1. Resistance – Learning Outcomes  Define resistance and give its unit.  Solve problems about resistance.  State Ohm’s Law.  HL: Derive the formulas for resistors in series and parallel.  Solve problems about resistors in series and parallel.  Give the factors that affect the resistance of a conductor.  Use an ohmmeter.  Solve problems about resistivity.

2. Resistance – Learning Outcomes  Discuss light-dependent resistors (LDRs) and thermistors.  Demonstrate LDRs and thermistors.  HL: Describe wheatstone bridges.  HL: Solve problems about wheatstone bridges.  HL: Discuss uses of a wheatstone bridges.  HL: Use a metre bridge.

3. Resistance

6. Ohm’s Law

7. Types of Resistor

8. Resistors in Series

10.  e.g. Calculate the total resistance of the following resistors:

11. Resistors in Parallel

13.  e.g. Calculate the total resistance of the following resistors:

14. Resistance in Circuits

17.  e.g. What is the total resistance of the following resistors?

18. Factors Affecting Resistance – Temperature  We already know that the resistance of a conductor depends on temperature.  Increased temperature has two effects:  Heat releases extra electrons from the atoms, decreasing resistance.  Heat causes atoms to vibrate more, increasing resistance.  For metallic conductors, very few electrons are released, so resistance increases with increasing temperature.  For insulators and semiconductors, lots of electrons are released, so resistance decreases with increasing temperature.

19. Factors Affecting Resistance

20. Factors Affecting Resistance - Length

21. Factors – Cross-Sectional Area

22. Factors - Resistivity

23. Factors Affecting Resistance

25. Light-Dependent Resistor  A light-dependent resistor (LDR) is a semiconductor that decreases its resistance when light shines on it.  Light hitting the resistor releases electrons from the molecules, allowing them to conduct electricity.

26. Thermistor  A thermistor is a semiconductor designed to decrease its resistance as its temperature increases.  The heat energy frees electrons from the material, allowing them to be used for conduction.

27. Wheatstone Bridge

28. Wheatstone Bridge – Uses  Usually, one resistor is variable and set to monitor something, while the galvanometer is replaced by some circuit.  e.g. A thermistor can be used to monitor room / oven temperature and the galvanometer can be replaced by a heater.  When the thermistor unbalances the circuit due to falling temperature, a current flows and activates the heater.  Also used in fail-safe devices.

29. Wheatstone Bridge  e.g. If the bridge pictured is balanced, what is the value of R?

30. Metre Bridge