Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013

4

Electricity can be understood through an analogy with the flow of water (borrowed from

(Thompson 2005)). Picture two tanks connected by a pipe. One tank has water in it; the other is

empty. Potential energy is created by the presence of water in the first tank. The water flows

through the pipe from the first tank to the second with some intensity. The pipe has a certain

amount of resistance to the flow of water as a result of its physical properties, like its size. The

potential energy provided by the full tank, reduced somewhat by the resistance of the pipe,

results in the power of the water flowing through the pipe.

By analogy, in an electrical circuit we have two voltages connected by a conductor.

Analogous to the full tank of water, we have a voltage – an excess of electrons – at one end of

the circuit. Let’s say that at other end of the circuit we have 0 voltage, also called ground or

ground potential. The voltage at the first end of the circuit causes pressure, or potential energy,

as the excess electrons want to move toward ground. This flow of electricity is called the

current. The physical connection between the two halves of the circuit provides resistance to

the flow. The connection might be a copper wire, which offers little resistance and is thus called

a good conductor. On the other hand, something could intentionally be inserted into the circuit

to reduce the current – a resistor for example. The power in the circuit is determined by a

combination of the voltage and the resistance.

The relationship among potential energy, intensity, resistance, and power are captured in

Ohm’s law, which states that intensity (or current) is equal to potential energy (or voltage)

divided by resistance:

Equation 4.1 Ohm’s law

Power is defined as intensity multiplied by potential energy.

Equation 4.2 Equation for power

Combining the two equations above, we can represent power as follows:

Equation 4.3 Equation for power in terms of voltage and resistance

Thus, if you know any two of these four values you can get the other two from the equations

above.

Volts, amps, ohms, and watts are convenient units to measure potential energy, current

resistance, and power in that they have the following relationship:

1 V across 1 of resistance will generate 1 A of current and result in 1 W of power

The above discussion speaks of power (W), intensity (I), and potential energy (V) in the

context of electricity. These words can also be used to describe acoustical power and intensity as