How does a force get transmitted through power lines?

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entropy1
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Main Question or Discussion Point

Why does a turbine in an energy plant experience more resistance if the electrical load on the other end increases (electrical resistance decreases)?

What I don't understand is how the force that drives for instance a washing machine gets through the power lines that don't seem to experience any force. Yet the turbine load gets increased by a washing machine on the other end somehow.
 
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  • #2
Merlin3189
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I suppose there may be many ways of answering this, but a simple way is to look at it as an electrical generator and an electric motor.

When you turn a generator, the force you have to apply depends on how much current is allowed to flow. If the generator is open-circuit and no current can flow, then there is little resistance. When you connect a load and allow current to flow, the current flowing in the coils causes a force resisting the rotation. The more current, the greater the force.
The wires just transmit this current.
When it gets to the motor, an unloaded motor spins fast and generates a large back-emf, opposing the electricity supply and limiting the current. The motor is doing no work, allowing very little current to flow, so the generator experiences little resistance.
Once we put a load on the motor, it slows down, the back-emf drops and the current increases. So now the generator experiences more resistance and you do more work to drive it.

So the wires just have to transmit the current and voltage. The motor and the generator convert these into forces.
 
  • #3
entropy1
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So what's puzzling me is how the force gets transmitted through the wires. Or even more puzzling still: why doesn't there get a force transmitted through (or exerted on!) the wires? (that is equal to the force exerted in the motor?)

(And what is back-emf exactly? :cool: )
 
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  • #4
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What I don't understand is how the force that drives for instance a washing machine gets through the power lines that don't seem to experience any force.
Force is not conserved, so there is no sense in which the force is transmitted. What happens is that when there is a load attached there is a requirement for greater current. It takes more force at the generator to deliver that greater current.
 
  • #5
Merlin3189
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I wonder if you are mixing up force and energy?

The force you apply to the generator is simply to turn the rotor round. End of pushing. End of mechanical force.
After that it's electric current until it gets to the motor.
Then the current makes a magnet and that provides the force to turn the rotor there.
Along the wires, no mechanical forces (unless you consider incidentals, like pylons to hold it up in the air !)

What is transmitted along the wires is energy.
The generator converts mechanical energy into electrical energy, that goes along the wires, then the motor converts electrical energy into mechanical energy again.
Energy has many interconvertable forms, each with its own characteristics. At the power station, the chemical energy of coal is converted to heat by burning. The heat boils water to create pressurised steam, which goes through a turbine to become rotational mechanical energy. That turns the generator to produce electrical energy and so on as above. The motor could then go on to convert the energy into moving air as in a vacuum cleaner, or lifting weights as in a crane or hoist, whatever.

Electric energy is transmitted by voltage and current along wires, without mechanical forces.
Rotational mechanical energy is transmitted along shafts, belts, chains, etc without electrical forces.
Compressed gasses transmit energy along pipes without electric currents and without any other moving parts.
Heat energy will pass through a copper rod with no moving parts, solid, liquid nor gas, no mechanical forces and no electrical forces.

Hope that helps a bit. It's rather simplistic, but I hope ok for now
 
  • #6
entropy1
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Perhaps my question is posed a little too naively. I understand practically everything written above. I just have no recollection of my education in highschool. So energy is transported. Energy tends to produce force, right? Why then does the electrical energy not produce a force in the transport wires? I understand it doesn't, but why not just ask: "Why?" :wink: Perhaps this is better put. :wink:

I suspect it has something to do with electricity, electrons, mechanics etc.
 
  • #7
Merlin3189
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The electric current in transmission lines will produce mechanical forces, because it is in the earth's magnetic field. But that is very weak compared to the magnetic fields in motors and generators.
So the forces are not important and have nothing to do with the transmission of energy.
The force on a wire due to current and magnetic field is perpendicular to it, so it doesn't push in the direction the current or energy is travelling.

Two wires carrying current in parallel directions also create a force between them, but again unless they are close and the currents are large, the forces are small.

Electric current can also cause heating, but the whole design of electricity transmission is to avoid converting electrical energy into other forms (mainly heat), because that would be waste energy that did not reach the users.
 
