Ohm's Law

Revisiting Ohm's Law

At least twice per week someone walks in to our store and asks to by a switch or a power supply. First words out our mouths are, "how many Amps?" The typical response is, "12 Volts". So we ask, "How many Amps" Always followed by a blank stare a pause and, "I don't know, its only 12 Volts"

Ohm's law applies to electrical circuits; it states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.


The mathematical equation that describes this relationship is:


I = V / R

A basic electronic circuit has three main properties; Voltage, Resistance and Current. These three properties lead to a great deal on confusion with folks who don't often have to think about them or deal with them.

Consider This simple circuit, a 10Volt battery and a light bulb. If we connect the bulb to the battery via some wire, current will flow through the wire and through the bulb making it light.

The battery provides voltage (V), electric pressure; also know as EMF, Electro Motive Force or Potential Difference.

For the beginner it is best to think of Voltage as electric pressure. ( Note, Sometimes V is represented with E for EMF so, I=E/R is the same as I=V/R )

The bulb and the wire provide Resistance, (R). Resistance impedes current, ( I ) flow.

Resistance is measure in Ohms. Current is measured in Amps.

If the bulb has a resistance of 10 Ohms the current in the circuit will be, 1 amp…

10/10 = 1

If the bulb has a resistance of 20 Ohms the current will be ½ or .5 Amps.

10/20 = .5

If we double the Voltage to 20V and the bulb remains 20 Ohms we get back to 1 Amp.

20/20 = 1


So, EMF in Volts divided by Resistance in Ohms equals current in Amps.

We can solve for any of the three variables as long as we Know the other two.

R=I x V, V = I x R

One way to think about this is to make a comparison to plumbing. Voltage is sort of like Water pressure in PSI. Current is similar to the amount of water flowing in a system and resistance is similar to the size of the pipe.

If the water pressure remains the same and the size of the pipe grows smaller, Less water will flow in the system, It the same or at least a very similar relationship.

What about Watts?

When electric current flows in a circuit, it can transfer energy to do mechanical or thermodynamic work. Devices convert electrical energy into many useful forms, such as heat (electric heaters), light (light bulbs), motion (electric motors), sound (loudspeaker) or chemical changes.


By the definitions of electric potential (volt) and current (ampere), work is done at a rate of one watt when one ampere flows through a potential difference of one volt.


1W=1V×1A

Electric power, like mechanical power, is represented by the letter P in electrical equations. The term wattage is used colloquially to mean "electric power in watts."


In direct current resistive circuits, instantaneous electrical power is calculated using Joule's Law, which is named after the British physicist James Joule, who first showed that heat and mechanical energy were interchangeable. Joule's Law is


P = V x I


where P is the electric power, V the potential difference, and I the electric current.
Joule's law can be combined with Ohm's law (V = RI) to produce two more equations:


P = I^2 x R


and


P = V^2/R


This is by no means a complete explanation and we are only talking about DC, (Direct Current) circuits AC, (Alternating Current) can get thing a lot more complicated. If interested, there are plenty of books and on-line resources that explain in more detail.

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123 W 1st Ave.

Mitchell, SD 57301