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Key areas

Key areas. A.C as a current which changes direction and instantaneous value with time Calculations involving peak and r.m.s. values Determination of frequency from graphical data. What we will do today:. Revise the two types of electrical current, a.c. and d.c.

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Key areas

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  1. Key areas • A.C as a current which changes direction and instantaneous value with time • Calculations involving peak and r.m.s. values • Determination of frequency from graphical data

  2. What we will do today: • Revise the two types of electrical current, a.c. and d.c. • Give examples on peak voltage, Vpeak, and peak current, Ipeak.

  3. Background revision (a.c. and d.c.) • There are two types of electrical current: • Direct Current (d.c.) where the current only travels in one direction eg a battery. • Alternating Current (a.c.) where the current is constantly changing direction eg the mains socket.

  4. Background revision – oscilloscope • An electronic device called an oscilloscope can be used to draw a graph of the electrical signal of both a.c. and d.c. a.c d.c (above AND below) (above OR below)

  5. Using an oscilloscope • Oscilloscopes have dials on them to allow you to calibrate two things about the signal produced: • On the x-axis – time per cm (time base often given in ms) • On the y-axis – volts per cm (voltage gain often given in mV)

  6. Period of a wave • The period of a wave is the time taken for one complete wave to pass a point. • It has the symbol T and is measured in s. • For example, if we have an oscilloscope set to 1ms cm-1 (a time of 1ms for each gridline 1 cm apart) as shown:

  7. Then we can see that one complete wave is made after 4 boxes. • Therefore the period of 1 wave, T = 4 x 1ms = 4ms: • T = 4 x 10-3 s

  8. Measuring the Frequency of a wave • Once we have established the period of a wave, T, we can find the frequency of a wave, f, by the equation: • f = 1 T

  9. Measuring the Frequency of a wave

  10. Definition of Peak (read) • When an athlete is at their “peak”, we say they are at their absolute best. • Will an athlete always be at their peak in every performance? No! • Therefore, peak is the absolute maximum!

  11. Peak Voltage • When we consider voltage there are two values that we must consider: • Peak voltage • R.m.s. voltage (root mean square voltage) • Peak voltage is “always bigger” than r.m.s. voltage • The value quoted for the mains (230V) is the r.m.s. value. • The r.m.s. voltage of an a.c. value is equivalent to the d.c. value e.g. an r.m.s of 230V a.c = 230V d.c.

  12. Peak voltage • Oscilloscope traces show the peak voltage of a wave, consider the following oscilloscope trace: • (the y-gain setting is set to 0.1 V cm-1) • The amplitude shown is 2 cm. Therefore the peak voltage is 2 x ‘volts per cm’ setting on the control. • 2 x 0.1v cm-1 = 0.2 V

  13. Peak and r.m.s.formulae • The peak voltage and r.m.s. voltage are related by: • Vpeak = √2 Vr.m.s • Peak current and r.m.s current have a similar relationship: • Ipeak = √2 Ir.m.s

  14. 2004 Qu: 12

  15. 2005 Qu: 9

  16. 2007

  17. 2003 Qu: 25

  18. Past Paper • 2000 Qu: 26(a) • 2006 Qu: 26(a)

  19. What we will do today: • State what is meant by Total Internal Reflection and the Critical Angle. • State the relationship between the refractive index and the critical angle. • Carry out calculations on the above.

  20. Total Internal Reflection and Critical Angle

  21. Revision from last day • sin θ1 / sin θ2 = λ1 / λ2 = v1 / v2 • n = sin θ1 / sin θ2 • We can express this as: n2 / n1 = sin θ1 / sin θ2 NB The refractive index of air is 1.

  22. Experiment • Draw round a semi-circular block. • Draw in the normal line through the centre. • Shine a light ray along the normal to the centre of the block. • Continue to shine light ray at the centre and slowly move the light ray outwards (diagram 1). • Keep going until the angle of refraction is 900 (diagram 2). • Measure the angle of incidence at this point.

  23. Total Internal Reflection • Diagram 1 – light is refracted. • Diagram 2 – Light is refracted at 900, the angle of incidence in this case is called the critical angle, Θc • Diagram 3 – Any angle bigger than the critical angle will show Total Internal Reflection

  24. The Critical Angle • The critical angle,Θc is the angle of incidence when the angle of refraction is 900. • It is the smallest angle of incidence above which Total Internal Reflection occurs. It is often given the symbol,Θc

  25. Total Internal Reflection • Takes place when all of a light ray is completely reflected and none of it is refracted. • This takes place at angles above the critical angle, Θc

  26. Curved Surface • Note that there is no refraction at the curved surface because a radial ray strikes the surface at normal incidence (i.e. perpendicular - 90º). • This is why a semi-circular block is used to find the critical angle.

  27. Uses of Total Internal Reflection • Total internal reflection is used to send light signals along optical fibres. • This can be used to send telecommunications such as internet, TV and phone. • This is how Virgin Media provide their services.

  28. Critical Angle and Refractive Index n = 1__ sin θc

  29. n = 1.4 Θc = ? n = 1 sin Θc 1.4 = 1 sin Θc sin Θc = 1 1.4 Θc = (sin-1) 1 1.4 Θc = 45.6o Example

  30. Complete questions 6 – 11 from section 6: Refraction of Light in class jotter Questions

  31. 2008 Qu: 16

  32. 2009 Qu: 15

  33. 2001 Qu: 27(b) 2002 Qu: 27 2010 Qu: 28(b) Old Higher Past Paper Questions

  34. 2001 Qu: 27(b)

  35. 2002 Qu: 27

  36. 2002 Qu: 27

  37. Revised Higher Past Paper Questions

  38. 2014 Qu: 15

  39. Revised Higher Past Paper Questions • 2012 Qu: 29 • 2013 Qu: 29

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