating in the cables. So isn't it still safe to touch live cables?
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Fu-Kwun Hwang
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Reply #5 on: September 26, 2010, 12:45:29 am » posted from:,,Taiwan

The high voltage transimission line do reduced the current flow so that the power loss due to cable is minumize.
For the same power P=VI , hight voltage means less current. But it does not mean very small current (or safe).


For 200kV power line, if 108W is delivered throught the power line, it means nearly 500A current is flow throught the power line.

1-10mA of current could cause people dead.

Never touch a hot wire. It is too dangerous! Shocked
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leeyiren
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Reply #6 on: September 27, 2010, 07:00:12 pm »

"Live wire is the dangerous wire because it carries a very high voltage while the neutral wire carries close to zero voltage."

So is the neutral wire safe to touch?
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Fu-Kwun Hwang
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Reply #7 on: September 27, 2010, 09:24:50 pm » posted from:,,Taiwan

Yes. It is safe to touch the neutral wire at home from power plug.
However, you have to make sure it is the neutral wire. If you touch the wrong one, it could be deadly.
I would not suggest you try it because you are a novice, and it is very possible that you could make mistakes.
So please do not try it!
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leeyiren
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Reply #8 on: September 28, 2010, 02:06:12 am »

Is it true that current is still flowing back through the neutral wire since the neutral wire connects to the source? Hence shouldn't be still dangerous?

Considering the fact that most electrical supply is alternating current. Hence neutral wire can become live wire, live wire become neutral wire. So still dangerous.

Correct me if I am wrong? thanks
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Fu-Kwun Hwang
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Reply #9 on: September 28, 2010, 11:37:03 am » posted from:Taipei,T\'ai-pei,Taiwan

Current need a loop to complete the circuit.

If you just touch the neutral wire, the voltage between your body and the neutral wire is very small.
So thel current flow through your body is very small (even there could be large current flow through neutral wire. 

The bird can stand on one of the hot wire safely. However, the bird will be deadly if it's wing touch another wire by accident at the same time.
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leeyiren
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Reply #10 on: September 28, 2010, 04:48:04 pm »

Quote
The bird can stand on one of the hot wire safely. However, the bird will be deadly if it's wing touch another wire by accident at the same time.

any simulations?

the current in the circuit could be 10A. can it flow through the live wire, the appliance the neutral wire then to you and electrocute you?

is the earth connected to the live wire to form a loop and complete the circuit?

thank you.
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Reply #11 on: September 28, 2010, 05:56:22 pm » posted from:Taipei,T'ai-pei,Taiwan

The resistance of your skin could be larger than 100kΩ.
It mean that for 10A to flow through your body, the voltage need to be larger than 100kV.

Normally, you only have 110/220V at home.
However, 1-10mA could cause your heart beat to stop.  The resistance could be reduced to 10kΩ If you hand is wet. So it is dangerous to touch the hot wire.

Voltage is the potential difference between two reference point.
The earth provide a ground reference potential. The current could also formed due to capacitor coupling (which you might not have learned).
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