(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 = R₁ + R₂ + R₃

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 R₁, R₂ and R₃, then:

1/R_T = 1/R₁ + 1/R₂ + 1/R₃

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 R₁ and R₂ are in parallel, then:

1/R_T = 1/R₁ + 1/R₂

1/R_T = (R₁ + R₂)/(R₁R₂)

R_T = (R₁R₂)/(R₁ + R₂)

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