Quick Question on Lenzs Law (1 Viewer)

[rama_v]

Hi, I think im getting quite confused here...lol

The rod AB is moving upwards through the magnetic field B going into the page as shown. Which end of the rod is negative?

 

[gordo]

b will become negative

the system wants to oppose the motion of the rod, so it will induce a current to the force the rod down, this current will flow from B to A, by convetion we say positive charges move, which means positives flow from B to A, leaving B negative and A positive

 

[rama_v]

Yeah I get how that works...but i thought the right hand palm rule takes into account the direction of conventional current right? Because then conventional current would be going from right to left..?

 

[mattchan]

umm, yeah, well theres no doubt that the current is travelling from B to A, that is conventional current. Doesnt taht mean that B is positive and A is negative. But electrons will still flow from A to B....leaving A negative

 

[Will Hunting]

Hey, dude!

I looooove these questions! There are so many different ways to go about them. I'll show you my two fave ways! (The negative end is A, by the way )

1. The scenario you're describing has electrons moving up the page (and therefore protons, or positive charge, moving down the page). But, by Lenz' Law, current must be induced to flow in the opposite direction to this in order to oppose the initial force on the wire, which is acting up the page. In other words, the protons will end up moving up the page, and the force on the wire will end up acting down the page.

Here's where I apply the right hand thumb rule to determine the direction of current flow through the wire. However, I do not use the rule on the whole wire, but rather on the motion of one of the protons I've just described above (remember that a moving charge in a magnetic field experiences a force the same as does a current-carrying conductor). Applying the rule, the protons are found to experience a force to the left (i.e. current flows from right to the left). From the definition of current as a flow of positive charge, A must be the negative pole (since protons migrate to the negative terminal).

2. By application of the right hand thumb rule on the wire, it should be clear in which direction current will flow (I don't actually use the rule at all. I prefer using the catapult method in my head!). Under the circumstances described by the diagram, current can be considered to flow from A to B (i.e. B is the negative pole). However, by Lenz' law, a current will be induced to oppose the initial force on the wire (i.e. the opposite will happen - the induced current will be induced to flow from B to A, and A will be the negative pole).

Note that no current actually exists until it is induced by the change in magnetic flux (or the motion of the wire up the page). However, whenever you have a magnetic field oriented into the page, and a force acting up the page - and you are given all this information, it follows logically that you can find the missing quantity (and its direction). This is what all these hand rules are for. This helps to come to grips with conditions before the EMF is induced and, therefore, by Lenz' law, conditions afterward.

Good luck, dude.

 

[rnitya_25]

i think b is negative, since this opposes conventional current, so its going from right to left, so its from b to a, so b is negative

 

[rama_v]

Hey, dude!

I looooove these questions! There are so many different ways to go about them. I'll show you my two fave ways! (The negative end is A, by the way )

1. The scenario you're describing has electrons moving up the page (and therefore protons, or positive charge, moving down the page). But, by Lenz' Law, current must be induced to flow in the opposite direction to this in order to oppose the initial force on the wire, which is acting up the page. In other words, the protons will end up moving up the page, and the force on the wire will end up acting down the page.

Here's where I apply the right hand thumb rule to determine the direction of current flow through the wire. However, I do not use the rule on the whole wire, but rather on the motion of one of the protons I've just described above (remember that a moving charge in a magnetic field experiences a force the same as does a current-carrying conductor). Applying the rule, the protons are found to experience a force to the left (i.e. current flows to the left). From the definition of current as a flow of positive charge, A must be the negative pole (since protons migrate to the negative terminal).

Thanks for the detailed post. I inititaly thought that A was negative, we had this question in our half yearly and my teacher was convinced that B was negative.

 

[Will Hunting]

Nah, it's A, unless you are defining current differently. By the syllabus definition, it's A.

Hang on... I've given you the polarity after the current is induced. But you've just asked for what the polarity is under the circumstances as they are in the diagram

Ha ha

I turned a 1 second question into a 1 minute one! The negative pole is B, but, if you think about what happens when a force is applied (see my post) the polarity will reverse.

 

[rama_v]

ahhh I see now, so it all boils down to the positive charges moving from right to left therefore leaving B negative when the current is induced...its a tricky question.

 

[Will Hunting]

leaving B negative and A positive

leaving B positive and A negative.

umm, yeah, well theres no doubt that the current is travelling from B to A, that is conventional current. Doesnt taht mean that B is positive and A is negative. But electrons will still flow from A to B....leaving A negative

No, it's travelling from A to B. It will end up travelling from B to A after electromagnetic induction has taken place. Make this one change and everything else you've said is fine.

i think b is negative, since this opposes conventional current, so its going from right to left, so its from b to a, so b is negative

Yep, you've answered the question.

