Current Flow Basics: Bulb, Wire, Insulators

[Crazymechanic]

Ok you got me lost in a forest of 2 and a half trees , now if the current stays the same after like 1000 miles of wire then what decreases? Just don't say nothing and everything stays the same as at the start of the line?

 

[sgstudent]

Well the current density is indeed proportional to the diameter of he wire or conductor it is going through , but explain then DrZoidberg what causes the loss of current in long transmission lines where the effects are measurable and also in every other conductor component even though usually they are too short to measure such a small loss.
Now if the current stays the same as you say then why do all the transmission line and power station operators try so hard to keep the lines as short as possible and if not short then atleast with high voltage so that the current could be decreased and the loss from transfer minimized?

Maybe I'm getting you wrong here but according to what I read I could conclude that after 2000 miles of wire the current would be the same as at the input? But this clearly is not the case.It is not the case because every wire if not superconducting is a resistor , a small one but it has resistance and that resistance takes away the potential of each electron slowly over distance and so every next one can create a lower em field which indeed has less power to push the electricity forward.
Electricity is like the dominos , when you start the dominos from one end they fall quite slowly until they reach the end of the line and so does electrons flow slowly in a DC circuit and they have no net movement in an AC one but now imagine the dominos being pushed not by the one before the one that fell but by an unseen force which travels much much faster han the dominos themselves so that every next dominos can "fall" before the last one has even touched the next one physically and that is the photon of the EM field.

Whoever said here in this thread that the speed of electricity is slow is wrong the speed of electricity through perfect vacuum would be hat of the speed of light or c. In a copper cable it is something like 95%, according to a fast google search.But I believe the figure should be about right.

Hi thanks for the reply

For the domino anology, would we imagine all of the blocks fall together before they even touch each other? But actually why would resistant take away the potential? Because in my syllabus (which is only a secondary level physics haha) just tells us that in a series circuit the potential difference divided by the total resistance would give us the current. Perhaps my syllabus is too simplified?

 

[Astronuc]

Ok you got me lost in a forest of 2 and a half trees , now if the current stays the same after like 1000 miles of wire then what decreases? Just don't say nothing and everything stays the same as at the start of the line?

There is a voltage drop, and for long distance, there may be current leakage from the transmission line. Think about a transmission line in terms of a mechanical analogy in a water-filled pipeline. The water (current) in = water (current) out, but along the pipe is a pressure (voltage) drop.

 

[DrZoidberg]

You mean in a dc circuit without any corona losses?
What decreases is the potential. If you have a battery and two wires each 1000 miles long connected to the two terminals of the battery and a resistor connected at the end, the potential difference i.e. the voltage across the resistor is smaller than the one across the battery because part of the voltage drops across the wires.

 

[Crazymechanic]

And voltage is proportional to current so what do we get ? :)

@sgstudent No the blocks don't fall all at once the speed of Em waves is that of c or almoust c depending on the medium they are traveling but c even though the highest speed physically is still not infinite remember that.
Light and electricity travel very very fast but still they travel and it takes time and goes over a certain distance.

 

[ZapperZ]

And voltage is proportional to current so what do we get ? :)

This is a severe misunderstanding. It is a POTENTIAL DIFFERENCE, not an absolute potential value! That is what is in V=IR. The drop in the potential across a resistor is dictated by the value of the resistor itself. Do a Kirchoff Law around a simple series loop and see how you can get a consistent result even when I is a constant all around the circuit.

Zz.

 

[sgstudent]

And voltage is proportional to current so what do we get ? :)

@sgstudent No the blocks don't fall all at once the speed of Em waves is that of c or almoust c depending on the medium they are traveling but c even though the highest speed physically is still not infinite remember that.
Light and electricity travel very very fast but still they travel and it takes time and goes over a certain distance.

Oh sorry about that :) I meant they start to fall together instead of hsving one domino pushing down the next domino.

As for the formation of a lightning I read the Wikipedia article on it and I don't quite get their explanation. I don't really understand where the ionization would take place and whether the excess negative charges from the cloud actually go down the air. And where would the positive ions go (also because I'm not too sure where those ions start to form as well..)

Thanks so much for the help :) seems like this thread sparked off a lot of discussion (pun intended haha)

 

[DrZoidberg]

And voltage is proportional to current so what do we get ? :)

A constant current but different voltages across the components of the circuit.
For example. Say the battery has 10V and you have 3 resistors in series connected to it.
2 Ohm, 3 Ohm and 5 Ohm. Now the total resistance is 10 Ohm so the current is 10V/10Ohm = 1A
Voltage drop across the 2Ohm resistor is 2Ohm*1A = 2V. The 3Ohm resistor has 3V dropping over it and the 5Ohm resistor 5V. The sum of all the voltages is 10V.
If you want to include the wires into that calculation, well wires are just resistors.

 

[ZapperZ]

Oh sorry about that :) I meant they start to fall together instead of hsving one domino pushing down the next domino.

As for the formation of a lightning I read the Wikipedia article on it and I don't quite get their explanation. I don't really understand where the ionization would take place and whether the excess negative charges from the cloud actually go down the air. And where would the positive ions go (also because I'm not too sure where those ions start to form as well..)

Thanks so much for the help :) seems like this thread sparked off a lot of discussion (pun intended haha)

Unfortunately, and this is my personal opinion, it has distracted from your original question, which is really straightforward and one that I hope has been answered.

I just hope you were able to weed through the responses you received and able to figure out which ones are the correct answers to your question.

Zz.

 

[sgstudent]

Unfortunately, and this is my personal opinion, it has distracted from your original question, which is really straightforward and one that I hope has been answered.

I just hope you were able to weed through the responses you received and able to figure out which ones are the correct answers to your question.

Zz.

Yup that is true. I managed to understand the mechanism of a simple circuit and a little on lightning as a bonus. I learned that generally electrons in a circuit would all move equally around the circuit back to the other end of the battery unless the wire increases in size, then accordingly the electrons speed of movement might decrease or increase to match the current. So it allows the total electrons leaving the battery to be equal to the total number of electrons entering the battery.

Should wait to learn more about capacitors to get a better understanding before I continue asking about the mechanics of a lightning strike?

And lastly, thanks so much everyone for all their replies I gained a lot from this post Haha :)

 

[DaveC49]

The field inside the resistor is different from the one in the wire, but the field inside the wire is the same on both sides of the resistor. Potential difference is not a measure of the strength of the electric field. It is the integral of the electric field over a distance. Inside the resisitor the field is stronger because the battery is not the only field source. There is also static charge that accumulated on the surface of the wire that produces a field of it's own. At the ends of the resistor you have a "pile up" of charge. The field created by that charge strengthens the field inside the resistor but weakens it in the wire. The integral of the e. field over the entire circuit stays the same of course and is equal to the P.D. of the battery.
Inside the resistor the drift velocity may be different but after the electrons leave the resistor their velocity goes back to their previous value. The e. field is constantly transferring energy to the electrons and the electrons are constantly tranferring energy to the atoms around them. However their kinetic energy at any point in time is virtually zero. Because they move very slowly and are very light. It's like pulling a heavy object with a rope. The rope never has much kinetic energy.
Waves do not occur in a static situation i.e. when the current is constant. So no waves are being reflected.