B Voltage & Current: Explaining the Basics for Beginners

[hendrix7]

TL;DR Summary

V = IR

This is a very basic beginner question but one which confuses me.
We are told that voltage = current x resistance but I've also read that we can have a voltage without current. But, if current = zero, then if V = IR, V must be zero. Can someone explain where am I going wrong?

 

[Orodruin]

If current is equal to zero then conductance $G = 1/R$ is also zero, leading to Ohm’s law taking the form $VG = I$ expressing zero equal to zero.
In terms of the more familiar form of Ohm’s law, resistance is infinite in this scenario.

 

[phinds]

An open circuit can have a voltage but no current flows through it (R is infinite).

A short circuit can have current flowing but with no voltage across it (R is 0).

 

[Dale]

TL;DR Summary: V = IR

We are told that voltage = current x resistance but I've also read that we can have a voltage without current.

That equation is called Ohm's law and it only applies for conductors/resistors. It does not apply for capacitors, inductors, voltage sources, current sources, and many other devices.

 

[tech99]

TL;DR Summary: V = IR

This is a very basic beginner question but one which confuses me.
We are told that voltage = current x resistance but I've also read that we can have a voltage without current. But, if current = zero, then if V = IR, V must be zero. Can someone explain where am I going wrong?

To answer the question, if you are finding a voltage without a current then the resistance must be infinite. This is the normal situation with an open circuit.

 

[tech99]

That equation is called Ohm's law and it only applies for conductors/resistors. It does not apply for capacitors, inductors, voltage sources, current sources, and many other devices.

However, Ohm's Law for AC is very useful, where the current flowing in an impedance is given by I = V/Z. This is irrespective of the phase angle.

 

[DaveE]

However, Ohm's Law for AC is very useful, where the current flowing in an impedance is given by I = V/Z. This is irrespective of the phase angle.

OK, yes for AC (steady state). But it doesn't work with initial conditions and transients (e.g. instantaneous measurements) very well. Definitely not "basic level". Way too confusing for students that are still figuring out resistors and DC circuits.

 

[tech99]

My experience is that I = V/Z is a useful formula for many simple tasks, but often thought to be incorrect as it does not mention phase angle.

 

[Herman Trivilino]

We are told that voltage = current x resistance but I've also read that we can have a voltage without current.

The former is true when there is a current, the latter when there isn't.

But, if current = zero, then if V = IR, V must be zero. Can someone explain where am I going wrong?

You're not going wrong. If you don't apply a voltage $V$ across a resistor you won't get any current flowing through that resistor.

 

[DaveE]

My experience is that I = V/Z is a useful formula for many simple tasks, but often thought to be incorrect as it does not mention phase angle.

The only reasonable interpretation of $I=\frac{V}{Z}$ when phase isn't mentioned is $|I|=\frac{|V|}{|Z|}$, which works fine for what it says. It's also more appropriate for a thread titled.

"Very basic electric circuit question about Ohm's Law..."​

where the OP never asked about or mentioned AC, phase, Z, or impedance. So what's next for this thread, Laplace Transforms?

 

[alan123hk]

TL;DR Summary: V = IR

But, if current = zero, then if V = IR, V must be zero. Can someone explain where am I going wrong?

I don't think your thinking is wrong, but you might be overlooking something. When the current is zero, there is a condition for the voltage to be non-zero, which is when the resistance is infinite. Since zero multiplied by infinity can be seen as equal to an indeterminate number, this means that the voltage can be any possible value.

 

[osilmag]

TL;DR Summary: V = IR

This is a very basic beginner question but one which confuses me.
We are told that voltage = current x resistance but I've also read that we can have a voltage without current. But, if current = zero, then if V = IR, V must be zero. Can someone explain where am I going wrong?

There could be an electric field and the voltage there would be $V=Ed$ where d is the distance between two points and E is the electric field.