  • #8
entropy1
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Perhaps this illuminates what I'm trying to ask a little:

If I have a firm rope, and tie one end to a cart, and hold the other end in my hand, then at first the rope lies limp on the floor. When I start to pull the rope and exert force on the cart, the rope is tight under strain. So at first there is no energy involved, and then I put energy in the pulling, each of which is reflected in a different state of the rope. You can see from the rope if there is energy transfer.

I don't know if this is a stupid example, but I wonder why the rope's state differs according to the energy flow, and a powerline stays limp under the flow of current while the current has enormous energy involved with it.

I hope it is clear! :smile: It just seems odd if I could pull the cart while the rope stayed limp! :wink:
 
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  • #9
Merlin3189
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The rope's state depends on the tension (force), not on energy. The greater the tension, the less it sags. Sag also depends on flexibility, density and direction. A rope hanging vertically does not show sag.
Energy requires force and movement. For a constant rate of movement, the sag might indicate force and hence energy transfer.
If there were no movement (of the truck), then there is no energy transfer, so the force and sag indicate nothing.
If you were pulling the truck which was accelerating and the rope started to sag more, you would have to do a calculation to work out whether the rate of energy transfer were increasing or decreasing. The sag alone would not tell you.

You can't see electric current and it can pass through some conductors with very little noticeable effect. So there aren't even as many clues as with your rope.
There may be some heating with no obvious source and you may guess it's due to an electric current. Even if you're right, you know that electric energy is flowing into and along the wire, but not how much. For that you'd need to know the voltage as well. Also invisible.

To put the boot on the other foot, why would anyone expect it to be obvious that energy is being transmitted?
If I look at a shaft between two machines, I can't tell which way the energy is flowing, nor even if it is flowing at all. A motor could be driving the shaft, but no load is on the other end.
Radio waves are carrying energy hither and thither all around us and we are completely oblivious to them all. When you peer into your microwave oven, until signs of heat appear on the food, you have no idea whether the machine is actually working or not. (You may hear the fan and transformer hum, see the light and the table rotating, but you've no idea if the magnetron is working.)
Look at your central heating radiator. Is it on? No way you can see. You may hear creaks from the expansion as it first heats up, but not much else.
Can you see whether water is flowing through a pipe?
Even light, you'd think you could see. But you can't ! If energy is coming into your eye, yes, but a beam of light passing transversely in front of you is invisible, unless it happens to hit a bit of dust or something and get deflected into your eye. The light going past could be any intensity and going in either direction and you would not know.
If you see a cyclist riding a fixie on a rolling road, can you tell if he's driving it, it's driving him or he's just pedalling at the same speed as the road?
The rope's state depends on the tension (force), not on energy. The greater the tension, the less it sags. Sag also depends on flexibility, density and direction. A rope hanging vertically does not show sag.
Energy requires force and movement. For a constant rate of movement, the sag might indicate force and hence energy transfer.
If there were no movement (of the truck), then there is no energy transfer, so the force and sag indicate nothing.
If you were pulling the truck which was accelerating and the rope started to sag more, you would have to do a calculation to work out whether the rate of energy transfer were increasing or decreasing. The sag alone would not tell you.

You can't see electric current and it can pass through some conductors with very little noticeable effect. So there aren't even as many clues as with your rope.
There may be some heating with no obvious source and you may guess it's due to an electric current. Even if you're right, you know that electric energy is flowing into and along the wire, but not how much. For that you'd need to know the voltage as well. Also invisible.

To put the boot on the other foot, why would anyone expect it to be obvious that energy is being transmitted?
If I look at a shaft between two machines, I can't tell which way the energy is flowing, nor even if it is flowing at all. A motor could be driving the shaft, but no load is on the other end.
Radio waves are carrying energy hither and thither all around us and we are completely oblivious to them all. When you peer into your microwave oven, until signs of heat appear on the food, you have no idea whether the machine is actually working or not. (You may hear the fan and transformer hum, see the light and the table rotating, but you've no idea if the magnetron is working.)
Look at your central heating radiator. Is it on? No way you can see. You may hear creaks from the expansion as it first heats up, but not much else.
Can you see whether water is flowing through a pipe?
Even light, you'd think you could see. But you can't ! If energy is coming into your eye, yes, but a beam of light passing transversely in front of you is invisible, unless it happens to hit a bit of dust or something and get deflected into your eye. The light going past could be any intensity and going in either direction and you would not know.
If you see a cyclist riding a fixie on a rolling road, can you tell if he's driving it, it's driving him or he's just pedalling at the same speed as the road?

