24-08-2013, 02:45 PM
Voltage, Current, Energy and Power
Overview
In this part, we will cover:
Definitions of current and voltage
Hydraulic analogies to current and voltage
Reference polarities and actual polarities
Current: Formal Definition
Current is the net flow of charges, per time, past an arbitrary “plane” in some kind of electrical device.
We will only be concerned with the flow of positive charges. A negative charge moving to the right is conceptually the same as a positive charge moving to the left.
Mathematically, current is expressed as…
Hydraulic Analogy for Current
It is often useful to think in terms of hydraulic analogies.
The analogy here is that current is analogous to the flow rate of water:
Charges going past a plane per time
– is analogous to –
volume of water going past a plane in a pipe per time.
Water flow Current
So, if we put a plane (a screen, say) across a water pipe, and measure the volume of water that moves past that plane in a second, we get the flow rate.
In a similar way, current is the number of positive charges moving past a plane in a current-carrying device (a wire, say) in a second.
The number of charges per second passing the plane for each [Ampere] of current flow is called a [Coulomb], which is about 6.24 x 1018 electron charges.
Why bother with reference polarities?
Students who are new to circuits often question whether this is intended just to make something easy seem complicated. It is not so; using reference polarities helps.
The key is that often the actual polarity of a voltage or current is not known until later. We want to be able to write expressions that will be valid no matter what the actual polarities turn out to be.
To do this, we use reference polarities, and the actual polarities come out later.
Sign Conventions or Polarity Conventions
To determine whether power and energy are delivered or absorbed, we will introduce sign conventions, or polarity conventions.
A sign convention is a relationship between reference polarities for voltage and current.
As in all reference polarity issues, you can’t choose reference polarities wrong. You just have to understand what your choice means.