Wheatstone Bridge Circuits

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Presentation transcript:

Wheatstone Bridge Circuits 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… R1 R2 R3 R4 Vs V

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

A Wheatstone Bridge Circuit Activity (Practical) Aim: To find out the unknown value of the resistor Rx, using a Wheatstone Bridge Circuit. Method: Circuit V S R1 R2 A R3 Rx 2. Alter R3 until the ammeter reads 0 mA. 3. Note the value of R3 .

Calculation The value of Rx can be calculated now using Rx = R2 R3 R1

R3 (W) DR3 (W) Bridge current (mA) -800 -600 -400 -200 +200 +400 +600 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) Bridge current (mA) -800 -600 -400 -200 +200 +400 +600 +800

Out of Balance Graph Bridge current (mA) DR3 (W)

V a DR I a DR 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.