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Lab 5: Wheatstone Bridge Only 7 more labs to go!! The Wheatstone Bridge is a commonly used circuit for making electrical measurements. When will the current through the ammeter be equal to zero? This will happen if we choose the resistor to be just right! If we assume no current flow through the ammeter then: Vb VD I b Rb I D RD Ia Ra IC RC A Ib Rb ID RD Va VC I a Ra I C RC Since no current flows through the ammeter: I a I b and I C I D This means we can divide the above equations and get this ratio: Ra RC RD RC Ra Rb RD Rb Remember that: L R A Where is the resistivity, L is the length and A is the cross sectional area. Using this relationship we can rewrite: LD RD LD A RC Ra Ra Ra Lb Rb Lb A Metal Bar Ra RC A Resistance Wire Lb LD LD RC Ra Lb R1= 10 What is the equivalent resistance? First reduce the parallel array to one resistor. 12 V R4= 45 R3= 30 R2= 20 1 1 1 1 R234 R2 R3 R4 1 1 1 1 964 R234 20 30 45 180 R234 180 9.5 19 R1= 10 12 V 12 V R234= 9.5 R1234 = 19.5 I = 0.62 A What is the current through R1 ? V 12V I 0.62 A R 19.5 R1234 = 19.5 12 V I = 0.62 A What is the voltage dropped by R1? V1 IR 0.62 A 10 6.2 V So this means that the voltage dropped by the parallel resistors is: R1= 10 6.2 V R234= 9.5 5.8 V 12 V V234 12V 6.2V 5.8V Can you find the current through each resistor? Use Ohm’s Law V I R