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3 DC Circuits G482 Electricity, Waves & Photons 3 DC Circuits G482 Electricity, Waves & Photons Series and Parallel Circuits – Kirchhoff’s second law Series and Parallel Circuits – Kirchhoff’s second law Mr Powell 2012 Index Practical Circuits Practical Circuits

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Mr Powell 2012 Index Practical Notes....

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Mr Powell 2012 Index Practical Circuits Assessable learning outcomes a) draw a simple potential divider circuit; b) explain how a potential divider circuit can be used to produce a variable p.d.; c)select and use the potential divider equation d)describe how the resistance of a light dependent resistor (LDR) depends on the intensity of light; e)describe and explain the use of thermistors and light- dependent resistors in potential divider circuits; f)describe the advantages of using dataloggers to monitor physical changes Assessable learning outcomes a) draw a simple potential divider circuit; b) explain how a potential divider circuit can be used to produce a variable p.d.; c)select and use the potential divider equation d)describe how the resistance of a light dependent resistor (LDR) depends on the intensity of light; e)describe and explain the use of thermistors and light- dependent resistors in potential divider circuits; f)describe the advantages of using dataloggers to monitor physical changes V1V1 V2V2 VsVs R1R1 R2R2

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Mr Powell 2012 Index a) Potential Dividers Explained… Take a simple series circuit with uniform current flow and two equal resistors. The p.d. drop across each is the same Then ‘open-out’ the cell to show as a “rail” Then label the supply as V s and the 0V as ground rail, the resistors and voltmeters as 1 & 2 (you could use a & b) V V V V V1V1 V2V2 VsVs R1R1 R2R2

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Mr Powell 2012 Index a) What is the output voltage…… We can use this circuit to be able to find the output voltage across R 2 so we can see a change in a component such as a thermistor. So add the output voltage V out Output voltage is the same as the voltage across R 2 i.e. V 2 = V out V1V1 V2V2 VsVs R1R1 R2R2 V out

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Mr Powell 2012 Index b) Calculations Since the current is the same through both resistors we can define VsVs R1R1 R2R2 V out I V1V1 V2V2 NB: Now we can express Vout as ratio of resistance multiplied by Vs

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Mr Powell 2012 Index I b) Alternative Maths...

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Mr Powell 2012 Index b) Quick Test... ? ? ? ? ? ? ? ? ? ? ? ? A B C D ? ?

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Mr Powell 2012 Index b & c) Temperature Sensors? Redraw the circuit shown and label the variables according to the rules. Work out the V out voltage for the two temperatures to verify the formula that we have just derived;

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Mr Powell 2012 Index d) Thermistor The resistance of a thermistor decreases as the temperature increases so if we look at it from the VI perspective it is the opposite of a bulb!

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Mr Powell 2012 Index e) How do they work? The exact conduction mechanisms are not fully understood but metal oxide NTC thermistors behave like semiconductors, as shown in the decrease in resistance as temperature increases. The physical models of electrical conduction in the major NTC thermistor materials are generally based on this theory; A model of conduction called "hopping" is relevant for some materials. It is a form of ionic conductivity where ions (oxygen ions) "hop" between point defect sites in the crystal structure. The probability of point defects in the crystal lattice increases as temperature increases, hence the "hopping" is more likely to occur and so material resistivity decreases as temperature increases. Only need the outcome in red for AS Physics

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Mr Powell 2012 Index e) Temperature Sensors? They are inexpensive, rugged and reliable. They respond quickly to changes and are easy to manufacture in different shapes. An example could be made from a combination of Fe 3 O 4 + MgCr 2 O 4 (metallic oxides) A NTC thermistor is one in which the resistance decreases with an increase in temperature. The circuit shows how you can use the thermistor as a potential divider. As the temperature changes the division of voltage or energy will change. You need the 5k resistor or the voltage would be that of the cell a constant 3V. A common use is the glass heat sensor in a car or the temperature sensor in a conventional oven.

