7 Potential Dividers
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Transcript of 7 Potential Dividers
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Potential Dividers
You will be familiar with the use
of a variable resistor to varycurrent.
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M
The variable resistor acts as a control overthe flow of current. It is being used as arheostat. The result is a control over the
speed of the motor.more resistance
less current
slower motor
The nextcircuit is verydifferent..
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R 1
R 2
V1
V2
V
V does not change
V = V1 + V2
The two resistors aredividing up thepotential (or voltage) -(the Potential Divider)
21
1
1
RR
RVV
21
2
2
RR
RVV
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What happens if:
R1 increases and R2 isunchanged?
R1 decreases and R2 remains the same?
The values of bothR1 and R2 aredoubled? The values of both R1
and R2 are halved?
The supply voltage V istrebled?
V1 V2 V
x 3 x 3 3V
V
V
V
V
V1
V1
V2
V2
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Uses of the potential divider
• To supply a variable voltage
• To make an input sensor from othercomponents - many detectors will switch on as avoltage goes above or below a certain threshold(such as in a thermostat).
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Consider the thermistor
-its resistance decreases as the temperature increases
R 1
R 2
V1
V2
V
R1 is a thermistor. Suppose a heater switcheson if V2 “went high”.
As it gets hot, R1 xxcreasesand so V1 xxxxs.
This means V2 mustxxxx. ie it “goes ?”
The thermostat turns on.
Falls
decreases
Rise
Could you explain what
would happen if you put thethermistor where R2 is?
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Now using theoriginal circuit as inthe diagram, whatwould be the effect ofmaking R2 a variableresistor?
R 1
R 2
V1
V2
V
It would effectively
vary the switching
temperature.
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An LDR (light dependent resistor) has a
resistance which decreases with increasedillumination. Sketch a circuit to show how you
could use a power supply, voltmeter, LDR and
a fixed resistor to measure light intensities.
What would be the point in replacing the fixed
resistor with a variable resistor?
How would you modify your circuit to measure
temperature instead of light intensity?
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R 1
R 2
V1
V2
V
Replace with
an LDR
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What would be thepoint in replacing thefixed resistor with avariable resistor?
How would youmodify your circuit to
measure temperatureinstead of lightintensity?
Change the lighting level at
which it switched
Use a thermistor
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Measuring PD with a potential divider.
V
S
A B
2 volt
Moving the slider S from Ato B will make the
voltmeter read from zeroup to the supply voltage -in this case from 0V to 2V.
The resistance AB could be
1m of “resistance wire”.
If AS was 25cm, what
would the voltmeter read?2Vx(25cm/100cm) = 0.5V
If SB was 20cm, what would
the voltmeter read? 2V((100cm-20cm)/100cm) = 1.6V
We don’t really need thevoltmeter to know what that
voltage is!
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We don’t really need the voltmeter to know what thatvoltage is!
This is only because the current through the wire AB isthe same along its length and we assume none flows
through the voltmeter.
S
A B
2 volt
Centre Zero
galvanometer - a very
sensitive ammeter.
V
For practical purposes a resistor is
frequently placed in series with the galvoto protect it from high currents.
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Suppose the voltage from A to S is the same as the emf of
the second cell.
S
A B
2 volt
Centre Zero
galvanometer - a very
sensitive ammeter.
V
The voltage drop from A to S due to the 2 V cell will matchthe emf from the other cell so no current will flow throughthe galvo.
If we move S to the left, VAS falls, so V will drive currentthrough the galvo.
If we move S to the right, VAS rises and forces currentback through the galvo and cell, the other way.
We now know that V = VAS.
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QuestionIf the wire AB is 1m long and a “balance point” is
reached when AS = 20cm, What is the value of V ?
V = 2volt x (20cm/100cm)V = 0.4volt
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Why might you do the following?
S
A B
2 volt
V
Add this
resistor
here?
S
A B
2 volt
V Add this switch and
resistor here?
This type of circuit is frequentlyused to measure very small emfssuch as those generated bythermocouples - what are they?
It is called a null method as areading is made when the galvois reading zero.
It is more accurate to look for thebalance point when making theconnection at S produces nodeflection of the galvo - why?
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The
End