Challenge Questions

C'mon, this is Physics 102. It's a force of attraction.

How about a real challenge, like a fundamental proof of Euclidean parallelism.

Good point, but remember, ALL SPACE is curved and convoluted!

Exactly true. There is, of course, no possible proof of Euclidian parallelism, as Reimann demonstrated.

And there was I remembering high-school experiment when I was 15... - I'm not certain what grade you would call that.

But GA from me, anyway - if only because you got there first.

repulsion. both conductors are generating an identical polarity EMF field. like charges repel. Were one conductor flowing opposite the other the force would be attraction.

There would actually be two forces. The first would be a magnetic field pushing the two wires away from each other. The second would be a very small Lorenz force on each wire as the electrons flowed through the magnetic field generated by the other wire. The Lorenz force would be at a right angle to the flow of current and would obey the right hand rule.

-Doug

I need to revise my previous answer. I was wrong when I said they repelled each other. They actually do attract. The Lorenz force portion should still apply.

If anyone is interested there is a good link that demonstrates this.

http://qbx6.ltu.edu/s_schneider/physlets/main/bcompass.shtml

-Doug

Doug, I believe the three laws of robotics were expounded by Isaac Asimov in his robot series of stories

By the left hand rule the magnetic flux lines would join causing attraction.

ChazL

Force of repulsion! That is why multi stranded wires are twisted so that they force aways from each other.

WHAT??? This makes no sense!

Not what...........Watt.............or killerwatt.

This question cannot be answered simply on the available information. it needs a little more clarity.

Is it ac or dc current?

Are the wires in close proximity of each other or separated by distance but still electrically in parallel?

Is the source of the current the same? if not then they are physically in parrallel but not electrically - the question does not identify which.

I assume nothing - I need more data.

A response to a few of your questions.

It shouldn't matter if it is AC or DC current. If, as the problem states, the currents are flowing in the same direction then the answer would be the same if it was direct current or synchronized AC currents.

The distance also doesn't matter. It would be very significant if we were asked the magnitude of the forces created, but we were only asked for the direction. As the wires get further and further away from each other the force would get infinitesimally small, but it would still be in the same direction relative to each other. (Note - there could be some argument here if we consider a Plank type of minimum value for the field but I am unaware of any such concept so I am ignoring it.)

It really doesn't matter if the current is from the same source or different sources. It all comes down to moving electrons creating a magnetic field.

Sorry KRW43, I didn't meant to step all over your post, but I did want to point out some of the physics behind the problem.

-Doug

I agree with the first post, the wires would attract. The right hand rule applies. Between the two wires, the magnetic vector of the left hand wire would be away from the viewer and the magnetic vector of the right hand wire would be toward the viewer. They are in opposite directions, so consequently they would attract.

Dr. Ed

Dr. Ed is correct. A lot of nonsense was elicited by this challenge question! Of course, all challenge questions may elicit a lot of nonsense, but this is one I know the answer to!

PS: The left-hand rule applies as well, and guess what -- gives the same answer!

Torsion.

Two counter-rotating elctric fields = Attraction

A force of attraction.

Greg

force of repulsion

This question has generally been answered using the relationship of the force imposed on moving charged particles in one wire by the magnetic field caused by the moving charged particles in the other wire (right hand rule). This shows that when the charges are moving in the same direction along parallel wires the resultant force is attractive. It is also possible to derive the same result without recourse to any magnetic field but by using the theory of relativity and the normal electrostatic attractive/repulsive forces between charges. As this question is not very challenging can anyone derive the answer (qualitatively - no maths required) using relativity and electrostatic forces?

The explanation I was looking for invollved lorentz contraction of charged particles moving on one wire relative to charged particles on the other wire. The starting point is that there is equal numbers of positive and negatively charged particles on the wire along any given length, ie, the average spacing between the negative (electrons) and positive charges along a unit length of the wire are the same. If this wasn't true then sections of the wire would have a net positive or negative charge which is not possible for a good conductor. Now if there is a current flowing along both wires the electrons are moving (drifting very slowly) along the wire and the positive charges are stationary (this will work same way if you consider positive charges as moving). Now relative to the moving electrons on one wire the moving electrons on the other wire are stationary, however, the positive charges are moving and because they are moving the average separation between the charges wil be length contracted by an amount >0 (this will only be a miniscule amount for any velocity much less than the speed of light, but since there are a huge number of charges even a miniscule length contraction will result in a net charge difference) which will mean the electron will see slightly more positive charges per unit length of wire than negative ones and so will be attracted. Same analysis can be done from frame of reference of the positive charges and will get same result. If currents are flowing in opposite directions the same analysis shows that there will be a repulsion. If this is done rigorously with mathematics the values obtained by analysing in this way will agree with the traditional values obtained using the magnetic field. Bottom line is that magnetic field can be considered as just relativistic result from electrostatic field with moving charges and not as separate type of field.

Whoa. Ok, now apply that reasoning to the non-lame challenge question proposed by Guest in Response # 21.

