# 5.01 Basic d.c. circuit Problems.

This section contains some interactive basic d.c. circuit (cct) problems, that allow you to step through the solutions one stage at a time.

For these problems you will be required to calculate all of the currents and voltages in the cct. In order to do this you need to know how to add resistors in series and parallel, as well as how to use Ohm's law and Kirchoff's current and voltage laws.

Technique is very important. Some students have difficulty with these ccts, even though they know all of the laws and equations that are required. For example, there may be several different currents, voltages and resistances in the cct., so you are likely to have to use Ohm's law several times. A common mistake is to put voltages and currents from different parts of the circuit into an equation producing an incorrect value. Labelling all of the currents, voltages and resistances in the circuit with sensible labels, will help to avoid this mistake. In all of the cct problems in this section, this has already been done.

For each problem a table shows all of the possible equations*, highlighting the values which are known. This helps you see which equation must be solved first, (you can only solve an equation when there is only one unknown value). As you progress through each problem this table is updated. This allows you to see which equation to use next. At some stages you may have a choice of equations which can be solved. In general, it does not matter which order these are done. Also when there are two alternative equations available to calculate a circuit value, it is good practice to choose one to perform the calculation and the other to check your answer.

* To try to keep the layout compact, the equations for adding resistors are not shown in the table, however they are displayed during the calculation.

In these examples a method called the product over sum rule is used for adding two parallel resistors, (this rule is explained in the next section). However the more general equation 1/RT = 1/R1 + 1/R2 can be used to give the same answers.