Tuesday, June 24, 2008


Electricity is the flow of electrons in a conductor and there are four quite intuitive quantities help to characterize it. voltage, current, resistance and power

The first is voltage. This term refers to the level of energy electrons have relative to some reference point (often called ground in a circuit). The higher the voltage, the more energy electrons have to do work as they travel through the circuit. In general, if two points are at a different voltage relative to each other, electricity will flow from one to the other if they are connected by something that conducts electricity. The unit of measurement of voltage is the Volt (V). As a sample of voltages our wall outlets at home (at least in the US) are 110V, the AA, C & D cells we buy at the corner store are all rated at 1.5V, and the electronics on Teleo Modules requires 5V.

The next quantity is current. This is an expression of how much charge is travelling through the conductor per second. The unit of measurement for current is the Amp (A). You can see that voltage and current are separate things: you can have a very small current at a very high voltage, a huge current at a very high voltage and so on.

The next quantity is resistance. Resistance is an expression of the degree to which electron flow will be impeded through a conductor. The unit is the Ohm (). In simple circuits resistance determines the relation between voltage and current. At the extremes, a short piece of wire will have a resistance of nearly zero Ohms, while an air gap (for example in an open switch) has very large resistance (millions of Ohms). Intuitively a couple of relationships will hold: in a conductor, a voltage difference between the two ends will cause a current to flow. How much current will be determined by how much resistance the conductor offers. If there's less resistance more current will flow. In fact, given a power source of high enough capacity, if you half the resistance, you will double the current. Conversely, if you double the resistance, you will half the current.

The final quantity is power. The unit of power is the Watt. It's an expression of the overall energy consumed by a component. It is worked out by multipling the voltage and the current together - P = VI. For example if a motor was running at 12V and the current it was drawing was 2A, the power it would be dissipating would be 24W.

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