**(New tips are continually
added to these pages. Check back in a few months' time for more)**

**TOPIC
15: Electric Circuits**

**Tip 1:**

To find the total resistance of several resistors connected in series, simply add up their resistances. This is pretty straight forward, as follows:

*R*_{T} =
*R*_{1} + *R*_{2} + *R*_{3}

For resistances in parallel, some short-cut formulae exist which can save both time and effort.

We know that for
resistances in parallel, let's say *R*_{1}, *R*_{2}
and *R*_{3}, then:

1/*R*_{T} =
1/*R*_{1} + 1/*R*_{2} + 1/*R*_{3}

If the 3 resistors have
the same value, eg. *R*, then:

1/*R*_{T} = 1/*R* + 1/*R* + 1/*R*
= 3/*R*

Hence, *R*_{T} =
*R*/3

In general, if there are
*N*
number of resistors *R* in parallel, their combined resistance is *R*/*N*

Another short-cut:

If two resistors *R*_{1}
and *R*_{2} are in parallel, then:

1/*R*_{T} =
1/*R*_{1} + 1/*R*_{2}

1/*R*_{T} =
(*R*_{1} + *R*_{2})/(*R*_{1}*R*_{2})

*R*_{T} = (*R*_{1}*R*_{2})/(*R*_{1}
+ *R*_{2})

ie. their combined resistance is equal to their product divided by their sum.

eg. resistors 3W and 6W are in parallel, their combined resistance is:

(3 x 6)/(3 + 6) = 18/9 = 2 W

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