Voltage & Current: Explaining the Basics for Beginners

In summary: This is only possible when current is flowing and the resistance is infinite. 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.My experience is that ##I=\frac{V}{Z}## is a useful formula for many simple tasks, but often thought to be incorrect as it does not mention phase angle. However, Ohm
  • #1
hendrix7
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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?
 
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  • #2
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.
 
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  • #3
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).
 
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  • #4
hendrix7 said:
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.
 
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  • #5
hendrix7 said:
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.
 
  • #6
Dale said:
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.
 
  • #7
tech99 said:
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.
 
  • #8
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.
 
  • #9
hendrix7 said:
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.

hendrix7 said:
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.
 
  • #10
tech99 said:
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?
 
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  • #11
hendrix7 said:
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. :smile:
 
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  • #12
hendrix7 said:
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.
 

FAQ: Voltage & Current: Explaining the Basics for Beginners

What is voltage?

Voltage is the measure of the electrical potential difference between two points in a circuit. It is typically measured in volts (V) and is responsible for the movement of electrical charge, also known as current.

How is voltage different from current?

Voltage and current are closely related but have distinct differences. While voltage is the measure of potential difference, current is the measure of the flow of electrical charge. Voltage is like the force pushing the charge, while current is the actual movement of the charge.

What is the relationship between voltage and current?

According to Ohm's Law, the relationship between voltage and current is directly proportional. This means that as voltage increases, current increases, and vice versa. This relationship is represented by the equation V=IR, where V is voltage, I is current, and R is resistance.

What is the unit of measurement for current?

The unit of measurement for current is the ampere (A), which is defined as the amount of electrical charge passing through a point in one second. It is named after the French physicist André-Marie Ampère.

How does resistance affect voltage and current?

Resistance is the measure of how difficult it is for current to flow through a material. As resistance increases, the voltage needed to maintain the same level of current also increases. This is why higher resistance materials, such as rubber or plastic, are often used to insulate wires and prevent electrical shocks.

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