3 DC Circuits

G482 Electricity, Waves & Photons

3.3.1 Series and Parallel

Circuits – Kirchhoff’s second law

KS5 OCR PHYSICS H158/H558Mr Powell 2012

Index

3.3.2 Practical Circuits

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Practical Notes....

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3.3.2 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 equationd) 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

sVRR

RV

21

22

V

1

V

2

Vs

R1

R2

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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)

V

V

V

V

V1

V2

Vs

R1

R2

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a) What is the output voltage……

• 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

V1

V2

Vs

R1

R2 Vout

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b) Calculations

• Since the current is the same through both resistors we can define

221

2

221

2

21

21

VVRR

RV

RRR

VV

RIV

IRR

V

RRIV

IRV

sout

sout

out

s

s

Vs

R1

R2 Vout

I

V1

V2

NB: Now we can express Vout as ratio of resistance multiplied by Vs

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I

b) Alternative Maths...

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b) Quick Test...

?

?

?

?

?

?

A B

CD

?

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b & c) Temperature Sensors?

sout VRR

RV

21

2

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;

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d) ThermistorThe 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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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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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.

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Practical Idea?

V1, V2

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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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AC Audio inputVariable voltage

output to a loud speaker

A simple volume control

AC Audio input

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Theory Summary

sout VRR

RV

21

2

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.

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Summary of Uses….

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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 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)

iSlice Explaining

V

I

R

R

9 .0 V

1

2

1) If the emf of the battery = 9.0V, R1 = 120 and R2 = 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 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)]

21

2inout RR

RVV

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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 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)

iSlice Explaining

V

I

R

R

9 .0 V

1

2

1) If the emf of the battery = 9.0V, R1 = 120 and R2 = 60, calculate the current I flowing in the circuit. (3)

2) Calculate the voltage reading on the voltmeter. (1)

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Summary Questions…

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Plenary Question....

1. 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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More Simple examples to try...

0V

12V

VOUT

0V

100

100

0V

1.5V

VOUT

0V

50

45

0V

50V

VOUT

0V

10

75

0V

3V

VOUT

0V

75

25

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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

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