Why Power Lines Buzz: Explaining the Motor Principle

[physicsgal]

Homework Statement

explain why high-voltage power lines give off a buzzing sound using what you know about the motor principal to explain the source of the buzzing.

Homework Equations

so the motor principal has to do with when a force is exerted on a current-carrying conductor when it is in a magnetic field.

The Attempt at a Solution

the field lines changed their shape (or position) and this causes a frequency that produces sound?

any help is appreciated!

Amy

 

[berkeman]

I googled to check, and both effects that I suspect are discussed in this thread:

http://ask.metafilter.com/mefi/22992

The one I'm most familiar with is corona discharge, especially on a wet/humid day. The 60Hz power waveform produces 120Hz peaks in voltage, and the 120Hz buzz is very audible.

The the thread I linked to also discusses the effect that you are being asked about in your question. To help you get farther -- calculate the force on typical high voltage power transmission wires. That force will also peak at a 120Hz rate in the US. Use the typical spacings and voltage and power levels for a transmission line to see how much deflection you might get in the wires.

 

[physicsgal]

thanks. I am still not sure how to word my answer so it relates to the motor prinicipal though.

Amy

 

[berkeman]

Can you define what is meant by "the motor principal"?

Do you know how to calculate the force on current-carrying wires that are in a magnetic field? Do you know how to calculate the magnetic field generated by current carrying wires? Do you know how those effects are used in electric motors?

 

[physicsgal]

Can you define what is meant by "the motor principal"?

when a conductor carries a current at right angles to a magnetic field experiences a force at right angles to both the current and the direction of the field.

Do you know how to calculate the force on current-carrying wires that are in a magnetic field? Do you know how to calculate the magnetic field generated by current carrying wires? Do you know how those effects are used in electric motors?

theres been no calculations so far. I am just suppose to give my best reasonable guess about the cause of the buzzing sound.

Amy

 

[berkeman]

What course is this for? Is there a later chapter in your book that describes the magnetic field? In most introductory E&M books, in the middle of the first chapter on the magnetic field, they show how forces are generated between parallel current carrying wires. If you have a text like that, look ahead a little to get a better picture of what is going on. The currents flowing in the power transmission line generate AC magnetic fields, which interact with the currents in the wires to generate some forces.

 

[ND3G]

They are working on Grade 11 Physics. I had the same question when I was doing a correspondence course through www.ilc.org (Ontario, Canada).

I'll take a look at answer I provided when i get home. One thing though, I do remember when I mentioned the 60 Hz buzz peaking at a rate of 120Hz the teacher incorrectly changed the figure to 60Hz. Sometimes you just can't win... =)

In my text, the very next chapter actually mentioned what causes the buzz, maybe you will be equally as luckly...

 

[physicsgal]

thanks. his is just an introductory hs course. nothing fancy. here's what i wrote:

When a current flows through a conductor, it creates a magnetic field. This field interacts with the field produced by the magnets. These fields fluctuate, and cause the current to interact with the wires of the power lines. This leads to resistance, which causes some of the electricity to be lost as heat and sound energy (buzzing).

Amy

 

[physicsgal]

thanks ND3G. the last chapter does relate the buzzing to the resistance. ) This chapter doesn't mention any math involved (no 60 hz or anything)

Amy

 

[berkeman]

thanks. his is just an introductory hs course. nothing fancy. here's what i wrote:

When a current flows through a conductor, it creates a magnetic field. This field interacts with the field produced by the magnets. These fields fluctuate, and cause the current to interact with the wires of the power lines. This leads to resistance, which causes some of the electricity to be lost as heat and sound energy (buzzing).

Amy

Just a brief clarification. The "magnets" part would apply to many actual motors, but not to the powerline example. The magnetic field from one line interacts with the electrons flowing in the other line to generate a force on the other line, and vice-versa. It will make a lot more sense when you get to studying magnetic fields in detail in college.

 

[physicsgal]

thanks again

Amy