Generator stuff (1 Viewer)

[CrashOveride]

In jacaranda on page 140 where its got the determing plarity of a generato'rs terminals the current is found to be going from L to K. But then shouldnt it emerge from terminal A so terminal A should be positive? Also if its going from L to K then using the right hand rule with current going from L to K and the magnetic field directed to the right, the force would act on side LK downwards causing anticlockwise motion. Yet the roatation is shown as clockwise ?

Similar eg. in Excel if you have got it, on page 76.

 

[Jase]

Ah yes this one.. took me about a year to realise what was going on.

Now theres two ways to find this thing.. one is with Lenz's law and the other is a test point.

Test charge - on KL by using the right hand palm rule. First, you have to use Lenz's law. The motion of that side is upwards, so it needs to induce an emf that will oppose that motion. Put your palm down and fingers going N to S. So the thumb makes the current go clockwise. The current goes A to B as you said.
Here's the important bit.
The current you just found is the conventional current, so POSITIVE charge 'accumulates' at terminal B. And the electrons accumulate at A.

Now there's a crazier method, the right hand GRIP rule. See, you know the solenoid thing? Treat the coil as a solenoid. Okay so as the coil is rotating 'out' of the field, the number of field lines cutting it is increasing (and it's cutting thourgh it vertically). So the the e.m.f is produced in such a way that it needs to oppose this increase by reducing the number of field lines. So it creates a field that goes downward into the page. Point thumb downards, you get you fingers curling in clockwise way, which is the same result as first method.

 

[CrashOveride]

thanks Jase
just quickly, i remember reading somewhere ages ago that if u got +ve and -ve terminals conven. current flows from + to - but the actual charges travel from electrons to the positive charges. so if its travelling from the electrons then electrons go out of the + terminal. im prolly completely wrong ?

 

[Xayma]

I find it easier to think of it as a battery, in that the positive is where it is leaving. It will then complete a circuit and go to the more negative end.

 

[JayWalker]

Lol Xayman, let me respond to some, you allready got enuf posts and rep

 

[CrashOveride]

I find it easier to think of it as a battery, in that the positive is where it is leaving. It will then complete a circuit and go to the more negative end.

Ok then from this, if the positive charges leave from the positive terminal and travel towards the negative end, then since the force on the charges is from L to K the charges are travelling then to the more negative end, which is terminal B? In contrast with the suggested answer that terminal B is the positive terminal?

 

[Xayma]

No. Don't consider the generator to be part of the circuit. Just like a battery, the current flows out of the positive terminal goes through a circuit and then into the other side. Just think about which way you would connect a diode if you know electronics.

 

[CrashOveride]

got it, thanks xayma

 

[reecepilgrim]

xayma is definently wrong all listen to ilovechildren7

 

[NichNich]

Ah yes this one.. took me about a year to realise what was going on.

Now theres two ways to find this thing.. one is with Lenz's law and the other is a test point.

Test charge - on KL by using the right hand palm rule. First, you have to use Lenz's law. The motion of that side is upwards, so it needs to induce an emf that will oppose that motion. Put your palm down and fingers going N to S. So the thumb makes the current go clockwise. The current goes A to B as you said.
Here's the important bit.
The current you just found is the conventional current, so POSITIVE charge 'accumulates' at terminal B. And the electrons accumulate at A.

Now there's a crazier method, the right hand GRIP rule. See, you know the solenoid thing? Treat the coil as a solenoid. Okay so as the coil is rotating 'out' of the field, the number of field lines cutting it is increasing (and it's cutting thourgh it vertically). So the the e.m.f is produced in such a way that it needs to oppose this increase by reducing the number of field lines. So it creates a field that goes downward into the page. Point thumb downards, you get you fingers curling in clockwise way, which is the same result as first method.

Thanks for explaining that Jase, does this only apply to generators? I mean, if this was a motor, and current flowed from L to K to N to M, terminal A is positive, while Terminal B is negative right? This is the reverse in generators?

Many thanks!

 

[Jase]

induction and motor are different things. Evidently, induced back e.m.f in motors opposes the voltage applied, whereas in generators, there is no applied voltage so to speak.. so they are different.. although i wouldn't say reverse.