# How is voltage by itself important?

1. Oct 2, 2009

### user111_23

Lets say I have circuit with resistance 4Ohms and the current 100mA. If I use Ohm's law, I get 0.4V. Why is it important to know the voltage if there is already a current?

Also, when you know the energy available per unit charge (voltage) and current, I can calculate the power. In that case, wouldn't it be useless to know the voltage when you aleady know how much energy is being used (power)?

2. Oct 2, 2009

### A.T.

In every relationship like a * b = c knowing two of the variables gives you the third. But why do you pick on voltage? You could just as well call any of the other two useless, based on the same logic.

3. Oct 2, 2009

### user111_23

Voltage causes current. Why should I know the voltage if there is already current? You know that the current is not dangerously high, lets say, so why worry about voltage?

4. Oct 2, 2009

### S_Happens

Voltage does not CAUSE current. If you have a current, then you know that you have a voltage.

Edit- It's mostly semantical. I simply mean that you can have a voltage without current and therefore voltage alone is not enough to cause a current. I agree that if you already have a current, then you know you have a voltage.

In your example (since you mention danger) it is important to also know voltage or the resistance. What happens if you decide to take that same current to ground through your body? Do you assume that the current will stay constant? Let's say the resistance of your body is much less than the circuit that you measured the current in, what will happen?

The three variable equation allows you to describe one of the variables in terms of the other two. At worst it can be considered redundant to calculate the voltage when you know the current and resistance, but it is hardly useless.

Last edited: Oct 2, 2009
5. Oct 2, 2009

### cabraham

You need to examine all 3, I, V, & R, actually 4, since power, P is also important. Among P, I, V, & R, if you know 2, you can compute the other 2.

As S_Happens already stated, V does not cause I. nor does I cause V. One can exist w/o the other only under static conditions. Under dynamic conditions, they are inclusive, i.e. neither can exist w/o the other.

They coexist. If you know 2 of the 4 quantities, you can compute the other 2. If P= 2.0W, & R = 8 ohms, then it's easy to find I & V. Of course, I = 0.5A, & V = 4.0V.

Does this help?

Claude

6. Oct 2, 2009

### user111_23

Let me clarify:

Voltage=energy available per unit charge

Power=transferring of energy per unit of time

If the circuit is already on, what is the importance in knowing the voltage? I'm talking about DC circuits here.

7. Oct 2, 2009

### Staff: Mentor

....you need it to find power. (sorta). Or maybe you want to know how amperage changes with a change in resistance? Depends on the problem

8. Oct 5, 2009

### Good4you

I think to best answer your question; you need to first think of why you are asking.

Beyond mathematics, and semantics, the important part to me would be to think of it in terms of what unit everyday items are rated in. Each type of compenent is rated by a different unit, because that unit of measure best explains what you are trying to do with the items.

For example:
Voltage: This unit is used for current conductors (wires.) Wires are rated in voltage because high voltages require expensive insulation. Also; voltage drop, is based on wire size. That is; high voltage can use smaller wire size, but will need expensive insulation.

Current: Most load devises are rated in "amps." This is because most devices are designed to draw a certain number of amps. Increasing the voltage will increase the horsepower (or watts). Many motors are controlled by VFD (Variable frequency drives, which adjust the 60hz cycle or the voltage. this is also true of light bulbs. So in most loads the work (watts) can be variable, so can the voltage, but typically the amperage (current) is constant.

Power: Watts is the preferred unit of measure for generators. This is important because watts measures the amount of work that can be performed. A generator can use a transformer to change the voltage or the amperage to virtually anything. Wattage is the only thing that is constant, and consequently the most important unit because it measures the amount of work that can be done (e.g. the number of light bulbs, houses, air conditioners, etc. that can be run.)