SIMPLE DC CIRCUITS

10 06 2008

Three Resistors in Series.

In a practical circuit consisting of just three resistors, connected in series across a battery, four circuit parameters can be measured using a simple multi-meter. Firstly the current I flowing which is determined by inserting an ammeter in series with the resistors and then the three voltage drops across the individual resistors.

The current is a result of the applied voltage divided by the total series circuit resistance. Apply the formula for series resistance to determine the total resistance R.
Individual resistor voltage drops are each found by applying Ohm’s Law. Resistance R1, R2 or R3 multiplied by the series circuit current. Adding the individual voltage drops together will always equal the applied battery voltage

SIMPLE DC CIRCUITS: Two Parallel Resistors.

Shown are three components connected together forming a circuit. A battery or source of electric current and two resistors. From this diagram a number of circuit parameters may be found. Some are known others need to be calculated.

Considering the current as being ‘conventional’ where it flows from the battery positive terminal and divides into two branches. The amount of current in each branch will be inversely proportional to the resistor value, i.e. the larger resistor value the less current flows.

Firstly we need to find the total current I, but before we can do this we require the equivalent circuit resistance from the formula. We know the applied battery voltage. However each individual branch current could just as easily be calculated by applying Ohm’s Law and the two current values added together which will equal I. Current is never lost (Kirchhoff’s Law) the sum of individual currents will always equal the total current.

As you can see there are several ways of solving this problem. If you were given the total current and the resistor values, could you have found the battery voltage?


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