# 3.3.1 Series and Parallel Circuits – Kirchhoff’s second law

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3.3.1 Series and Parallel Circuits – Kirchhoff’s second law
3 DC Circuits G482 Electricity, Waves & Photons 3.3.1 Series and Parallel Circuits – Kirchhoff’s second law 3.3.2 Practical Circuits Ks5 OCR Physics H158/H558 Index Mr Powell 2012

Practical Notes.... Practical Skills are assessed using OCR set tasks. The practical work suggested below may be carried out as part of skill development. Centres are not required to carry out all of these experiments. In this module, there is much potential for experimental work and improving instrumentation skills. Use a multimeter as an ohmmeter to ‘verify’ the rules for total resistance for series and parallel circuits. Use a calibrated light-meter to plot the variation of resistance of an LDR against intensity. Determine the internal resistance of a chemical cell. Use a potential divider circuit to show the validity of the potential divider equation in Design a light-sensing circuit based on a potential divider with a light-dependent resistor. Design a temperature-sensor based on a potential divider with a thermistor. Monitor the output potential difference from either light-sensors or temperature-sensors using a data-logger.

3.3.2 Practical Circuits Vs V1 R1 V2 R2 Assessable learning outcomes
draw a simple potential divider circuit; explain how a potential divider circuit can be used to produce a variable p.d.; select and use the potential divider equation describe how the resistance of a light dependent resistor (LDR) depends on the intensity of light; describe and explain the use of thermistors and light-dependent resistors in potential divider circuits; describe the advantages of using dataloggers to monitor physical changes V1 V2 Vs R1 R2 Students can set up and investigate a potential divider using two fixed resistors or a variable resistor or a rotary potentiometer. Students can investigate how to use a potential divider as a source of variable p.d. Students can construct potential divider circuits for a simple light-sensor and a temperature sensor.

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 Vs and the 0V as ground rail, the resistors and voltmeters as 1 & 2 (you could use a & b) V1 V2 Vs R1 R2 V V Orientation to ptl circuit

a) What is the output voltage……
Vs R1 R2 Vout We can use this circuit to be able to find the output voltage across R2 so we can see a change in a component such as a thermistor. So add the output voltage Vout Output voltage is the same as the voltage across R2 i.e. V2 = Vout Vout point across the lower resistor

b) Calculations Since the current is the same through both resistors we can define Vs R1 R2 Vout I V1 V2 Calculation of the output voltage NB: Now we can express Vout as ratio of resistance multiplied by Vs

b) Alternative Maths... I

b) Quick Test... B A ? ? ? D C ? ? ? ?

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

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!

Only need the outcome in red for AS Physics
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

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 Fe3O4 + MgCr2O4 (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.

Practical Idea? V1, V2

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

Variable voltage output to a loud speaker A simple volume control AC Audio input AC Audio input

Theory Summary V1 V2 Vs R1 R2 Vout
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 Vout 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. V1 V2 Vs R1 R2 Vout

Summary of Uses….

Plenary Question…. 3) The circuit shown in the diagram acts as a potential divider. The circuit is now modified by replacing R1 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) In the circuit shown, the battery has negligible internal resistance. Basic Calc (temperature increases, resistance decreases), total resistance decreases (1) current increases (1) voltage across R2 increases (1) or R2 has increased share of (total) resistance (1) new current is same in both resistors (1) larger share of the 9 V (1) or R1 decreases (1) Vout decreases (1)] If the emf of the battery = 9.0V, R1 = 120  and R2 = 60, calculate the current I flowing in the circuit. (3) Calculate the voltage reading on the voltmeter. (1) iSlice Explaining

Plenary Question…. 3) The circuit shown in the diagram acts as a potential divider. The circuit is now modified by replacing R1 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) In the circuit shown, the battery has negligible internal resistance. Basic Calc If the emf of the battery = 9.0V, R1 = 120  and R2 = 60, calculate the current I flowing in the circuit. (3) Calculate the voltage reading on the voltmeter. (1) iSlice Explaining

Summary Questions…

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

More Simple examples to try...
0V 12V VOUT 100  0V 50V VOUT 10  75  0V 3V VOUT 75  25  0V 1.5V VOUT 50  45 

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 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 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

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