why does current flow through something that is earthed? Watch

physics4ever
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for example when birds perch on pylon wires ,no current flows through them,but if a mans fishing rod line reaches the wire the current flows through the line to him to the earth? ive never understood why the current flows to the earth. any explanations?
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Xabier
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Your name is physics4ever, shouldn't you know this? We did this in GCSE Physics a couple weeks ago and I think it was because when the bird is on a live wire it's at the same voltage as the wire but no current flows into its body. A body is a poor conductor compared to copper wire, and current goes along the path with the least resistance (I think).
But if a bird (or a power line worker) accidentally touches an electrical "ground" while in contact with the high-voltage wire, she completes an electrical circuit. The earth, and anything touching it that can conduct current, is the ground. And I'm guessing the earth route has less resistance? Sorry for the poor explanation.
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WGR
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Current is pushed down a voltage (potential )gradient because voltage is a sort of "electrical pressure" The potential of earth is 0 V and always stays at 0 V because it is too massive to have it's charge changed noticeably by a human power cable. Therefore current will just keep on flowing into it. Earth is basically a "bottomless charge sink". With the bird on the wire now it's at the same potential as the wire so current won't be pushed through it
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physics4ever
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(Original post by KingKumar)
Your name is physics4ever, shouldn't you know this? We did this in GCSE Physics a couple weeks ago and I think it was because when the bird is on a live wire it's at the same voltage as the wire but no current flows into its body. A body is a poor conductor compared to copper wire, and current goes along the path with the least resistance (I think).
But if a bird (or a power line worker) accidentally touches an electrical "ground" while in contact with the high-voltage wire, she completes an electrical circuit. The earth, and anything touching it that can conduct current, is the ground. And I'm guessing the earth route has less resistance? Sorry for the poor explanation.
i did get taught this but they never explained why, and it only occured to me that i didnt know why lol
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physics4ever
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(Original post by WGR)
Current is pushed down a voltage (potential )gradient because voltage is a sort of "electrical pressure" The potential of earth is 0 V and always stays at 0 V because it is too massive to have it's charge changed noticeably by a human power cable. Therefore current will just keep on flowing into it. Earth is basically a "bottomless charge sink". With the bird on the wire now it's at the same potential as the wire so current won't be pushed through it
how does the bird gain a potential thats the same as the wire?
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uberteknik
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(Original post by physics4ever)
for example when birds perch on pylon wires ,no current flows through them,but if a mans fishing rod line reaches the wire the current flows through the line to him to the earth? ive never understood why the current flows to the earth. any explanations?
Pylon conductors carry electric current which is another way of saying a flow of electrons - incredibly big quantities of them.

Those electrons are negatively charged particles and will be attracted by opposite charges. The bulk of the earth provides an almost infinite sink for attracting electrons.

But they need a path to flow along. Air is a good insulator (very high resistance to current) so no electrons will flow to the earth via that path.

A bird sitting on a high voltage cable is insulated from the earth by the air in the same way, so again no current will flow through the bird.

However the fishing line, when touching the cable and the angler, is a much lower resistance path to earth. Consequently a current will flow via that path.

Because the voltage (electric pressure) of the cable is very high (>100,000 volts), even a modest resistance will still allow a very sizeable current to flow to earth. Easily enough to substantially heat up anything in the path of the current, resulting in combustion or even local explosion.
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WGR
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(Original post by physics4ever)
how does the bird gain a potential thats the same as the wire?
If you touch a charged object you become charged yourself via electron transfer.
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uberteknik
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(Original post by physics4ever)
how does the bird gain a potential thats the same as the wire?
It's the word 'potential' that holds the key.

By touching the wire, the bird is connected to a source of potential energy carried by the cable.

Voltage is defined as Joules/Coulomb. i.e. a voltage is a measure of the energy transferred per coulomb quantity of charge.

A battery voltage (for example) therefore is a measure of how much energy per unit charge can be supplied. But until there is a conduction path, the energy stored in the battery stays as just a potential energy. Likewise, the bird by virtue of touching the cable, has gained potential energy.

A good analogy is gravitational potential energy: if you sat on a ledge high above ground, you have a lot of gravitational potential energy by means of your height.

That potential energy can only be converted to kinetic energy when you jump off the ledge.
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Clip
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Next week : "so where does that antineutrino come from?"
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uberteknik
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(Original post by WGR)
If you touch a charged object you become charged yourself via electron transfer.
True, but don't confuse charge with voltage potential. See previous explanation.
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physics4ever
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(Original post by Clip)
Next week : "so where does that antineutrino come from?"
i dont get much explanation from teachers, they usually just give me what i need to know for the exam no derivations or proofs and explanations, at least im curious enough to go online and search for explanations
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physics4ever
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(Original post by uberteknik)
It's the word 'potential' that holds the key.

By touching the wire, the bird is connected to a source of potential energy carried by the cable.

Voltage is defined as Joules/Coulomb. i.e. a voltage is a measure of the energy transferred per coulomb quantity of charge.

A battery voltage (for example) therefore is a measure of how much energy per unit charge can be supplied. But until there is a conduction path, the energy stored in the battery stays as just a potential energy. Likewise, the bird by virtue of touching the cable, has gained potential energy.

A good analogy is gravitational potential energy: if you sat on a ledge high above ground, you have a lot of gravitational potential energy by means of your height.

That potential energy can only be converted to kinetic energy when you jump off the ledge.
thanks! good analogy
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