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

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)

Resistors in parallel Total resistance = R1 x R2 R1 + R2 HT 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

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

Try these 1. 2. 3. 4.

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

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

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

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.

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

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

Practical applications - LDRs 0V 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

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

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

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)

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

PC research http://www.bbc.co.uk/schools/gcs ebitesize/science/triple_ocr_gatew ay/electricity_for_gadgets/sharing/ revision/4/