What is different between electrostatic voltage and circuit voltage?

1. Jan 12, 2013

okami11408

I'm confused about this because voltage or potential difference is defined as work done by an external source in moving a unit charge from one point to another in an electric field.

However, in circuit analysis things seem to be different because we measure voltage from V=IR.

I want to know the different between these two methods. Or Is it the same thing?

Sorry for my English.

2. Jan 12, 2013

Studiot

The physical quantity which has the units of 'Energy per unit Charge' is known as the electric (electrostatic) potential and and is measured in volts.

Now this definition can be shrunk to a point so you can have a defined 'voltage' at any point in space.

If you have two or more points you can define potential difference as the difference (also measured in voltage) of the electric potential between these two points.
This is the same idea as elevation (height) above the floor. The seat of your chair is at one height, the table at another, but both are measured in metres. The Height Difference between the chair and table is also measured in metres as in the voltage situation.

These points may be points in free space or points in a material or points at the ends of a component, say a resistor.

Notice I have only mentioned charge. If the charge is moving you have a current and can apply ohms or other laws. Obviously movment implies time so you have bring time into the equationa so current = charge per second.

So all voltages refer to the same physical phenomenon, but the circumstances are different so you require different equations.

Does this help?

3. Jan 12, 2013

It is the same concept. It is the minimum work you would have to do to move a unit charge through the resistor (even if the resistor didn't affect the charge at all).

4. Jan 12, 2013

tiny-tim

hi okami11408!
not necessarily

V is not always equal to IR (that is true only if the material is "Ohmic")

V is always equal to power divided by current … P = VI
from the pf library on voltage

Two ways of defining voltage:

voltage = energy/charge = work/charge = force"dot"distance/charge = (from the Lorentz force) electric field"dot"distance, or dV = E.dr

but also voltage = energy/charge = (energy/time)/(charge/time) = power/current, or V = P/I​

5. Jan 13, 2013

DrZoidberg

Voltage is always energy per charge. When a current flows through a resistor an electric field is created across that resistor. Just like when air flows through a small hole a pressure difference is created.
Well, actually it's the other way around, the electric field causes the current. And the stronger the electric field is, the higher the energy per charge and the higher the current that gets pushed through the resistor.