 

[gordo]

your first and last quotes contradict each other

 

[richz]

its (b) using the right hand palm rule

 

[KFunk]

its (b) using the right hand palm rule

[EDIT]:*sigh* I hate it when I use the wrong hand rule... It's induced current so you can't use the right hand palm rule. You need to use the right hand bitch slap rule (back handed slap) . Conventional current flows from B to A. Hence a positively charged particle would flow from B to A. Positive flows towards negative. Hence A is negative.

 

[KFunk]

your first and last quotes contradict each other

You're first post contradicts itself...

positives flow from B to A, leaving B negative

 

[Will Hunting]

your first and last quotes contradict each other

You said B was negative. This is true.

But you said B was negative because positives flow from B to A. This is false.

 

[mattchan]

omg, so what the hell is the answer here

 

[gordo]

not wen dealing with conventional current...


everyone knows electrons move (well according to current accepted theories anyway), but by conventional current, u deal with which direction a positive particle would travel. same with electric fields.

if all the positives "move" from B to A, wats gonna happen to B?

don't question by hot 96 hsc physics

 

[KFunk]

That thing has always bugged me actually. Is it positive because positive charges move away from it or is it left negative from the lack of positive charge... The physicists who created all the current and charge definitions really messed up.

 

[rama_v]

I checked a similar question in one of the old physics books i have at home:

"A rod XY (X is the left end and Y is the right end) is moved downwards through a magnetic field going into the page. Which direction will the current flow?"

The answer in the book states: "In the conductor, electrons will be forced towards end X, making it negatively charged. End Y will become positively charged. Therefore a current will flow clockwise in the circuit. Current will flow from X to Y in the conductor."

Hence it would seem that B is indeed the negative end in my question. To be 100% sure if someone has one of those success one books, this question was asked in the 2002 hsc physics paper, can u please post up what it says.

 

[Will Hunting]

b will become negative
the system wants to oppose the motion of the rod, so it will induce a current to the force the rod down

You are wrong. By your logic, A will become negative, which is, incidentally, what I have already explained three times now. Try throwing off the shackles of an indefensible narcissism and listening to others for a change. You might save yourself some embarrassment. Conventional current is defined as a flow of positive charge (your word usage in positives, by the way... dude.. no). If B becomes negative, as you have said, that means electrons will move from B to A (since electrons migrate to the positive pole) i.e. current will flow from A to B. But this will cause a force acting up the page. How can applying an intial force up the page induce a current to produce a second force that also acts up the page, not opposing, but rather adding to the intial force (and therefore energy)? Your proposition defies the law of conservation of energy. You are wrong.

this current will flow from B to A

No, it won't. The electron current will flow from B to A, but that definition of current is inexpedient to this situation. We are dealing with conventional current in this instance, which will flow from A to B (Conventional current is defined as a flow of positive charge). Besides, finding the direction of current flow after induction wasn't even the question.

which means positives flow from B to A, leaving B negative and A positive

See my other post. Also, note that they don't actually move (Judging by your evident, and seemingly lingering, shakiness with this concept, this reinforcement should serve you well). Even if current (conventional, remember) did end up flowing from B to A, which it doesn't, this would make B positive and A negative (why would it leave something it's attracted to?)

your first and last quotes contradict each other

No, they don't, hack. You misconstrued both. You weren't looking deeply enough into the post. With the last comment, I was saying that B was negative. This is correct. With yours, I was saying B was not negative if current (conventional) flowed from B to A, as you incorrectly concluded. It's quite a simple difference. Do you get it, or will I need to post again?

not wen dealing with conventional current...

The word is when, and not wen, by the way, idiot. If I were you, I'd be steering your argument away from conventional current, dude, since your mixing up of ideas thereof has led to its being corrected in the first place.

everyone knows electrons move (well according to current accepted theories anyway)

Well, clearly not everyone (if you are implying, with this, that positive charge is stationary)...

but by conventional current, u deal with which direction a positive particle would travel; same with electric fields

It's reassuring to see that you know something, even if it happens to be the most obvious point to have ever shown its face in the syllabus; now, all you need to do is apply it correctly! But, don't rush yourself... take little, baby steps and you'll get there eventually

if all the positives "move" from B to A, wats gonna happen to B?

Positive charge doesn't move. B isn't losing positive charge, as you are implying. This cannot happen, and has nothing to do with the definition of conventional current (which is a wholly academic concept, by the way), nor with electromagnetic induction.

don't question by hot 96 hsc physics

Uhhh... is this supposed to impress me? I can tell just by receiving your messages that you're not at all hot, man. Why should I care about a mark, which, incidentally, isn't that good, and whose calculation, for all I know, was subjected to all forms of bias, partiality, luck and misalignment, when I can evaluate your calibre right here and now? You ain't got nothing, man, and certainly can't dance with the big guns. I've seen some of your other posts.

So, yeah, I have no problems questioning you.