(b) In a strain gauge, the increase in resistance ΔR depends on the increase in length ΔL. The variation of ΔR with ΔL is shown in Fig. 7.1. 8 ΔR/Ω 6+ 4 2- 0 0 2 4 6 8 10 ΔL/10⁻⁵ m Fig. 7.1 The strain gauge is connected into a circuit incorporating an ideal operational amplifier (op-amp), as shown in Fig. 7.2. +6.00 VO strain gauge +5 V +2.00 V OVO 153.0 Ω A + -5 V X Y Fig. 7.2

1. (a)Describe, with a labelled diagram, the structure of a metal-wire strain gauge.[3]
2. (b)(i)The strain gauge is initially unstrained with resistance 300.0 Ω. Use data from Fig. 7.1 to calculate the increase in length ΔL of the strain gauge that gives rise to a potential of +2.00 V at point A in Fig. 7.2.[3]
3. (b)(ii)The strain gauge undergoes a further increase in length beyond the value in (b)(i). State and explain which one of the light-emitting diodes, X or Y, will be emitting light.[4]

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