Transcript Wheatstone Bridge Circuit

Notes p.10
Wheatstone Bridge Circuits
A “Wheatstone Bridge” circuit is simply a
parallel circuit with 2 branches. Each branch
is a potential divider.
The “bridge” connects the branches as
follows…
Vs
R1
R2
V
R3
R4
A Wheatstone Bridge is “balanced” when the
voltmeter reads 0V (or ammeter reads 0A).
This happens when the resistor ratio on the top
branch = the resistor ratio in the bottom branch.
R1 = R3
R2
R4
Wheatstone Bridge Circuit Activity (Practical)
Aim: To find out the unknown value of the resistor Rx, using a Wheatstone
Bridge Circuit.
Method:
1. Circuit
V S
R2
R1
R3
A
2. Alter R3 until the ammeter reads 0 mA.
3. Note the value of R3 .
Rx
Calculation
The value of Rx can be calculated now using
Rx = R2
R3 R 1
Out of Balance Wheatstone Bridge
Starting with your bridge balanced, change the variable resistor by small
amounts and record the reading on the mA meter.
R3 (W)
DR3 (W)
-800
-600
-400
-200
0
+200
+400
+600
+800
Bridge current
(mA)
Out of Balance Wheatstone Bridge
Bridge current
(microamps)
40
20
0
-1000
-20 0
1000
-40
-60
Change in Resistance of R3
(ohms)
Series1
Poly. (Series1)
Bridge current (mA)
Out of Balance Graph
0
DR3 (W)
Out of Balance Conclusion
For small changes to one resistor, R, in a balanced
Wheatstone Bridge, the out-of-balance voltage (or current)
is directly proportional to the change in resistance.
V a DR
I a DR
The Wheatstone Bridge
Any method of measuring resistance using an
ammeter and voltmeter involves some error
unless the resistance of the meters are taken
into account.
To remove these errors the meter would have to
read zero.
A Wheatstone Bridge Circuit makes use of a
zero reading (null deflection) to find an unknown
resistor value.
Watch the demo of a metre bridge and complete
the “Metre Bridge” sheet.
Quick Problems
Calculate the value of the unknown resistor that
would balance the following Wheatstone Bridges.
a) R1 = ? , R2 = 10 W, R3 = 20 W, R4 = 80 W.
b) R1 = 50 W , R2 = ? , R3 = 50 W, R4 = 25 W.
c) R1 = 300W , R2 = ? , R3 = 200W, R4 = 170W.
d) R1 = 180W , R2 = 200W, R3 = ? , R4 = 400W.
e) R1 = 320W , R2 = 60W, R3 = 2kW, R4 = ? .
Problems: Continue with Problems up to 32.
Revision questions for Higher Physics,
page 53, Q. 1 – 7.
Quick Problems
Calculate the value of the unknown resistor that
would balance the following Wheatstone Bridges.
a) R1 = 2.5 W , R2 = 10 W, R3 = 20 W, R4 = 80 W.
b) R1 = 50 W , R2 = 25 W , R3 = 50 W, R4 = 25 W.
c) R1 = 300W , R2 = 255W , R3 = 200W, R4 = 170W.
d) R1 = 180W , R2 = 200W, R3 = 360W , R4 = 400W.
e) R1 = 320W , R2 = 60W, R3 = 2kW, R4 = 375 W .
Problems: Continue with Problems p.11, Q.21-23.