Newbie in calculating circuits need help!

Thread Starter

simply shady

Joined Jun 11, 2017
4
Hello, I need help understanding how ground works in a circuit. How do I calculate total current, total resistance and power when ground is involved?

Example: are these two the same?


Would my calculations for total current, resistance and power change, if the circuit has 4 grounds? Or 3 grounds or 2 or 1 ground indicated?
 
The two circuits are mathematically the same. Everything connected with the same ground symbol are understood to be at the same voltage. This is equivalent to the components being connected together.
 

OBW0549

Joined Mar 2, 2015
3,411
Hello, I need help understanding how ground works in a circuit. How do I calculate total current, total resistance and power when ground is involved?

Example: are these two the same?
Yes.

"Ground" in a circuit (a.k.a. "common") is simply a name given to one particular circuit node, usually because that node is used as a reference point with respect to which other voltages are measured or defined. The calculations are the same, whether you call a node "ground" or not.
 

WBahn

Joined Mar 31, 2012
25,785
The "ground" symbol (in a circuit such as this it is better to use a "common" symbol, but we can discuss that separately) is merely a shorthand to indicate a common node to which all of those wires are connected. You can draw a physical wire connecting each of those symbols and then delete them, leaving you with what you started with.
 

Thread Starter

simply shady

Joined Jun 11, 2017
4
The two circuits are mathematically the same. Everything connected with the same ground symbol are understood to be at the same voltage. This is equivalent to the components being connected together.
Thanks for the quick reply!

I need to calculate also the current passing through each resistance.

Would I simply just use the formula: I=V/R?

I am confused because we just learned ohms law and the formulas: P=I*V and V=I*R. We have not covered how ground works in circuit and some of the drawings for homework has them. I do not know what the point is in including them when the above pictures are technically the same. I thought maybe the "ground" would affect the calculations. That's why I'm asking also If I need to calculate the current passing each resistor, if I just use the same formula and treat it as if the "ground" doesn't exist.
 

WBahn

Joined Mar 31, 2012
25,785
Again, the ground symbol is a shorthand notation. It is generally used for the node that is the reference for the entire circuit and, as a result, there are lots of things connected to this node. When your schematic gets a lot more complicated, such as the schematic for the electrical system in a car, for instance, then the diagram would be extremely cluttered if you had to draw an additional wire from nearly every light bulb and many other components all the way back to the negative terminal of the battery. Plus, in working with the schematic, you would always be having to trace that wire for dozens or hundreds of components all the way back through the schematic just to discover that it is connected to the negative terminal of the battery and, hence, is at your 0 V reference. By having the ground symbol, you know both of those things at a glance.
 

Thread Starter

simply shady

Joined Jun 11, 2017
4
Again, the ground symbol is a shorthand notation. It is generally used for the node that is the reference for the entire circuit and, as a result, there are lots of things connected to this node. When your schematic gets a lot more complicated, such as the schematic for the electrical system in a car, for instance, then the diagram would be extremely cluttered if you had to draw an additional wire from nearly every light bulb and many other components all the way back to the negative terminal of the battery. Plus, in working with the schematic, you would always be having to trace that wire for dozens or hundreds of components all the way back through the schematic just to discover that it is connected to the negative terminal of the battery and, hence, is at your 0 V reference. By having the ground symbol, you know both of those things at a glance.
I see. Thank you so much for a very in depth reply! It is my first time seeing that symbol in a circuit. I'll carry on with my homework and post what I got to make sure I understood it correctly.
 

WBahn

Joined Mar 31, 2012
25,785
Thanks for the quick reply!

I need to calculate also the current passing through each resistance.

Would I simply just use the formula: I=V/R?

I am confused because we just learned ohms law and the formulas: P=I*V and V=I*R. We have not covered how ground works in circuit and some of the drawings for homework has them. I do not know what the point is in including them when the above pictures are technically the same. I thought maybe the "ground" would affect the calculations. That's why I'm asking also If I need to calculate the current passing each resistor, if I just use the same formula and treat it as if the "ground" doesn't exist.
If you look in your text, there is probably some mention made of the ground symbol and it's meaning somewhere before they start using it -- but it might be very early in the text. Look in the index for "ground" as see what you can find (or search for "ground" if you have an electronic version of the text).

You use P=I*V and V=I*R just as before, always being careful to note that it is not just any voltage, any current, or any resistance that you throw at those equations. Ohm's Law relates the resistance of a specific resistor to the voltage across THAT resistor and the current through THAT resistor.

You are not treating it as though the "ground" didn't exist -- doing that would be treating it as though those wires weren't connected at all. They are.

Since voltage is fundamentally a quantity that describes the difference in potential energy between two points in a circuit, it only makes sense to talk about the voltage AT a particularly point if you have defined some point within the circuit as a reference for such claims. You get to pick any one point in the circuit as your reference. By long standing and nearly universal convention, the ground symbol is a way of telling the reader that that node has been declared to have a voltage of 0 V and that the voltage on any other node is to be understood as being the voltage relative to the ground node.

Don't make it more than it is.
 
Top