Lesson 2: Simple Circuits & Ohm's Law.



The three readable elements of an electrical circuit are Current measured in Amps, Resistance measured in Ohm's, and Electromotive force measured in Volts. There is a direct relationship between these three, which was first defined by the German scientist Georg Ohm in 1827. The formula for the relationship is E=IR. Expanding the letters; E stands for Electromotive force (measured in Volts), I represents Current (measured in Amps), and R stands for Resistance (measured in Ohms), therefore the formula could just as accurately be written Volts=Amps x Ohms.

Simple circuit illustrating Ohm's Law. In the simple circuit on the right the voltage across the resistor could be easily calculated if we know the value of the resistor, and can measure the current flowing through it. More often than not though, because current can only be read by a meter which is itself a part of the circuit, (which requires opening the circuit to insert the current meter), it is usually the current which is the calculated value. A simple mathematical manipulation changes the formula to E/R=I. So, with 10Volts being read across a 200Ohm resistor it becomes 10v divided by 200ohms which equals 0.05amps.
There is a fourth element in circuits called Power. Power is a derived element whose unit is the "Watt", and can not be read by a meter. It is calculated by one of two formulas. The first is P=VI (Power = Volts x Amps). (Goodness knows why Volts is represented by V in this formula, whereas Ohm's law uses E). Anyway, our simple circuit shows that the power used is calculated as 10volts x 0.05amps, which equals 0.5 Watts.

The other formula for power comes from substituting the V with the E in the Ohm's law, so then P=V x I is the same as P = E x I, which is the same as P = (I R) x I, and, as there are two "I"s its the same as I2, so we end up with P=I2R. In our circuit, P = 0.05amps x 0.05amps x 200ohms which once again equals 0.5 Watts.

Now, just a little definition explanation; the term Watts has also become common use when referring to the amount of sound produced from a speaker, as well as the amount of light that a light bulb produces, as well as the amount of power an electric motor produces, and the amount of heat an electric heater produces. These are all measurements of power, but in most cases the power rating refers to the amount of electricity it uses to produce the output. A 40 watt filament light bulb uses the same amount of electricity as a 40 watt flourescent tube, and yet it produces a very different amount of light. The power rating of Speakers is actually the maximum amount of electricity the voice coil can take before it melts down. The amount of sound that comes out depends entirely on the efficiency of the speaker to convert the electricity into movement.





We're now ready to move on to the next lesson. Click here to go to the lesson about parallel and series circuits, or click here to return to the Electronics for Sound Techs index page.