MEC Physics review Electronics & photonics

Themes Interpreting & decoding questions: –Marks per question. –Types of questions. –Patterns in questions. Short answer questions: –Using dot points. –Idea per mark.

1 mark questions For 1 mark there are no calculations or interpretation to make. Simply find the x axis value and read off the corresponding y axis value. 100 ‘06 Q6.

Q9, ‘09

2 mark questions - Numeric 1 mark for voltage divider equation and correct substitution and 1 mark for final solution of 750Ω. This question is a bit mean since the finding of 1500Ω from 20 lux should be worth 1 mark as well. 750 ‘07 Q8.

Q1 ‘09

2 mark questions - Graphical (draw) Voltage gain given as V -1.6V 1 mark for inverting the waveform and 1 mark for multiplying 20 (and -20) by 80. Only 1 mark for the calculation as no formula is involved. ‘07 Q1.

2 mark questions - Short answer Decrease- 1 mark At increased lux R LDR will be lower. R must be lowered to increase voltage drop across LDR to 4.0V.- 1 mark Make sure you answer the question explicitly. Make sure you have 1 clear idea per mark after that. ‘07 Q9.

3 mark questions - Graphical (read) + numeric 1 mark for finding 400Ω from graph, 1 mark for voltage divider equation and substitution, 1 mark for 3kΩ. This is a fair allocation of marks. ‘06 Q7.

3 mark questions - Numeric 20V RMS 50Ω 150Ω 1 mark for finding the equivalent voltage divider circuit, 1 mark for voltage divider equation and substitution, 1 mark for 15V. Finding the equivalent circuit appears to be a lot of work for 1 mark but, really, it’s maths you should be able to do in your head. ‘05 Q4.

Q2 ‘09

Q3 ‘09

3 mark questions - Short answer ‘10 Q7. Make sure you have one clear idea per mark. As light gets less bright resistance of LDR goes UP. As resistance of LDR goes up, ratio R LDR :R goes UP. As ratio goes up V OUT goes UP.

Increased As T , R  Maintain Ratio ‘09 Q11.

4 mark questions - Graphical (draw) ‘05 Q10. VV II 1 mark for each graph and 1 mark for each set of correctly labeled axes. This is a reasonably simple question for 4 marks.

‘06 Q8. 4 mark questions - Short answer Modulation occurs at P where the electrical signal from the microphone is converted into a light signal of varying intensity before being transmitted along the fibre. Demodulation occurs at Q where the varying intensity light signal is converted back into an electrical signal before being passed to the speaker. Modulation and demodulation involve the conversion of one type of signal to another. Make sure you describe what form the signal is being converted from and to.

Example 1. ‘05 Q3: 2 marks

Example 2. ‘06 Q5: 2 marks

Example 3. ‘07 Q5: 3 marks

Example 3. ‘05 Q6: 3 marks 10V dropped across RC leaves 20V.

Example 4. ‘07 Q3: 3 marks Saturation is when the maximum collector current for the circuit is flowing. The amplifier is no longer in its linear region and a further increase in base current will not result in a further increase in collector current. When collector current is at max, Vout is at minimum (~0V). As Vin increases Vout will stay at zero and the output will be clipped with the bottom of the waveform cut off.

Example 4. ‘07 Q6: 3 marks 6.0 Diode does not conduct when reverse biased so no current flows. With zero current the voltage drop across resistor is zero so voltage across diode is 6.0V

Short answer Where most marks are lost (not gained). Often the difference between getting an A and getting a B. Very easy to “muck up”.

Common errors Verbal diarrhea. –Trying to say too much. You repeat yourself (waste of time and space) You contradict yourself (can lose marks) You write about irrelevant things

Common errors Copy from reference sheet. –Relying on pre-prepared responses. Usually does not address the question Usually reads like a dictionary description Examiners know they are not your words.

Common errors Lack of detail in your response –Too short Does not address criteria Does not show required depth of understanding. Leaves Examiners asking “but why”.

Examiners reports

Numerical questions These have one correct answer. Usually have one best solution. Common errors: –Not showing enough working. –Consequential errors.

Strategy Find the appropriate formula and write it out. –This shows the examiner you know which tool to use. Substitute immediately. Do not do any algebra. –This shows the examiner you know how to use the tool –Eliminates any chance you might stuff up the algebra.

Final strategies 1. Pace yourself. –How many marks in how many minutes? –Plan for checking time at the end. –Work on strategies to give you some “Oh bugger!” time. 2. Easy marks first. –Build confidence. –Buy time.

Final strategies 3. Use data sheets wisely. –Worked examples of hard questions. –Reminders of your common mistakes. –Not formulas – these are given to you. The best preparation is practice papers.

MEC Physics review Electronics & photonics