Electric circuits and the hydraulic analogy.
Introduction: charge, voltage, and current
Electric circuits are very much like hydraulic systems.
Here we have a hydraulic system,
a pump pumping water through a pipe,
and that is very much like a battery
pushing charge through a wire.
These are the hydraulic and electric quantities
and units.
The volume of water is like the charge in an electric circuit.
The flow rate is liters per second in a hydraulic system, or
coulombs per second in the electric system, or amps.
Potential is pressure in kiloPascals for a hydraulic system,
or voltage in volts for an electric system.
Energy is measured in joules in both cases,
and power is in watts in both cases.
Let's take a look at a hydraulic circuit.
Here, we have a reservoir of water,
sitting at zero pressure
This is a pump that
pressurizes the water to 300 kiloPascals.
The water flows through this fairly fat pipe,
so there's very little pressure drop along
this pipe until we reach this narrow point.
When the water squirts through here and drops
from 300 kPa to 0 kPa.
And if the flow rate is 1 liter per second,
then the resistance of this narrow point is
300 kPa
per liter per second.
That's the pressure drop divided by the flow rate.
Here we have an electric circuit
that's very similar.
You start with charge
that's at zero volts in the ground.
The battery draws the charge up through
and pressurizes it, so to speak, to 1.5 volts.
The charge flows through this wire.
The wire is a good conductor,
so there's very little voltage drop
through the wire until we reach this resistor,
which partially blocks the flow of charge.
The charge drops to zero volts.
If the flow is 1 amp,
then the resistance of this resistor is
1.5 volts per amp,
1.5 volts divided by 1 amp,
and there's a special term for that, ohms.
1.5 ohms means 1.5 volts per amp
the power is the flow rate
multiplied by the potential
In the case of a hydraulic system,
power equals flow rate times pressure
In this example,
we have the flow rate of 1 liter per second
and a pressure change of 300 kPa
equals 300 watts,
so you have 300 watts of power,
electric power going into this pump
and you have 300 watts of heat energy
dissipated by this narrow spot.
Now, that's not very useful energy.
We could hook up a water wheel
to get some useful energy out of this.
So 300 watts in, 300 watts out
Similarly, in the electric circuit,
power equals current times voltage.
Current is like the flow rate
voltage is like the pressure
If we have a flow of 1 amp
and a voltage change of 1.5 volts,
we have a power of 1.5 watts.
That means this battery is producing 1.5 watts of power
and this resistor is dissipating 1.5 watts as heat
Again, if you wanted to get something more useful out of this,
you could hook up a DC motor
and do some work
or a light bulb and produce some light
In summary then, charge,
which is measured in coulombs,
is like water volume in liters
Current, which is measured in amps,
or coulombs per second,
is like water flow rate in liters per second
Voltage, measured in volts,
is like pressure measured in kiloPascals
Power is the flow rate times the pressure
in a hydraulic system, or current
times voltage in an electronic system,
and the units are watts in both cases.