OK. From a moving frame of reference that is moving at same speed as the electrons in Q21 the electrons can be considered as stationary at a fixed distance in space. They would at first only feel the electrostatic repulsive force, however, as soon as they started to accelerate away from each other as a result of that force they would start to radiate energy (accelerated charge) which would then act to counteract the acceleration, ie, they would feel a resistance to their acceleration. This resistance could be interpreted as the effects of a magnetic field. You can find a better (less handwaving) description of relativistic interpretation of magnetism in this web site:

http://physics.weber.edu/schroeder/mrr/MRRtalk.html

... and to think that I thought that my arm-waving was impenetrable.

It's school physics. As most have already said, there is an attractive force on the electrons flowing in the magnetic field caused by the electron flow in the other wire.

I could only add, that there is also another one force, repelling this time, due to the fact that the electrons tend to accumulate towards the side of the wire facing the other wire: this causes the wires to get slightly polarized. Unfortunately I'm too busy right now to brush up the formulas and wrap it up, but I have a feeling that this force might be significant under some special wire configurations (maybe wires too thick and too close to each other). I doubt that the total force could ever be repellent, but can somebody nail it down using maths?

It's seems very easy.

I made a draw.

It shows the magnetic field produced by the one wire and the force which is applied to the other wire -due to this magnetic field and the current- (with the help of the "three fingers rule"). It's easy to see that the same kind of force (attractive) is applied to the other wire too. So the two wires attract each other. If currents of opposite directions flow inside the two wires (it's, also, easy to show that) the two wires will repel each other.

It's such an easy "CR4 chalenge". Is there any trap???

How did Human Kind make it to the moon.......

Attraction

The conductors would attract.

So, here's another one. If two electrons were moving in a parallel direction (constituting an electric current), would there be an attractive magnetic force (in addition to the repulsive electric force) between them?

If yes, then if you were moving at the same speed as the electrons they would appear stationary. Where did the magnetic force go?

Maybe from the protons in the nucleus of the atoms that make up the conductor in which the electrons are moving.

No, just an observed relativistic reduction in the apparent repulsion.

They attract!

For those needing that "special" visual explanation with photos, pictures and movies, please review the following web site:

http://www.magnet.fsu.edu/education/tutorials/java/parallelwires/index.html

Thank you Florida State University!

The spiral looks fun. But:

Are they still allowed to use the mercury contact? and

Why do the bubbles in the battery seem to appear at the same rate when the switch is open?

Does this answer the question???

Only if you already know the answer. The diagrams show some kind of arrows representing magnetic polarities, but in the diagram in which the flow is in the same direction, the arrows don't make a compelling case for attraction. In the diagram on the left, the arrows make a compelling case for repulsion, so anyone with a triple-digit IQ should be able to see the converse on the right. But I don't think the diagrams tell the story very well. Plus, they're using the Left Hand Rule. What's THAT about??

It will be attraction. Since the direction of the magnetic field aroung both conductors is either clockwise or counter-clockwise, the the direction of the fields between the conductors will be in the opposite direction. So the force between the conductors will be attractive.

my god, is it a challenge? try to ask a student in 12 grades. he must be able to answer it. no problem.

I have to agree with Kinsale (response #1) This is a physics 102 question and not worthy to be a challange question. How about next time you ask does the sun rise in the east or west.

After reading some 25 responses, the answer is still not unanimous . What we need for questions or "challanges" like this are two boxes; #1 attract; #2 repel, and everyone can add a vote in the appropriate box. There are so many unstated options: size (diameter) of wires, current carried by each, distance between wires, insulated or not, composition of wires (Cu, Al, etc) and so on.

None of your "unstated options" would change the answer. This "challenge" question is so fundamental, you don't need any specific information to answer it -- just knowledge of the Right Hand Rule (or the Left Hand Rule, if you're a communist), and the understanding that magnetic fields of like polarity repel, and of opposite polarity attract.

one wire stretches out with its feelings and feels the force; it is attraction

http://www.jedisanctuary.org/pages/teachings/teachings-from-starwars.htm

I find this whole blog "Repulsingly attractive!" do you all not agree......?

More info needed.Are the voltages and currents in phase with each other? What is the amplitude of the voltage and current? What is the physical distance between them? What is the insulation medium between them? Are the voltages and currents of the same amplitude?Are these currents flowing at the same time? Forces between the conductors can vary depending on these factors. Assuming equal conductor length , properly insulated, and currents,in phase,simultaneous flow,the same magnetic field will be present in both conductors at the same time, therefore a repulsive force will be developed between them. If not insulated, and in contact with one another, you have a stranded wire, and electrons tend to repel one another, so most flow will be in outer skin of conductor. If these conductors are inside of a metallic conduit,there will be an increased eddy current heating of the conduit,which, with sufficient current,can be hazardous.This is the reason why the National Electrical Code requires all phases to be in the Same metallic conduit when paralleling conductors.If in Plastic conduit, the conductors can be single-phase paralleled in each conduit.----------ShoeShineBoy---------------

Force of repulsion if the current flow is DC.

