Tuesday, June 24, 2008



We mentioned earlier that there is a relationship between voltage, current and resistance. It turns out that this relationship is a mathematical one and it can be expressed very simply by way of Ohm's Law.

Ohm's Law observes that in a simple resistive circuit, voltage (V) , resistance (R) and current (I) are related in the following way:

V = IR

This expression can be rearranged algebraically to find other ways to use it as follows:

I = V / R (dividing both sides by R)
R = V / I (dividing both sides by I)

The idea is that if you know two of the quantities, you can work out the third by using one of the equations.


In the circuit fragment above, a resistor is connected between a 5V supply and ground (0V). We can use this to check the assertions we made earlier about the effects of doubling the resistance on a circuit and so on. If the resistor's value is 10, we can work out what current will flow as follows. We know the voltage (5V) and the resistance (10), so the form of the equation we need is:

I = V / R

Substituting our values in we have:

I = 5 / 10

The current I in our circuit will be 0.5A which can also be expressed as 500mA.

Now if we were to double the resistance (to 20), let's confirm that we halve the current:

I = V / R

Substituting our values in we have:

I = 5 / 20

The current I in our new circuit will be 0.25A (or 250mA), which is indeed half the previous current.

Ohm's Law is a very important relation to remember how to use. Finding current from resistance and voltage or finding voltage from current and resistance is something that is required very frequently in electronics.

The energy of the electricity passing through the resistor is being converted by the resistor into waste heat. The ability of the resistor to do this depends on how big it is. If it's too small it will not be able to get rid of the heat fast enough and it may start to overheat and eventual burn.

The resistor's power rating is stated in Watts. Recall that the power is calculated by multiplying the voltage by the current:

P = VI

In this last case, the voltage across the resistor was the same, 5V, but the current was halved to 0.25A. Putting those values into the equation gives:

P = (5)(0.25)

P = 1.25W

Thus, the resistor chosen must be rated at least 1.25 W - which is rather a large resistor in the context of digital electronics.

As a side note and another nice use of Ohm's Law, the V term in the power expression P = VI can be substuted by IR, making P = IR, which a very convenient way to calculate power when the voltage drop across something is not known.

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