1. P6 – Electricity for Gadgets
Lesson 4 – Resistors in parallel
Learning aim: Demonstrate an understanding of how resistors in parallel circuits, thermistors and LDRs work

2. Success Criteria Learning Objectives
Understand how LDRs and thermistors work, and how they are used in potential divider circuits.
Understand how resistors in parallel reduce the overall resistance of the circuit.
Calculate the total resistance for resistors in parallel.
Recognise and draw the circuit symbols for LDRs and thermistors and recall how they work.
(Grade E-D)
Describe how the resistance of an LDR and thermistor varies.
(Grade C)
Recall how resistors in parallel differ from resistors in series.(Grade C)
Explain how LDRs and thermistors can be used in potential divider circuits.(Grade C-A)
Calculate the total resistance of resistors in parallel.(Grade C-A)

3. Resistors in parallel Total resistance = R1 x R2 R1 + R2HT
When resistors are connected in parallel, the total resistance can be calculated using:
Total resistance = R1 x R2
R1 + R2
What is the total resistance for this circuit?
2 
4 
Total resistance
= R1 x R2
R1 + R2
= 4 x 2
4 + 2
May see it written as in the book pg 235
= 1.33

4. Resistors in parallel 8  6  Total resistance = R1 x R2 R1 + R2HT
What is the total resistance for this circuit?
8 
6 
Total resistance = R1 x R2
R1 + R2
= 8 x 6
8 + 6
= 3.4 

5. Try these 1. 2. 3. 4.

6. Different types of resistors
Starter
Component
Circuit symbol
resistor
variable resistor
thermistor
light dependent resistor
diode
What could we use an LDR for
Write down the list of resistors and draw the symbol for each
Do as many as you can then us the book for Help –pg237

7. Thermistor
The resistance of a thermistor decreases as the temperature increases:

8. Thermistors
A thermistor is a component that has a high resistance when cold but a low resistance when hot.
The graph shows how the resistance of a thermistor decreases as its temperature increases.
This is the reverse of the normal trend for resistance.
This property makes thermistors useful in circuits that control and monitor temperatures. x x x x
Resistance /  x x x x x
Temperature / C

9. LDRs
In the dark and at low light levels, the resistance of an LDR is high, and little current can flow through it.
In bright light, the resistance of an LDR is low, and more current can flow through it.

10. Light dependent resistors
The resistance of a light dependent resistor (LDR) is not fixed and depends on the light intensity.
A LDR has a high resistance in the dark but a low resistance in the light.
The graph shows how the resistance of an LDR decreases as the light intensity increases.
This makes LDRs useful in circuits that are controlled by light intensity. x x x x
Resistance / k x x x x x
Light intensity

11. complete the sentences
When the light intensity increases on an LDR its resistance
When the temperature of a thermistor increases its resistance
An LDR could be used to
A thermistor could be used to
Decreases, decreases, street lights, heater control

12. Practical applications - LDRs0V
Vin
VOUT
Here’s a potential divider that is used to control light-activated switches…
When the light intensity on the LDR decreases its resistance will ________. This causes VOUT to _______ so if used with a bulb, it would turn _____. The variable resistor can be adjusted to change the ________ of the whole device.
Words – decrease, sensitivity, increase, off, on
15/11/2018

13. Plenary
What happens to the resistance of an LDR when it is bright sunlight?
What would happen to the current passing through the LDR in bright sunlight?
When the light levels slowly decrease what happens to the resistance of the LDR?
Sketch a graph of resistance against light level for a typical LDR.
Discuss answers in pairs and write answers on white board

14. answers
Q2a it is low • Q2b it would be quite high • Q2c increases • Q2d correctly sketched graph as shown on p236

15. Success Criteria Learning Objectives
Understand how LDRs and thermistors work, and how they are used in potential divider circuits.
Understand how resistors in parallel reduce the overall resistance of the circuit.
Calculate the total resistance for resistors in parallel.
Recognise and draw the circuit symbols for LDRs and thermistors and recall how they work.
(Grade E-D)
Describe how the resistance of an LDR and thermistor varies.
(Grade C)
Recall how resistors in parallel differ from resistors in series.(Grade C)
Explain how LDRs and thermistors can be used in potential divider circuits.(Grade C-A)
Calculate the total resistance of resistors in parallel.(Grade C-A)

16. Plenary
Use the information sheet to help you explain the resistance changes taking place in a thermistor and an LDR
Make a list of CONTROL circuits that could use a thermistor or LDR

17. PC research
ebitesize/science/triple_ocr_gatew ay/electricity_for_gadgets/sharing/ revision/4/