Basics of Electricity
9
4.5
Power
Power is measured in units of
Watts
.
A key concept:
Power is the
rate
at which work is done or energy is expended and
not
the
amount
of work done.
Or:
Power = energy
time
The power being delivered or used (in Watts) is measured by the current (in Amps)
multiplied by the voltage (in Volts) of the system being measured. I.E.
Watts = Amps
Volts
A battery has a fixed energy capacity or amount of work it can do. The rate at which it does
that work or dumps that energy is its
power output
and that power output can be varied by
the time over which the work is done.
For example, the work or energy a battery and motor do to roll an electric car up a 30 foot
hill is
the same
whether it takes a minute or an hour to do so. The power, however,
is 60
times greater in the second case!
Using the water analogy, you might fill a 10,000-gallon water tank (representing a fixed
amount of work/energy to be done) in two days with a small electric water pump. It would
take a pump attached to a “very powerful” V8 gas engine with a supercharger to fill it in
five minutes. Both need the same energy, but require very different powers. The difference
is the time the effort takes.
4.6
Work/Energy
From a physics point of view, work and energy describe the same thing. The word chosen
at a particular time depends on the point of view being emphasized by an author.
Hopefully, this will become clear as you read on.
People often, incorrectly, mix the concepts of power and energy/work as though they are
the same. They are different, but related by time. Understanding the difference is very
important toward an understanding of propulsion system performance.
Voltage does work to move charge. The battery charger worked to push charge against the
battery’s voltage into the battery. That work is stored as charge in the battery. Discharging
the battery does work on whatever is using the charge.
This work or energy is measured in
Watt-hours
by measuring the power (in Watts)
expended over some time duration (in hours).
Watt-hours = Watts (averaged)
hours
This is how much work the electricity has done.
The energy stored in a battery depends on the product of charge and voltage. I.E.
Energy (Wh) = voltage (V)
charge (Ah)
So while a 7.4 V and 14.8 V battery pack may both have the same charge of 2000 mAh, the
14.8 V pack has
twice
the energy and capacity to do work.
4.7
Conservation of Energy
This is a very important physics concept that will help you evaluate electric model power
systems.
Simply stated, conservation of energy means that energy isn't ever lost; rather it has gone
someplace "else."
Energy has many forms, Heat, Kinetic, Potential and Chemical being the most relevant to
RC.
"Watt's Up" & "Doc Wattson" Watt Meter and Power Analyzer User's Manual
RC Electronics, Inc.
