1. 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.

2. Revision (READ) Resistors in series: What was the relationship between the total resistance, R t, and three resistors in series, R 1, R 2 and R 3 ? R t = R 1 + R 2 + R 3

3. 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?

4. Resistors in Parallel We will measure three resistors separately and then measure them when combined: ResistorResistance (  ) R 1 (on its own) R 2 (on its own) R 3 (on its own) R T (all together)

5. 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.

6. Resistors in Parallel Construct the following table: 1R11R1 1R21R2 1R31R3 1 + 1 + 1 R 1 R 2 R 3 1RT1RT

7. Resistors in Parallel We can now see that the relationship for resistors in parallel is as follows: 1=1+1+11=1+1+1 RtR1R2R3RtR1R2R3 The total resistance is always smaller than the smallest resistor.

8. Example 1 Find R T below

9. Example 1 What to input into your calculator: How to lay out in the exam:

10. Example 2 Find R T below

11. Example 2 What to input into your calculator: How to lay out in the exam:

12. 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.

13. 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.

14. Example 1: both parallel and series Find R T

15. Example 2: both parallel and series Find R T

16. What have we learned today? For a parallel circuit: 1=1+1+11=1+1+1 RtR1R2R3RtR1R2R3 If we have resistors in series AND parallel: 1.Work out R t of parallel resistors. 2.Then work out R t of series resistors.

17. 2003, Qu: 11 C

18. 2004, Qu: 8 C

19. 2006, Qu: 10 D

20. 2004, Qu: 8 C

21. 2008, Qu: 25(b)(i)

22. 2009, Qu: 25(b)(i), (ii)

23. 2010, Qu: 24(a), (b)

24. 2005 Qu: 25