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# Electric Shocks watch

1. Why don’t birds get electrocuted on power lines? It is because they are not completing a circuit by being connected to earth for example? And why is a complete circuit necesssry for the electrocution? I understand that the bird represents a path with more resistance than the wire, but would not some current still pass through the bird and then continue through the wire? Thanks
2. Their cells do not offer a better conductor than the wires they are travelling through.

Edit: changed pylon to wires.
3. (Original post by -1!)
Why don’t birds get electrocuted on power lines? It is because they are not completing a circuit by being connected to earth for example? And why is a complete circuit necesssry for the electrocution? I understand that the bird represents a path with more resistance than the wire, but would not some current still pass through the bird and then continue through the wire? Thanks
Electric current is the killer.

There needs to be a circuit path for current to flow. The birds sitting on the power line are surrounded by air. Together the bird and the air offers and extremely high resistance to ground or one of the other power lines. It's more useful to measure the electric field strength needed to cause breakdown of the air, which then creates a low resistance circuit path. This is what happens to the air to cause a lightning strike.

The breakdown voltage of air at STP is around 2 million volts per meter. Since the HT cables of a pylon are at a voltage of 400kV, there is no danger of arcing through breakdown of the air and causing a high current to flow.

With the birds perched on the power line, in reality, an extremely minute current will still flow, but will be so small as to be imperceptible to the birds and completely safe for them.
4. And if a human were to do exactly the same as the bird, without touching anything else besides the wire, would he be unharmed like the bird? Is it only once there is contact with something with a low resistance (compared to air) that a lot of charge will flow through him?
5. (Original post by -1!)
And if a human were to do exactly the same as the bird, without touching anything else besides the wire, would he be unharmed like the bird? Is it only once there is contact with something with a low resistance (compared to air) that a lot of charge will flow through him?
Yeah it's not that birds have any special electrical properties - just that they can fly through the air, i.e. reach the wire without climbing up something connected to the earth and are good at balancing on wires (which I guess they think are like twigs)
6. This is a bit more general, but why would current continue to flow through a person at 0V to earth which is also at 0V? Would it not just stop in the person as there is no p.d between him and earth? Also, am I right in thinking that still only a fraction of the current in the wire will flow through the person to earth and most will continue to travel through the wire which is at a lower resistance? Again, thanks.
7. (Original post by -1!)
This is a bit more general, but why would current continue to flow through a person at 0V to earth which is also at 0V?
Current cannot flow if both ends of the circuit path are at the same voltage potential.

By definition 0V is a reference. When we use 'earth' as a reference, it means the bulk solid mass of our planet is, for all intents and purposes, a virtually infinite source of electrons and by inference, therefore able to take back any amount of electrons supplied to a circuit.

Voltage, by definition, is the ability to perform work in Joules per Coulomb of charge. If both points in a circuit are at 0V potential, there is no ability to perform work. Current (charge) cannot flow.

A difference in voltage measured wrt 0V means there is a potential for a charge to flow. 1V potential difference has the ability for work to be performed at the rate of 1 Joule per Coulomb of charge that passes between the two points. 1 Ampere means 1 Coulomb of charge is passing between the two points at a rate of 1 Coulomb per second. 1 Watt means work is being done at a rate of 1 Joule per second.

Would it not just stop in the person as there is no p.d between him and earth?
Correct. No current (charge) can flow. There is no potential for work to be performed.

Also, am I right in thinking that still only a fraction of the current in the wire will flow through the person to earth and most will continue to travel through the wire which is at a lower resistance? Again, thanks.
Correct. If the person touches the high voltage wire, but is insulated from earth (by a very high resistance) then simply by Ohms law, the current flowing to ground will also be very small. Almost no current flowing in the wire, will be diverted to ground.
8. If current isn’t flowing to earth as there is no work being done on charge when it gets to the man, why is it important that the man is grounded? Why is it that he will be electrocuted when touching earth, but not when he isn’t? Is it to do with the fact that touching earth completes the circuit? I’ll admit I don’t even know why a circuit needs to be complete, but also why is it a complete circuit anyways if the current is flowing to the man/earth and not back to the source?
9. Theres a cool video out there of people who work on power lines! They fly up in a helicopter, dude reaches out with like a metal pole thing and touches the cables (Im guessing making the potential zero so he doesnt get zapped when he then gets on the wire) its crazy sh it and something i struggle to understand
10. (Original post by Betelgeuse-)
Theres a cool video out there of people who work on power lines! They fly up in a helicopter, dude reaches out with like a metal pole thing and touches the cables (Im guessing making the potential zero so he doesnt get zapped when he then gets on the wire) its crazy sh it and something i struggle to understand
More stuff that needs explaining 😭
11. (Original post by -1!)
If current isn’t flowing to earth as there is no work being done on charge when it gets to the man, why is it important that the man is grounded?
Ahhhh. This is about safety and the way electrical devices/systems are designed to protect humans from electrocution. High voltage and household mains distribution cabling, have devices called earth leakage and residual current circuit breakers. Also, the outer casing of mains supplied equipment (if metal) is connected to earth via a very low resistance path.

