1. Starter Stick in the following (in the right places):
Tracking sheet – Alex only but some need to transfer this from their old books.
Atomic structure test review sheet – put score and grade into tracking sheet and write down how you can improve next time.
Ohm’s Law questions – please correct where needed.

2. Ohm’s Law questions
If R2 breaks what happens to the current in the circuit?
0.5 A
0.2A
0.3 A
0.0 A
0.3 A

3. Now complete questions from ‘Electricity Questions’ sheet

4. Answers:
1. a) 6v b) i) 2J ii) 6J 2. 12V 3. 3Ω 4. 20V 5. 2A 6. 15Ω Ω
Lesson Aims:

5. Lesson Goals To review how resistance combines in series and parallel
To be able to draw and explain how the resistance of a NTC thermistor changes with temperature.
To be able to draw and explain how the resistance of a LDR changes with light intensity.
To devise an experiment to test these observations.

6. Resistors in series and parallel?
When resistors are placed in series the total resistance of the circuit is the _______ of the individual resistances.
It is like increasing the ________ of wire the current flows through leading to more collisions between electrons and the metal ions, which resists flow.
When resistors are placed in parallel the total resistance of the circuit ______________.
It is like increasing the ______ _______ _____ of the wire and providing more paths for the current to flow.
sum
length
decreases
cross – sectional area

7. Thermistor
A thermistor is NOT made from metal, it is made from a semi-conductor.
It has a negative temperature coefficient which means:
The resistance of a thermistor decreases as its temperature increases

8. Resistance of Thermistor
Temperature [°C]

9. I-V Graph of a Thermistor
Which line represent a thermistor at a high temperature?
Current [A]
This one!
Potential Difference [V]
At high temperatures, resistance is low so is a higher gradient.

10. What about a resistor Be very careful
Which line represents a resistor at a high temperature?
Current [A]
Be very careful
This one!
Potential Difference [V]
At high temperatures, resistance is low so is a higher gradient.

11. Thermistors – what’s going on?
Thermistors are made out of a semiconducting material
Semiconducting martials experience two effects when a current is passed through them.
Increased ion vibration restricts the flow of current (increases R, similar to the effect in a filament bulb)
There is an increase in the number of charge carriers (electrons), this increases current.
The overall effect is that resistance decreases.

12. What can we use a thermistor for?
Thermostatic controllers
Temperature sensors – remember what you did in Year 8 with temperature sensors and logic gates.

13. Light Dependent Resistor
A light dependent resistor is used as a light sensor.
The resistance of a light dependent resistor decreases as the light intensity on it increases.
This means it has a higher resistance in the dark.

14. Resistance of Light Dependent Resistor
Light Intensity [Lux]

15. I-V Graph of a LDR Which line represent a LDR in daylight? Current [A]
This one!
Potential Difference [V]
At high temperatures, resistance is low so is a higher gradient.

16. What can we use a LDR for? Light sensors
Automatic night lights, camera flash controls etc.
Can you use a Light Dependent Resistor on its own to turn a light on automatically when it gets dark?