Enough?
 
  • #10
entropy1
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  • #11
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Perhaps this illuminates what I'm trying to ask a little:

If I have a firm rope, and tie one end to a cart, and hold the other end in my hand, then at first the rope lies limp on the floor. When I start to pull the rope and exert force on the cart, the rope is tight under strain. So at first there is no energy involved, and then I put energy in the pulling, each of which is reflected in a different state of the rope. You can see from the rope if there is energy transfer.

I don't know if this is a stupid example, but I wonder why the rope's state differs according to the energy flow, and a powerline stays limp under the flow of current while the current has enormous energy involved with it.

I hope it is clear! :smile: It just seems odd if I could pull the cart while the rope stayed limp! :wink:
Powerline is more like a cable than a rope.

https://www.sheldonbrown.com/cables.html
See chapter "How Cables Work"
 
  • #12
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I wonder why the rope's state differs according to the energy flow, and a powerline stays limp under the flow of current
The powerline’s state differs also. It just is not a state that corresponds to stiffness, and why should it be.

Force isn’t conserved, so there is no reason for it to be transmitted. Electromagnetism is different from mechanics, so there is no reason why a wire should become stiff under power.

There is no reason why either of these points you have raised should be true. So I have a hard time understanding why you think it is strange that they are not true.
 
  • #13
russ_watters
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Or even more puzzling still: why doesn't there get a force transmitted through (or exerted on!) the wires? (that is equal to the force exerted in the motor?)
Why would there be? The wires are not dissipating a significant amount of energy. If there is a force exerted, it would be exerted on the thing dissipating the energy!
 
  • #14
entropy1
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So if we have heat in a steam engine, there is energy contained in the compression chamber that exerts a force on the walls of it. If we have electrical energy in wires there is energy contained in it (I guess?) that (almost) does not dissipate in any way. Are mechanical energy and electrical energy radically different in this way?

(I would think: of course, but WHY and HOW?)
 
  • #15
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An electric wire consists of two part that can to move relative to each other, kind of like brake cables in bikes consist of two parts that can move relative to each orther.

A generator exerts a Lorentz force on one part of the wire and and an opposite Lorentz force on the other part of the wire. The wire stays slack, kind of like brake cable stays slack.
 
  • #16
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Are mechanical energy and electrical energy radically different in this way?

(I would think: of course, but WHY
And I would ask you WHY they should be the same? There is no reason you have given that they should be the same, so WHY do you ask WHY they are different?
 
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  • #17
russ_watters
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So if we have heat in a steam engine, there is energy contained in the compression chamber that exerts a force on the walls of it.
That force is not in the direction of motion of the steam, so there is no energy loss associated with it. The force has to be in the direction of motion for there to be an energy loss. Almost all of the energy loss is at the turbine, where it does useful work for you.
If we have electrical energy in wires there is energy contained in it (I guess?) that (almost) does not dissipate in any way. Are mechanical energy and electrical energy radically different in this way?
Electrical and mechanical energy are pretty different, but not in the way you are describing. You are looking for a loss where you shouldn't expect there to be one in either case. You really should stop focusing on the wires and pipes and start focusing on the pump and generator; where the power is actually applied and the turbine and motor or light, where it is "lost".

Electric motors and generators are the easiest to apply an analogy to mechanical power since, of course, they create or absorb mechanical power.
 
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  • #18
davenn
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If we have electrical energy in wires there is energy contained in it (I guess?) that (almost) does not dissipate in any way
The "electrical" energy is transported ( for want of a better word) in the electric field which is on the OUTSIDE of the wire.
And yes there is attenuation, the longer the run of the wire the more the attenuation. Any covering on the wire, eg. plastic
insulation, also causes attenuation.
 

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