Electronics: trying to understand grounding point

[Femme_physics]

Sorry for jumping through topics, I just have an electronics test tomorrow and I want to see I fully understand grounding points.

Homework Statement

So I made up this circuit and found its voltage drop at every point

http://img137.imageshack.us/img137/3352/volts1.jpg

Now if I set Vab = 0 (i.e. turn Vab to the grounding point), I'm trying to calculate the potential differences now, but my total potential difference between -10 and +10 is only 20v. I'm missing 10V somewhere!

http://img845.imageshack.us/img845/1601/volts2.jpg

 

[ehild]

Grrrrrr

Does point mean sector? Vab is the potential difference between points a and b.

ehild

 

[Femme_physics]

Does point mean sector? Vab is the potential difference between points a and b.

So you can't set a point = 0, only potential difference = 0?

 

[ideasrule]

So you can't set a point = 0, only potential difference = 0?

No, you can only set a point to 0, not a potential difference. Vab is a potential difference, so you don't get to choose its value; the Universe chooses it. You can, however, choose the potential at either "a" or "b" to be whatever you want.

Maybe an analogy would help. The height difference (analogous to potential difference) between the top and bottom of Mount Everest is a set value. It is what it is; nature has decided its value, and you can't change it. However, you can arbitrarily define the bottom of Mount Everest to be at height=0, or height=-1000 m; the top of the mountain would then be at 8000 or 7000 m (assuming the mountain is 8000 m tall).

 

[q11we]

Maybe an analogy would help. The height difference (analogous to potential difference) between the top and bottom of Mount Everest is a set value. It is what it is; nature has decided its value, and you can't change it. However, you can arbitrarily define the bottom of Mount Everest to be at height=0, or height=-1000 m; the top of the mountain would then be at 8000 or 7000 m (assuming the mountain is 8000 m tall).

Good example.
And yes setting Vab to zero would be OK, only when the current between a and b is zero.

 

[Femme_physics]

So no matter where I set my ground point, the potential difference between 2 points remains constant?

 

[Femme_physics]

I presume that the answer to my question above this post is "yes", in this case setting b = 0 I should be all correct here:http://img814.imageshack.us/img814/8268/thistime.jpg Correct?

PS calculator watches ftw!

 

[I like Serena]

I presume that the answer to my question above this post is "yes", in this case setting b = 0 I should be all correct here:

Correct?

PS calculator watches ftw!

Yes, the answer to your post above is yes. :)

And yes, Va = 10.
However, Vc = -20.

Voltage still goes down in the direction of the current.
Or perhaps I should say, that current flows down the mountain of voltage, like a mountain stream. ;)

 

[Femme_physics]

Yes, the answer to your post above is yes. :)

That clears up a lot, thanks! :)

And yes, Va = 10.
However, Vc = -20.

Voltage still goes down in the direction of the current.
Or perhaps I should say, that current flows down the mountain of voltage, like a mountain stream. ;)

I see, so I should've done

Vbc = Vb - Vc

And not

Vbc = Vc - Va

It's always the first minus the second, yes?

 

[I like Serena]

I see, so I should've done

Vbc = Vb - Vc

And not

Vbc = Vc - Va

It's always the first minus the second, yes?

I guess so, but that would be mostly about how you define it yourself.
However you define or use it, make sure that voltage goes down in the direction of your current.

 

[Femme_physics]

I guess so, but that would be mostly about how you define it yourself.
However you define or use it, make sure that voltage goes down in the direction of your current.

Makes perfect sense :) And, as it hit the source, it increases again, right?

 

[I like Serena]

Makes perfect sense :) And, as it hit the source, it increases again, right?

Yep. Guaranteeing that KVL works! :)

 

[Femme_physics]

Yep. Guaranteeing that KVL works! :)

Need...consume...more...science...knowledge.. *drools*

Thanks

 

[I like Serena]

Don't forget the item that increases your abilities!