Transcript Potential divider mod

POTENTIAL
DIVIDER
Outcomes:
• Recap V= IR
• Know how a potential divider works.
• Confidence with exam questions.
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V–I graphs for diodes
A diode is a component that stops current flowing in one
direction, but allows it to flow readily in the other, providing it
is over a certain voltage.
current (A)
What would a V–I graph for a diode look like?
voltage (V)
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Ohm’s Law summary
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Understanding voltage
Voltage is an electrical pushing force.
The voltage of a cell describes how much electrical potential
energy it gives the electrons: this pushes them around a circuit.
When voltage is measured across a component it records
the difference in electrical potential energy between the two
sides of the component. This is also known as the
potential difference.
Thus the voltmeter reading
of 4 V tells us that there is
4 V more electrical potential
energy on one side of the
resistor than the other.
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4.0
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Controlling voltage
Imagine your alarm clock’s battery is flat.
It requires 4 V to run successfully, but
you only have a 6 V battery. This will
overload its circuitry.
However, you do have a selection of
fixed resistors.
How can these resistors help you to
run the alarm clock from the battery
without damaging it?
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Series resistors and potential difference
If two resistors are connected in series with a power supply,
then the voltage is shared out between them.
6V
2.0
4.0
10Ω
20Ω
The voltage is divided between components in proportion to
their resistance. Thus the larger resistor has a larger share of
the power supply voltage.
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What is a potential divider?
VIN
This principal can be used to
power the alarm clock.
6V
The clock itself has a
resistance of 20 Ω.
When placed in series with a
10 Ω resistor, the battery’s
voltage is split between the
resistor and clock in a 2:1 ratio.
2V
4V
10 Ω
20 Ω
The voltage across the resistor is 2 V, while the voltage across
the clock is 4 V. The clock can now run safely.
A circuit that splits the voltage between components, to
produce a specific output voltage, is a potential divider.
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Drawing potential dividers
A potential divider uses series resistance to produce an output
voltage (VOUT) that differs to the input voltage (VIN).
Potential dividers are drawn in a slightly different way
to other circuits.
V
IN
The distance between the
horizontal lines represents the
potential difference between
different parts of the circuit.
This arrangement is designed
to visually demonstrate the
change in potential difference
across the resistors.
6V
R1
10 Ω
VOUT
4V
R2
0V
20 Ω
0V
potential divider diagram
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Fixed output potential dividers
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The potential divider equation
The output voltage (VOUT) of a potential divider depends on
the size of the resistors, and also the input voltage (VIN).
VOUT can be calculated using the potential divider equation:
VIN
R2
VOUT = VIN × (R + R )
1
2
R1
VOUT
VOUT and VIN are measured in volts (V).
R2
R1 and R2 are measured in ohms (Ω).
0V
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0V
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Potential divider questions
Calculate the output voltage, VOUT, for this potential divider.
R2
VOUT = VIN ×
(R1 + R2)
10 V
R1
60
= 10 ×
15 + 60
VOUT
60
= 10 ×
75
= 10 × 0.8
15Ω
R2
0V
60 Ω
0V
= 8V
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Potential divider questions
Calculate the output voltage, VOUT, for this potential divider.
VOUT = VIN ×
R2
(R1 + R2)
10V
R1
300
= 10 ×
75 + 300
VOUT
300
= 10 ×
375
= 10 × 0.8
75 Ω
R2
0V
300 Ω
0V
= 8V
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Potential dividers with a variable output
If a variable resistor is used in a potential divider,
VOUT becomes variable.
If R1 is a variable resistor…
VIN
R1
VOUT is low when the
resistance of R1 is high.
VOUT
R2
0V
0V
R1 has a high proportion of the
resistance, and thus a high
proportion of the voltage.
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Potential dividers with a variable output
What happens when R2
is a variable resistor?
VIN
R1
In this arrangement, the
relationship between the
resistance of the variable
resistor and VOUT inverts.
VOUT
R2
0V
0V
VOUT is high when resistance of
R2 is high.
R2 has a high proportion of the
resistance and thus a high
proportion of the voltage is at VOUT.
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Answers:
Complete:
V0 / V
R1 / Ω
R2 / Ω
V1 / V
V2 / V
12
5000
5000
6
6
12
9000
1000
10.8
1.2
12
1000
9000
1.2
10.8
230
2000
500
184
46
9
400
800
3
6
Exam questions
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Potential divider summary
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Glossary
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Anagrams
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Controlling current and voltage quiz
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