To harness the power of electricity we need to make a basic electrical circuit. The simplest kind of circuit is shown to the right consisting of a voltage source and a resistor through which a current I flows. A light bulb has a resistive element it such that when current flows through it that element heats up and radiates. By turning a switch on then current flows from the battery voltage source through the wire to light the bulb.

Suppose the Resistance of the lightbulb is R = 10 Ohms and suppose the Voltage of the battery is 3 Volts - how much current flows through the circuit?

  • V = I * R I = V/R

  • V = 3; R = 10 I = 3/10


Now let's consider this situation of powering a speaker from an amplifier.

  • Peak Current = V/R = 34/8 =4.25 amps

  • Peak Power = V x I = 34 x 4.25 = 144.5 Watts



Now because we use AC (alternating current) is the source of power outlet electricity. The historical reasons why we use AC as the standard instead of DC can be found here

In AC voltage the voltage is continuously varying so that one can define a Peak Voltage and an Average Voltage. The average voltage is usually called the root mean square (rms voltage). Note that 1.414 is the square root of 2.

Vrms = Vpeak/1.414


Referring back to the speaker example we would then find

  • Vrms = 34/1.414 = 24V
  • Irms = 4.25/1.414 = 3A
  • Prms = VxI = 24x3 = 72 Watts
    where these values can be thought of as the average Voltage, Current, and Resistance of the circuit.

Now its time to discuss how basic electrical circuits are constructed. The key concept to retain is that a circuit in which the resistors are connected in series means that there are no branching points for the current. A circuit which contains resistors in parallel means that the current can be branched and rerouted.



Resistors in Series:

Note that in the following, we use the symbol, (the electromotive force) to represent voltage.

The circuit in the figure above shows three resistors connected in series, and the direction of current is indicated by the arrow. Current flows through each resistor so current I1 slows through resistor R1 , etc.

Since there is only one path for the current to follow (i.e. there is no branching point in this circuit) then the current is everywhere the same in this circuit no matter where you are in the circuit.

I = I1 = I2 = I3



Across each resistor there is a voltage drop (think of this as the following: each resistor represents a point of energy dissipiation) The sum of all the voltage drops across the 3 resistors as to equal the total supplied voltage.



V = V1 + V2 + V3

The total resistance in this circuit is the sum of the individual resistances of all the resistors in series.

R = R1 + R2 + R3

For any given resistor, Ohms law is also satisfied. That is,

V1 = R1I1



Now, since I1 = I then we have the generic condition for each resistor n:

Vn/Rn = I



For any circuit where the resistors are in series. Series circuits are pretty easy to understand if you just visualize that the current only goes one way.