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Mr Powell 2012 Index Practical Idea? V 1, V 2

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Mr Powell 2012 Index Conclusions… A)The current through and p.d. across the bulb can be reduced to zero in a potential divider circuit B)The current & voltage can be made to minimum but not zero! 2

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Mr Powell 2012 Index AC Audio input Variable voltage output to a loud speaker Variable voltage output to a loud speaker A simple volume control A simple volume control AC Audio input

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Mr Powell 2012 Index Theory Summary V1V1 V2V2 VsVs R1R1 R2R2 V out A potential divider does just what is states. It divides a potential difference Think of a p.d. of 10V across a resistor. The p.d. will drop by 1V for each 10% of the resistor that the current passes through. From this theory two resistors will have a ratio which from the idea that V=IR will relate the output voltage on a resistor to the source voltage as shown. Obviously if resistor 1 and 2 are swapped V out also swaps. We can replace one of the fixed resistors with; Variable resistor, which could act as a volume control or sensor i.e. Thermistor or LDR.

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Mr Powell 2012 Index Summary of Uses….

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Mr Powell 2012 Index Plenary Question…. In the circuit shown, the battery has negligible internal resistance. Basic Calc 3) The circuit shown in the diagram acts as a potential divider. The circuit is now modified by replacing R 1 with a temperature sensor, whose resistance decreases as the temperature increases. Explain whether the reading on the voltmeter increases or decreases as the temperature increases from a low value. (3) iSlice Explaining 1)If the emf of the battery = 9.0V, R 1 = 120 and R 2 = 60 , calculate the current I flowing in the circuit. (3) 2)Calculate the voltage reading on the voltmeter. (1) (temperature increases, resistance decreases), total resistance decreases (1) current increases (1) voltage across R 2 increases (1) or R 2 has increased share of (total) resistance (1) new current is same in both resistors (1) larger share of the 9 V (1) or R 1 decreases (1) V out decreases (1)]

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Mr Powell 2012 Index Plenary Question…. In the circuit shown, the battery has negligible internal resistance. Basic Calc 3) The circuit shown in the diagram acts as a potential divider. The circuit is now modified by replacing R 1 with a temperature sensor, whose resistance decreases as the temperature increases. Explain whether the reading on the voltmeter increases or decreases as the temperature increases from a low value. (3) iSlice Explaining 1)If the emf of the battery = 9.0V, R 1 = 120 and R 2 = 60 , calculate the current I flowing in the circuit. (3) 2)Calculate the voltage reading on the voltmeter. (1)

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Mr Powell 2012 Index Summary Questions…

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Mr Powell 2012 Index

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Mr Powell 2012 Index

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Mr Powell 2012 Index Plenary Question Give an example of what you might use a potential divider for as well as a Light sensor. 2.What is the output voltage of this potential divider? 4.4V

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Mr Powell 2012 Index More Simple examples to try... 0V 12V V OUT 0V 100 0V 1.5V V OUT 0V 50 45 0V 50V V OUT 0V 10 75 0V 3V V OUT 0V 75 25

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Mr Powell 2012 Index Connection Connect your learning to the content of the lesson Share the process by which the learning will actually take place Explore the outcomes of the learning, emphasising why this will be beneficial for the learner Connection Connect your learning to the content of the lesson Share the process by which the learning will actually take place Explore the outcomes of the learning, emphasising why this will be beneficial for the learner Demonstration Use formative feedback – Assessment for Learning Vary the groupings within the classroom for the purpose of learning – individual; pair; group/team; friendship; teacher selected; single sex; mixed sex Offer different ways for the students to demonstrate their understanding Allow the students to “show off” their learning Demonstration Use formative feedback – Assessment for Learning Vary the groupings within the classroom for the purpose of learning – individual; pair; group/team; friendship; teacher selected; single sex; mixed sex Offer different ways for the students to demonstrate their understanding Allow the students to “show off” their learning Activation Construct problem-solving challenges for the students Use a multi-sensory approach – VAK Promote a language of learning to enable the students to talk about their progress or obstacles to it Learning as an active process, so the students aren’t passive receptors Activation Construct problem-solving challenges for the students Use a multi-sensory approach – VAK Promote a language of learning to enable the students to talk about their progress or obstacles to it Learning as an active process, so the students aren’t passive receptors Consolidation Structure active reflection on the lesson content and the process of learning Seek transfer between “subjects” Review the learning from this lesson and preview the learning for the next Promote ways in which the students will remember A “news broadcast” approach to learning Consolidation Structure active reflection on the lesson content and the process of learning Seek transfer between “subjects” Review the learning from this lesson and preview the learning for the next Promote ways in which the students will remember A “news broadcast” approach to learning

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