BTEC Level 3 Nationals in Engineering: Unit 1, Study notes of Engineering

Download BTEC Level 3 Nationals in Engineering: Unit 1 and more Study notes Engineering in PDF only on Docsity! BTEC Level 3 Nationals in Engineering: Unit 1 Your free sample of the student book: preparation for assessment (BTEC National Engineering: Student Book 1 (with ActiveBook), ISBN: 9781292141008) UNIT 1 Engineering Principles68 Getting ready for assessment This section has been written to help you to do your best when you take the external examination. Read through it carefully and ask your tutor if there is anything you are not sure about. About the test This unit is externally assessed using an unseen paper-based examination. Pearson sets and marks the examination. The assessment must be taken under examination conditions. As the guidelines for assessment can change, you should refer to the o cial assessment guidance on the Pearson Qualifi cations website for the latest defi nitive guidance. Make sure you are familiar with the command words and know how to apply them within the context of the examination. The table below shows the command words for each Assessment outcome. Assessment outcome Command words AO1 calculate, describe, explain AO2 calculate, fi nd, solve AO3 fi nd, calculate, describe, draw, explain AO4 calculate, draw AO5 calculate, draw, explain Remember that all the questions are compulsory and you should attempt to answer each one. Organise your time based on the marks available for each question. Set yourself a timetable for working through the test and then stick to it – do not spend ages on a short 1–2 mark question and then  nd you only have a few minutes for a longer 6–7 mark question. Try answering all the simpler questions  rst, then come back to the harder questions. This should give you more time for the harder questions. M01_ENG_SB_BTEC_1008_U01.indd 68 4/12/17 5:00 PM 71 Answering engineering principles calculation questions • Identify the variables from the information given in the question. • Write down the formula you need. • Substitute numbers into the formula. • Explain each step and show all your workings. • Express your answer in appropriate units. • Check your answer. Worked example An engineer is testing an inductor with an inductance of 0.5 H and a resistance of 8 Ω. It is connected to a 120 V 50 Hz a.c. supply. Calculate the current drawn from the power supply. (3 marks) Answer The solution will require the calculation of I rms . The values given in the question are: L  = 0.5 H, R  = 8 Ω, V rms  = 120 V, f  = 50 Hz Th e inductive reactance is X L  = 2π fL  = 2π × 50 × 0.5 = 157.08 Ω. The total impedance is given by Z  =  √ _______ X L 2 + R 2 =  √ ___________ 157. 08 2 + 8 2  = 157.28 Ω. So the a.c. current is I rms  = Vrms ____ Z = 120 ______ 157.28 = 0.763 A or 763 mA. Answering engineering principles short-answer questions – state • Read the question carefully. • Make sure that you make the same number of points as there are marks available in the question. Worked example State two sources of energy loss that aff ect the effi ciency of electrical transformers. (2 marks) Answer Eddy currents. Hysteresis losses. This answer provides two accurate points and full marks would be awarded. Command words such as ‘give’, ‘state’ or ‘identify’ can be answered in single words or brief statements. There is no need to write in full sentences because the examiner is only testing your ability to recall information. M01_ENG_SB_BTEC_1008_U01.indd 71 19/08/2016 09:02 UNIT 1 Engineering Principles72 Answering engineering principles short-answer questions – describe, explain Worked example Describe the process of heat transfer through conduction. (4 marks) Answer Heat transfer by direct contact between adjacent atoms: Heat energy is a measure of the vibrational kinetic energy possessed by atoms. These vibrations are passed between atoms that are in direct contact with each other. Heat transfer by the movement of electrons: Heat energy can also be distributed through a material by the transfer of vibrational kinetic energy possessed by free-moving electrons as they move around within a material. This helps to explain why metals are in general good thermal (as well as electrical) conductors. The mechanism of energy transfer through direct contact between atoms and the secondary mechanism involving energy transfer by free-moving electrons have been correctly identi ed. Each mechanism is then explained and expanded upon suf ciently for the award of the additional marks available. Answering synoptic questions Worked example A heat engine driving a combined heat and power system outputs mechanical energy to drive a generator. The energy is provided by a combustion process that uses air and a fuel with an energy content of 40 MJ kg −1 , which is supplied at a rate of 0.003 kg s −1 . The generator has a rotor that turns at 1500 rpm and has a torque of 255 N m. The output of the generator is 6.5 A at 400 V. a) Explain how energy loss processes in both mechanical and electrical equipment aff ect the effi ciency of the system. (4 marks) b) Calculate the effi ciency with which the heat engine provides mechanical work to the generator. (7 marks) c) Calculate the overall system effi ciency. (3 marks) Synoptic questions, such as the example given above, are basically a combination of interrelated short-answer and calculation questions based around a common complex system or theme. As such they can be broken down into individual elements that should be approached in a similar way to the worked examples already discussed. Synoptic questions will require you to make links between engineering principles from the full range of disciplines covered in the unit. M01_ENG_SB_BTEC_1008_U01.indd 72 19/08/2016 09:02