Current, Power, Potential, and Teaching
I just had a quick query in developing a model for Voltage and current in
power lines for a class I am teaching. I have been able to develop
suitable models for the explanation of Voltage in terms of joules per unit
charge, for current being the quantity of unit charge passing a fixed
point per second and how the current should increase with increased
electronic potential difference when an electric field is created using
say a battery for example. However I am having big problems coming up
with a particle model to explain transmission through power lines. I
understand the relationship between P=VI and P=(I*I)R and could simple use
this to explain why high voltages and low currents are used, however I am
having difficulty developing a model which will explain this at a particle
level. For example if the voltage is very high why does the current not
also increase? Does this not happen because P remains the same - at a
particle level why would this have any effect on the drift of an electron
or its energy due to the electric potential difference?
To explain what is going on here, you need a firm understanding of what is
"cause" and what is "effect". With a standard electrical circuit, battery
voltage is the cause. Current is the effect. Changing the voltage causes
the current to change. This, in turn, causes the power transfer to change.
The resistors are what limit the current. Resistors are where the power
leaves the circuit, usually as heat. If a light bulb provides the
resistance, the power leaves the circuit as light.
A particle passes through the battery. It passes from a low potential to a
high potential. The particle has received energy without being made to
speed up. All the energy is potential. The particle continues through the
circuit. Higher current means more particles passing through the battery
per second. More energy is taken from the battery per second. Power
(Energy per second) from the battery is higher.
The particles continue around the circuit. They pass through the resistor
or light bulb, dropping down to a lower potential, giving energy to the
resistor. This is usually from crashing into the resistor's molecules.
Kinetic energy of the particles goes to the resistor. Electrical potential
energy replaces the lost kinetic energy to keep the particle moving. The
particle continues on through the circuit. When it has passed through the
circuit and gotten back to the battery, the whole cycle begins again.
If voltage across a resistor is higher, the particles have more energy to
spare. They can crash harder into the resistor molecules and still get
through. They can move faster. As a result, higher voltage across a
resistor produces higher current. Particles moving too fast lose too much
energy. They lose more energy than the battery gives them. After a cycle,
they have been slowed. Likewise, particles moving too slow crash less
violently and lose less energy. The battery gives them more than they lose,
and they speed up. When the particles are at the correct speed, they have a
balance. This determines the resulting current.
Dr. Ken Mellendorf
Illinois Central College
Click here to return to the Physics Archives
Update: June 2012