# Voltage In An Electric Circuit

1. Nov 2, 2005

### Erwin Schrodinger

I need help with the idea of voltage as it relates to electric circuits. My book doesn't explain the concept well enough. All it says is that voltage is the force that moves charge through a circuit and overcomes resistance. Then it just states Ohm's Law, V=IR.

Here are the things I don't understand. If someone could give a brief explanation, that would be great.

1) If a cell moves charge through a circuit without any relevant resistance, is voltage from the cell still present but not doing anything, or is it not present at all?

2) Is voltage "used up"? If 6V cell moves charge through a resistor that requires 1V, then afterwards would there only be 5V left?

3) If a 5V cell is connected in series to a parallel circuit, would each "branch" of the parallel circuit have 5V?

I can draw some pictures to show you what I mean if my questions don't make sense.

2. Nov 2, 2005

### Integral

Staff Emeritus
If the circiut is open, or has infinite resistance, then no current flows. The cell potential is present. If the resistance is zero, as in "no relevant resistance" then the maximum current the cell is capable of will flow. In a real cell the output voltage will drop.
No voltage is not used up. In a real cell, with carefull measurements you can measure full cell voltage even if the cell is considered dead.
Yes, the voltage drop across parallel branches is equal.

3. Nov 2, 2005

### Erwin Schrodinger

Hmm I still need help with 1 and 2. I made some pictures to show you what I mean.

1. If you measured the voltage at point D, would it be 6V?

2. If you measured the voltage at point A, it would it be 6V? Point B would be 2V? Point C would be 0V?

4. Nov 2, 2005

### ranger

For 1, the voltage would not be six volts. Refer to the basic definition of voltage for hints as to why.
For 2, what are the values for R1 and R2.
$$V_1 = R_1 * I$$
$$V_2 = R_2 * I$$
Remember that current though a series circuit is always the same because there is only one path for it to flow.
By the way, when you say voltage at point B. That can be ambiguous. Voltage is measured across two points. For example on your circuit the voltage across $$R_1$$ could be stated as $$V_{ab}$$.

Last edited: Nov 2, 2005