If a fault develops in the equipment where a live wire contacts the outer casing, it immediately forms a low resistance circuit to earth and a large current will flow. This large current flows via the mains fuse in both the equipment supply plug (3-pin in the UK) and the distribution board circuit breaker. The fuse is designed to rupture when a moderate current flows and will cut off the mains supply. In addition, the circuit breaker detects the surge and also trips out isolating the supply from anything connected in that circuit.

i.e. the casing forms a much lower resistance path to earth than any person touching the equipment when the fault occurred. The bulk of the current flows through the equipment earth and not through the person and is transient for a very short duration (milliseconds) while the fuse ruptures and the circuit breaker trips out.
12. 1) How does a fuse located on the earth wire stop the mains supply when it melts?
2) How does it relate to the electrocution, and the fact the man needs to be touching earth to have a lot of current running through him when he touches the wire?
13. (Original post by -1!)
1) How does a fuse located on the earth wire stop the mains supply when it melts?
The fuse is never located on the earth wire, it is in-line with the 'live' high voltage wire from the supply. When the fuse ruptures, it breaks the circuit, No more current can flow.

2) How does it relate to the electrocution, and the fact the man needs to be touching earth to have a lot of current running through him when he touches the wire?
The man touching earth must also touch the live wire for the circuit through him to be completed.

In other words, he forms an unwanted parallel resistance path adjacent to the wanted path and a current from the live wire to earth will flow through him.

As little as 50mA is all the current needed to electrocute.
14. Ok thanks, I realised if I’d just looked at a plug I could’ve worked out the first part without wasting your time. The fuse breaks because the path along say the casing offers less resistance than the original path though the live wire and then say the bulb or whatever the appliance is on the circuit and so more current flows and melts the fuse. I still don’t understand the idea of the electrocution though. Why is the man, not touching earth (say he’s floating or something) not completing the circuit? Why is earthing necessary to complete the circuit if in both cases current is flowing to 0 p.d? Why is it the circuit complete when the circuit ends at earth?
15. I have the perfect video for you to watch that literally explain this whole thing in depth and the thing is it acc really fun to watch.
I discovered it many years ago but still remember it to this day and that’s why I was able to recall it as soon as I saw your question.
Hope this helps😂
https://www.thenakedscientists.com/p...ed-power-lines
16. (Original post by -1!)
I still don’t understand the idea of the electrocution though. Why is the man, not touching earth (say he’s floating or something) not completing the circuit? Why is earthing necessary to complete the circuit if in both cases current is flowing to 0 p.d? Why is it the circuit complete when the circuit ends at earth?
Ohms law: I = V/R

With the man not touching earth, the resistance path from the live to earth through him is extremely high. Which means the current through him is vanishingly small.

Say the voltage on the wire is 1,000 Volts and the resistance through the man via the surrounding air (remember he is not touching anything other that the live wire) of air is >100 Megohms, then the current to ground via the man will be <10 microamps. This level of current is completely safe.

The current through the man becomes dangerous when it exceeds a threshold of around 50mA. i.e. the resistance path through the man needs to be less than approximately;

R = V/I = 1000V / 50mA = 20 Kohms. For instance if the 50mA flows between his two arms and across his heart, then a cardiac arrest can be induced.

The higher the voltage, the more dangerous it gets. For instance if the voltage of an overhead railway power line of say 30,000V were bridged to ground by a man (the man touches both the live wire and also ground bypassing the air), then if his internal resistance were 10 Kohms say, a current of 3 Amperes would flow through him to ground. The additional problem is his own resistance would cause a huge amount of power to be dissipated in his body and burns are severe:

P = V x I = 30,000 x 3 = 90 K Watts of heat energy. The man suffers cardiac arrest and he spontaneously bursts into flames.
17. Thanks for the replies, I’m getting there 😂
Can anyone correct me when I’m wrong?

Current can only flow in a closed circuit (why?). When current is connected to earth, the circuit is complete as at some point the 0V neutral wire will be connected to earth and electrons from earth can be taken back to the positive terminal, through the neutral wire, as if they had gone through the circuit normally. When someone touches the live wire, while touching earth, the resistance of the new path he creates to earth is low enough for a harmful current to be supplied to him, however when he is not touching earth, the voltage is not high enough to cause the breakdown of the air so current cannot flow to earth and the only path to follow is the intended one through the wire.

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