What we will do today

• Complete and correct check test 5.• Look at resistors in parallel.• Define a formula for the total

resistance of resistors in parallel.

Revision (READ)Resistors in series:• What was the relationship between

the total resistance, Rt, and three resistors in series, R1, R2 and R3?

• Rt = R1 + R2 + R3

Combining resistors (Read)

• We are now going to investigate the effect of four resistors in a parallel circuit.

• What apparatus will be needed?• What will our circuit diagram look

like?

Resistors in Parallel• We will measure three resistors separately

and then measure them when combined:

Resistor Resistance ()R1 (on its own)R2 (on its own)R3 (on its own)RT (all together)

Resistors in Parallel• From our results it is hard to see a

relationship.• However, if we look carefully we

can see a pattern develop when the relationship is reciprocal eg. 1/R.

Resistors in Parallel

• Construct the following table:

1R1

1R2

1R3

1 + 1 + 1 R1 R2 R3

1RT

Resistors in Parallel• We can now see that the

relationship for resistors in parallel is as follows:

1 = 1 + 1 + 1Rt R1 R2 R3

• The total resistance is always smaller than the smallest resistor.

Example 1• Find RT below

Example 1• What to input

into your calculator:

• How to lay out in the exam:

Example 2• Find RT below

Example 2• What to input

into your calculator:

• How to lay out in the exam:

General Rule• In a series circuit, Rt is always

bigger than the biggest resistor.

• In a parallel circuit, Rt is always smaller than the smallest resistor.

What if we have both?• If we have both parallel and

series resistors in a circuit;1. Work out any resistors that are in

series in the same branch.2. Then work out resistors in parallel

and replace them with the value of one resistor.

3. Work out the resistors in series.

Example 1: both parallel and series

• Find RT

Example 2: both parallel and series

• Find RT

What have we learned today?

• For a parallel circuit:1 = 1 + 1 + 1Rt R1 R2 R3

• If we have resistors in series AND parallel:

1. Work out Rt of parallel resistors.2. Then work out Rt of series

resistors.

2003, Qu: 11

• C

2004, Qu: 8

• C

2006, Qu: 10

• D

2004, Qu: 8

• C

2008, Qu: 25(b)(i)

2009, Qu: 25(b)(i), (ii)

2010, Qu: 24(a), (b)

2005 Qu: 25