I am going with nothing happens, or no force at all. The conductors are copper and thus non magnetic.

I certainly could be wrong as I usually am in these instances.

Just my way of thinking outside of the box.

Go inside the box... ...

It doesn't matter if the conductors are made of copper. Any conductor which is flown by current, produces a magnetic field and can be considered as a magnet * .Because of this, such a conductor "feels" the influence (i.e. a force) by another magnetic field (e.g. produced by another adjacent conductor flown by current).

[ * As an example: a coil -flown by current- behaves as a magnet (even w/o an iron core) ]

Back in the box now.

Good thing I tagged that in advance that i would be wrong but then I have never gotten a really good understanding of magnetic fields.

I got a real suprise last night while watching Deconstructed, on the Science channel. They demonstrated how an electric meter worked to spin the dials and it seems that it would fit right into this discussion. Tried to find the video clip of the show but its not been released yet.

If you get a chance check it out. After seeing it I new my answer here was terribly wrong.

force of attraction......

with currents in opposite direction repel each other

c/ocds

Didn't anyone go to this web site and "SEE" the answer???????

http://www.magnet.fsu.edu/education/tutorials/java/parallelwires/index.html

Surely did..................very good animated display that clearly shows the answer.

They ATTRACT

Electrically that is in series.

REPULSION

It depends on the source. Let's assume that the two wires carrying current is from the same source. No force at all between them.

Al

I always try to read a few of the responses before blurting out an opinion. Sometimes the answer lies above.

Copied from Magnet Lab Web Site

Parallel Wires

The first person to discover evidence that electricity and magnetism are related phenomena was Hans Christian Ørsted. In the midst of a lecture on electricity, Ørsted noticed that a wire carrying current was able to deflect a compass needle. Unable to develop a plausible explanation, Ørsted published his findings in 1820. This news sent shockwaves through the scientific community and instigated numerous investigations into the matter. One of the scientists who immediately began to expand upon Ørsted's work was Frenchman André-Marie Ampère. Soon after, Ampère found that the magnetic fields created by parallel current-carrying wires interact with one another, as demonstrated in this tutorial.

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Goto website to see the Java interactive applet:

http://www.magnet.fsu.edu/education/tutorials/java/parallelwires/index.html

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The direct current circuit in this tutorial includes two parallel straight wires (in red). These wires can be arranged in a series circuit, as they are when this tutorial first opens, or in a parallel circuit. In the series circuit arrangement, the parallel wires are linked by a connecting wire into a circuit that allows current along a single path. As a result, the current travels one way down one wire, and in the opposite direction down the second wire.

You can change this to a parallel circuit by clicking on the radio button; in this scenario, the current forks when it reaches the parallel wires; some current goes down one of the wires, the rest down the second. The current down both wires travels in the same direction.

Watch how the parallel wires behave in each of these set-ups. When either circuit type is selected, the Knife Switch can be lowered to complete the circuit by clicking either the switch itself or the blue Run button. Notice a stream of yellow electrons traveling through the circuit; flowing (as they always do) from negative to positive – opposite the direction of conventional current. To halt the flow of current, click on the red Stop button or the knife switch. Clicking the blue Pause button will let you examine the process in mid-stream.

As you can see, the wires in the series circuit repel one another, while the wires in the parallel circuit attract one another.

This is explained by the right hand rule, which helps visualize how a magnetic field (depicted by the blue field lines above) around a wire travels. Extend your thumb in the direction of the conventional current, then allow your fingers to curve: The magnetic field circling the wire (represented by your thumb) travels in the direction that your curved fingers are pointing.

So if you have two current-carrying, parallel wires with magnetic fields circling around them in the same direction, they will attract each other, as shown in the tutorial; at the point at which their respective magnetic fields intersect, they are traveling in opposite directions, and opposites attract.

Similarly, if you have two parallel wires with current traveling in opposite directions, as you do in the series circuit, then the magnetic fields of the two wires will be traveling in the same direction at the point at which they intersect, and therefore repel each other.

Ampère was able to mathematically describe this type of magnetic force between electric currents, formulating what is known as Ampère's law.

Definitely attracks.

If you have ever seen an example of a fault on a pair of conductors where the current flow is opposite, the wires jump apart. This is also where the term "Bracing" in switchgear and other power distribution equipment comes into play. For load centers to survive electrical faults the busses inside need to be able to handle the mechanical forces of being riped apart.

If the wires are close enough for the magnetic fields to interact, the fields will couple and the wires will be drawn towards one another. The opposite is true if the currents are in opposite directions such as in a loop or the ends are connected in series.

a force of repulsion

Force of repulsion.

Much ado about nothing ? Nothing complicated

Although the substance of the answer (a force of attraction and also its magnitude) is correct, the analogy with a pair of bar magnets isn't helpful, as the field distributions are so different. (Just consider the field joining the centres of the magnets - straight from one pole to the other). There is no equivalent field for the wires - and if there were it would create a force that is orthogonal to the field and to the wires (upwards or downwards as drawn).