Bird Sitting on Power Line

[justadream]

“A bird landing on a high voltage wire on a rainy day is not shocked even though it is in contact with thousands of volts. What is the most probably explanation?”



Answer: The bird’s feet are both at the same potential, so there is no current flow.



So why can’t a human touch the wire? The human’s hand wouldn’t be “at the same potential”?

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[Cawolf]

The human would be standing on the ground. So there would be a potential difference between the hand and foot that creates a path to earth. This would allow current to flow as the person is "grounded".

The bird is not grounded and there is no path for the current to flow to a lower potential.

 

[justadream]

@Cawolf

Okay, then lets say a person drops from a parachute and lands on the wire. Would he be fine then?

 

[justadream]

Also, why does it matter what the person's potential is?

Can't you think of the bird as a "resistor" in the circuit? If so, wouldn't the current be passing through the bird?

 

[Cawolf]

It doesn't matter what the person's potential is. It matters that the person is touching the earth which is considered a ground - the easiest source for charge to flow. Anything touching a ground will have a potential difference - the bird isn't.

You could think of the bird as a resistor. Now this next part I am not 100% sure on - but I would imagine that if you put a resistor in parallel to a circuit with no resistance, no current would flow through the resistor. I think that the resistor would not even be drawn in the circuit diagram.

So here, the bird is not part of the circuit.

(I may be wrong on this aspect but it makes sense. )

I am pretty sure that the main point is that the bird has no difference in potential because it only touches the wire - where if it bumped a different wire or a pole - it would instantly have a potential difference and have current pass through it and kill it.

 

[justadream]

@Cawolf


So as long as the person is not touching the ground, he is free to touch any wire?

"You could think of the bird as a resistor. Now this next part I am not 100% sure on - but I would imagine that if you put a resistor in parallel to a circuit with no resistance, no current would flow through the resistor. I think that the resistor would not even be drawn in the circuit diagram."

What is the resistor (the bird?)?

 

[Cawolf]

Yes the person could be a bird and hang on the wire. If he bumped a pole he would not be fine anymore.

Can't you think of the bird as a "resistor" in the circuit? If so, wouldn't the current be passing through the bird?

You said the bird right there. I was just going off of your thought process.

 

[justadream]

""You could think of the bird as a resistor. Now this next part I am not 100% sure on - but I would imagine that if you put a resistor in parallel to a circuit with no resistance, no current would flow through the resistor. I think that the resistor would not even be drawn in the circuit diagram.""

Why would you be putting the resistor in parallel to anything?

I'm imagining a simple circuit with 1 resistor (the bird).

The current would flow through the resistor (the bird) right? But the bird would be fine?

 

[Cawolf]

Oh I see what you are saying. You are picturing the bird in series - like you are putting it into the circuit?

No that wouldn't be the case. With no potential difference the current would not enter the bird - it would not become part of the circuit.

 

[justadream]

@Cawolf

Wait it wouldn't enter the bird?

So what if I had a series circuit but added a random wire loop that left the circuit at point_1 and entered back into the circuit a meter later? Wouldn't the current enter the loop? If so, why can't you think of the bird as this random wire loop that provides another path for electron flow?

 

[Cawolf]

I don't know if that is an accurate analogy - so I will not hypothesize further there.

For current to enter the bird - there needs to be a potential (voltage). Voltage is the difference between two points - since both feet on wire V = 0. So I = 0.

In your example, the wire loop is a complete circuit. The bird is not a complete circuit as with no potential difference, there is no where for the current to go - so it will not.

 

[justadream]

@Cawolf

Not sure if this matters but are you thinking of the bird's two feet as one thing? What if they are separated by like 10cm? How are the bird's feet not providing a potential difference but a random wire loop is?

 

[Cawolf]

I don't think I can explore this any more without simply repeating myself. I don't think I have anything else to add.

Let me refer you to a helpful source:

http://www.allaboutcircuits.com/vol_1/chpt_3/3.html