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Nội dung text RVHS 2023 CT P4 Planning.pdf

RVHS 2023 CT P4 3 Creep is the name given to the slow deformation of solid materials over an extended period of time when the material experiences stresses, which are below that required to reach the elastic limit. An example of a situation where creep occurs is in the blades of a high temperature gas turbine. The operating temperature of the turbine is fairly close to the melting point of the material from which the blades are made. Therefore the blades are subject to creep and gradually become elongated as the turbine is used. This is illustrated in Fig. 3.1. Fig. 3.1 The clearance between the blades and the casing is very small. This clearance decreases during the life of the turbine due to the creep in the blades. Therefore, it is important to engineers to have information about the creep process so that the life expectancy of the blades can be determined and damage to the engine can be prevented. The length of a wire made of lead changes with time (i.e. creeps) as the temperature T of the wire and the load m which it supports are changed. The change in length Δd is related to the temperature T of the wire and the load m by the relationship Δd = k Tp mq where k, p and q are constants. Design an experiment to investigate how the change in the length of lead wires is affected by the temperature of the surrounding air and the mass the wire supports. In doing so, determine the values of p and q. You are provided with lead wires, a long box with toughened glass sides, some unknown masses, an electrical heater, and access to common laboratory apparatus.
Draw a diagram to show the arrangement of your apparatus. Pay particular attention to: • the equipment you would use • the procedure to be followed • the control of variables • the details on the measurements to be taken for the change in length of wire • any precautions that should be taken to improve the safety and accuracy of the experiment. ....................................................................................................................... ....................................................................................................................... ....................................................................................................................... ....................................................................................................................... ....................................................................................................................... ....................................................................................................................... ....................................................................................................................... ....................................................................................................................... ....................................................................................................................... .......................................................................................................................
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Answer Marks Diagram labelled diagram • Retort stand to hold one end of lead wire hanging vertically. The other end of lead wire should be attached to slotted masses. • The setup is in enclosed box with an electrical heater to change the temperature of the air in the box which leads to a change in temperature of wire. There should be a thermometer to measure the temperature of air in the diagram. • Metre rule (or other measuring instruments) should be shown in diagram on how Δd can be measured. Sample solution from student. Marker’s comments: Many candidates incorrectly position the lead wire horizontally and hang masses at the centre of wire. The measurement of the length of wire should done when the wire is heated and remains at that temperature. The experiment should be done with the wire in an enclosed box so that there is little temperature variation from the surrounding. 1 Defining the problem Part 1 T is the independent variable and Δd is the dependent variable or vary T measure Δd. and m to be kept constant. 1

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