Transcript Potential Dividers

Potential Dividers
You will be familiar with the use
of a variable resistor to vary
current.
M
The variable resistor acts as a control over
the flow of current. It is being used as a
rheostat. The result is a control over the
speed of the motor.
more resistance
less current
slower motor
The next
circuit is very
different..
V does not change
V = V1 + V2
R1
V1
V
R2
V2
The two resistors are
dividing up the
potential (or voltage) (the Potential Divider)
R1
V1  V
R1  R2
V2
R2
 V
R1  R2
What happens if:
V1
V
R1 increases and R2 is
unchanged?
V
R1 decreases and R2
remains the same?
V
The values of both
R1 and R2 are
doubled?
The values of both R1
and R2 are halved?
The supply voltage V is
trebled?
V2
V1
V
V2
V1
V
V2
x3
3V
x3
Uses of the potential divider
•
To supply a variable voltage
•
To make an input sensor from other
components - many detectors will switch on as a
voltage goes above or below a certain threshold
(such as in a thermostat).
Consider the thermistor
-its resistance decreases as the temperature increases
R1 is a thermistor.
Suppose a heater switches
on if V2 “went high”. decreases
R1
V
V1
Rise
R2
V2
As it gets hot, R1 xxcreases
and so V1 xxxxs.
Falls
This means V2 must
xxxx. ie it “goes ?”
The thermostat turns on.
Could you explain what
would happen if you put the
thermistor where R2 is?
R1
V
V1
R2
V2
Now using the
original circuit as in
the diagram, what
would be the effect of
making R2 a variable
resistor?
It would effectively
vary the switching
temperature.
An LDR (light dependent resistor) has a
resistance which decreases with increased
illumination. 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?
Replace with
an LDR
R1
V
V1
R2
V2
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?
Change the lighting level at
which it switched
Use a thermistor
Measuring PD with a potential divider.
2 volt
A
B
S
V
If AS was 25cm, what
would the voltmeter read?
2Vx(25cm/100cm) = 0.5V
Moving the slider S from A
If SB was 20cm, what would
to B will make the
the voltmeter read?
voltmeter read from zero
up to the supply voltage - 2V((100cm-20cm)/100cm) = 1.6V
in this case from 0V to 2V. We don’t really need the
The resistance AB could be voltmeter to know what that
1m of “resistance wire”.
voltage is!
We don’t really need the voltmeter to know what that
voltage is!
This is only because the current through the wire AB is
the same along its length and we assume none flows
through the voltmeter.
2 volt
A
For practical purposes a resistor is
frequently placed in series with the galvo
to protect it from high currents.
B
V
S
Centre Zero
galvanometer - a very
sensitive ammeter.
2 volt
A
B
V
S
Centre Zero
galvanometer - a very
sensitive ammeter.
Suppose the voltage from A to S is the same as the emf of
the second cell.
The voltage drop from A to S due to the 2 V cell will match
the emf from the other cell so no current will flow through
the galvo.
If we move S to the left, VAS falls, so V will drive current
through the galvo.
If we move S to the right, VAS rises and forces current
back through the galvo and cell, the other way.
We now know that V = VAS.
Question
If 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
Why might you do the following?
Add this
resistor
here?
2 volt
A
This type of circuit is frequently
used to measure very small emfs
such as those generated by
thermocouples - what are they?
B
S
V

It is called a null method as a
reading is made when the galvo
is reading zero.
2 volt
A
B
S
V

It is more accurate to look for the
balance point when making the
connection at S produces no
deflection of the galvo - why?
Add this switch and
resistor here?
The
End