Transcript Slide 1 - Animated Science

3 DC Circuits
G482 Electricity, Waves & Photons
3.3.1 Series
and Parallel
Circuits –
Kirchhoff’s
second law
3.3.2 Practical
Circuits
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Practical Notes....
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3.3.2 Practical Circuits
Assessable learning outcomes
draw a simple potential divider circuit;
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 lightdependent resistors in potential divider circuits;
f) describe the advantages of using dataloggers to monitor
physical changes
Vs
a)
b)
R2
V2 
Vs
R1  R2 
V
R1
1
V
R2
2
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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”
•
Vs
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
V1
R1
V2
R2
V
V
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a) What is the output voltage……
Vs
• 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
V1
R1
V2
R2
• Output voltage is the same as the
voltage across R2 i.e. V2 = Vout
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Vout
Index
b) Calculations
• Since the current is the same through
both resistors we can define
V  IR
Vs  I R1  R2 
Vs
I
R1  R2 
Vs
I
V1
V2
Vout  I  R2
R1
R2
Vout
NB: Now we can express
Vout as ratio of
resistance multiplied
by Vs
Vout
Vs

 R2
R1  R2 
Vout
R2

 Vs  V2
R1  R2 
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b) Alternative Maths...
I
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b) Quick Test...
B
A
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?
?
D
C
?
?
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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;
Vout
R2

 Vs
R1  R2 
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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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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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2
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!
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A simple
volume control
AC Audio input
Variable voltage
output to a
loud speaker
AC Audio input
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Theory Summary
A potential divider does just what is states. It
divides a potential difference
Vs
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.
V1
R1
V2
R2
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.
Vout
Vout
R2

 Vs
R1  R2 
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Summary of Uses….
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Plenary Question….
In the circuit shown, the battery has
negligible internal resistance.
I
Basic
Calc
R1
9.0 V
R2
V
1) If the emf of the battery = 9.0V, R1 =
120 and R2 = 60, calculate the
current I flowing in the circuit. (3)
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)
(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)
2) Calculate the voltage reading on the
voltmeter. (1)
or R1 decreases (1) Vout decreases (1)]
iSlice
Explaining
Vout  Vin
R2
R1  R2
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Plenary Question….
In the circuit shown, the battery has
negligible internal resistance.
I
Basic
Calc
R1
9.0 V
R2
V
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)
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)
iSlice
Explaining
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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...
12V
50V
100 
10 
VOUT
100 
75 
0V
0V
3V
0V
0V
1.5V
75 
50 
VOUT
25 
0V
VOUT
VOUT
45 
0V
0V
0V
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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
Consolidation
• 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
• 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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