Voltage (Potential) Dividers
A voltage (potential) divider is simply 2 resistors connected across a power supply, as shown:
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4Ω
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6V
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8Ω
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800 Ω
R1
10 kΩ
Ω
R1
5 MΩ
Ω
R2
400 Ω
R2
110 kΩ
Ω
R2
10 MΩ
Ω
0V
0V
0V
Wheatstone Bridge Circuits
A Wheatstone Bridge circuit consists of 2 voltage (potential) dividers connected to a power supply
with a voltmeter or sensitive ammeter connected between them, as shown:
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-
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R4
R2
R3
R1
R3
R2
R4
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If the voltmeter reads 0 V, we say the Wheatstone bridge is balanced.
This only happens when:
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The voltage of the battery voltage has no effect on this ratio - If you change the battery voltage,
the reading on the voltmeter will still be 0 V.
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40 Ω
R3
?Ω
R4
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6V
R2
600 Ω
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3Ω
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3Ω
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120 Ω
0V
2V
120 Ω
4 kΩ
Ω
0V
2V
9Ω
12 kΩ
Ω
10 kΩ
Ω
15 kΩ
Ω
0V
2V
RX
25 kΩ
Ω
3.6 kΩ
Ω
RX
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3 kΩ
Ω
9 kΩ
Ω
A
V
B
3 kΩ
Ω
3 kΩ
Ω
5 kΩ
Ω
10 kΩ
Ω
0V
+5V
A
V
2 kΩ
Ω
B
8 kΩ
Ω
0V
+ 10 V
3 kΩ
Ω
6 kΩ
Ω
A
2 kΩ
Ω
0V
V
B
4 kΩ
Ω
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10 kΩ
Ω
5 kΩ
Ω
V
2V
5 kΩ
Ω
12 kΩ
Ω
V
2V
10 kΩ
Ω
9 kΩ
Ω
16 kΩ
Ω
10 kΩ
Ω
V
2V
12 kΩ
Ω
7 kΩ
Ω
20 kΩ
Ω
14 kΩ
Ω
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Practical Application for a Wheatstone Bridge Circuit
When a Wheatstone bridge circuit is balanced, the potential difference (p.d.) across the
bridge is always __ V.
The formula connecting the resistors in the balanced Wheatstone bridge is:
=
However, when the resistance of any one resistor in the Wheatstone bridge circuit is
changed by a small amount, the bridge becomes ______________ and the above
relationship no longer applies - An "out of balance" potential difference (p.d.) is created
across the bridge.
The "out of balance" potential difference is directly proportional to the change in
resistance for the one resistor, as shown on the graph below:
out of balance p.d./ V
0
change in resistance/ Ω
This forms the basis for many sensor devices - For example, a light meter.
+ 12 V
variable
resistor
20 kΩ
Ω
0V
10 kΩ
Ω
V
LDR
In this Wheatstone bridge circuit, the variable resistor is
adjusted until the bridge is balanced
- The voltmeter will read ___ V.
As the light level changes, the resistance of the LDR
will change, so the Wheatstone bridge will now be
_________________.
An "out of balance" potential difference (p.d.) is created
across the bridge, i.e., the voltmeter will show either a
positive or negative voltage, depending on whether the
light level has increased or decreased.
This "out of balance" potential difference is directly proportional to the change in light level.
This allows us to calibrate the voltmeter (put a scale on it), so the voltmeter reading will give the
actual value for the light level.
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Power
The power of a circuit component (such as a resistor) tells us how much electrical potential energy the
component transforms (changes into other forms of energy) every second:
P=
E
t
The following formulae are used to calculate power (P):
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In each case, calculate the power of the resistor:
6V
2A
4Ω
5V
2.5 Ω
1.5 A