126359452-stpm-physics-experiment-7-wheatstone-bridge-second-term.docx

STPM Physics Experiment 7 (Second Term) Title: Wheatstone Bridge Results: No. 1 2 3 4 5 Average a

Length of wire Rx between terminals C and D, x/cm 38.00 38.00 38.00 38.00 38.00 38.00 Distance of the balance point from A, a/cm 57.30 56.90 42.80 43.20

a1 a2 a3 a4

No. 1 2 3 4 5 Average

b b1 b2 b3 b4

Diameter of wire Rx, d/mm 0.15 0.15 0.15 0.15 0.15 0.15

Distance of the balance point from B, b/cm 42.70 43.10 57.20 56.80 Zero error = - 0.04mm Actual d = 0.15 - (- 0.04) = 0.19mm

Calculation: 𝑇ℎ𝑒 𝑚𝑒𝑎𝑛 𝑓𝑜𝑟 𝑎1 , 𝑎2 , 𝑏3 𝑎𝑛𝑑 𝑏4 , 𝑙 =

57.30 + 56.90 + 57.20 + 56.80 228.20 = 4 4

= 57.050𝑐𝑚 𝑇ℎ𝑒 𝑚𝑒𝑎𝑛 𝑓𝑜𝑟 𝑏1 , 𝑏2 , 𝑎3 𝑎𝑛𝑑 𝑎4 , 𝑙2 =

42.70 + 43.10 + 42.80 + 43.20 171.80 = 4 4

= 42.950𝑐𝑚 Let 𝑅1 = 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝑅𝑥 𝑖𝑛 Ω while R 2 = 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝑅𝑠 𝑖𝑛 Ω 𝑅

𝑙

𝑙

2

2

57.050

Since 𝑅1 = 𝑙 , ∴ 𝑅1 = 𝑅2 (𝑙 ) = 5 (42.950) = 6.6414Ω 2

Let 𝜌 = 𝑟𝑒𝑠𝑖𝑠𝑡𝑖𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑜𝑓 𝑤𝑖𝑟𝑒 𝑅𝑥 𝑖𝑛 Ω 𝑚

𝑥 = 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑤𝑖𝑟𝑒 𝑅𝑥 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑡𝑒𝑟𝑚𝑖𝑛𝑎𝑙𝑠 𝐶 𝑎𝑛𝑑 𝐷 𝑖𝑛 𝑚 𝐴 = 𝑐𝑟𝑜𝑠𝑠 − 𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑤𝑖𝑟𝑒 𝑖𝑛 𝑚2 𝜋𝑑2 𝜋(0.19x10−3 )2 𝐴= = = 2.84x10−8 𝑚2 4 4 𝜌=

𝑅1 𝐴 6.6414 x 2.84x10−8 = = 4.96x107 Ω 𝑚 𝑥 38.00x10−2

Discussions: (a) Precautions: (i) Do not drag the jockey along wire J in order to retain constant crosssectional area of the wire. (ii) Do not coil the constantan wire. (iii) All connections of the wire and other devices must be tight. (iv) The experiment is carried out without necessary delay in the process to prevent wire from heating up which would increase the resistance of the circuit, affecting the accuracy of the result. (b) Sources of error: (i) Use of nominal value of resistance of Rs instead of determining its rating. (ii) Assumption that galvanometer is ideal. (iii) Zero error of the micrometer screw gauge used. (iv) End error of metre rule due to wear and tear. (c) Steps to overcome the weakness of the experiment (i) Use a new metre rule. (ii) Eliminate zero error by calculation.

Conclusion: The resistivity of the wire Rx, ρ is 4.96 x 10-7 Ω m.