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# Physics 1.2a The SI Units

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An Introduction to the units used in Physical Measurement

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### Physics 1.2a The SI Units

1. 1. Topic 1 – Physical Measurements 1.2a – The SI Units
2. 2. The Fundamental Units <ul><li>The SI system uses just 7 fundamental units.
3. 3. All other units can be derived from these 7.
4. 4. These fundamental units were originally based on standard definitions. </li><ul><li>The metre (quantity length [L] symbol m) – originally defined by two marks scored on a platinum-iridium bar stored in a vault in Paris. Now defined in terms of the distance travelled by light in a vacuum in a known time. </li></ul></ul>
5. 5. The Fundamental Units <ul><ul><li>The kilogram (quantity mass [M], symbol kg) – currently defined by reference to a platinum-iridium standard kilogram held in a vault in Paris.
6. 6. The second (quantity time [T], symbol s) – originally defined as 1/86400 of a mean solar day it is now defined as the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. </li></ul></ul>
7. 7. The Fundamental Units <ul><ul><li>The Ampere (quantity current [C], symbol A) – defined as that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in a vacuum, would produce between these conductors a force equal to 2 x 10 -7 Nm -1 of length.
8. 8. The kelvin (quantity Temperature [t], symbol K) – defined as being 1/273.16 of the triple point of pure water. </li><ul><li>A Celcius temperature is calculated using </li></ul></ul></ul>
9. 9. The Fundamental Units <ul><ul><li>The mole (quantity amount of substance [N], symbol mol) – defined as being the amount of substance which contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. </li><ul><li>This number is known as Avagadro's number (N A ) and is 6.022x10 23 mol -1 </li></ul><li>The candela (quantity luminous intensity [I], symbol cd) – defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10 12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. </li><ul><li>The candela is not included in the IB course!! </li></ul></ul></ul>
10. 10. <ul><li>All other units are derived from these fundamental units. </li><ul><li>Example: express the Newton in terms of fundamental units.
11. 11. Newton is the unit of force
12. 12. Force = mass x acceleration (amongst others)
13. 13. [N] = [kg] x [ms -2 ]
14. 14. N = kgms -2 </li></ul></ul>Derived units
15. 15. Derived Units <ul><li>State the following quantities in terms of fundamental units. </li><ul><li>Energy
16. 16. Concentration
17. 17. Frequency
18. 18. Power
19. 19. Pressure
20. 20. Electric Charge
21. 21. Voltage </li></ul></ul>
22. 22. SI Weirdness <ul><li>When writing SI units: </li><ul><li>Always use negative indices and not “/”
23. 23. When writing units in table headings and graph axes use /kg and not (kg) </li><ul><li>/ means has been divided by the unit which is technically correct. </li></ul><li>In general write the most positive index first. </li><ul><li>m 2 kgA -1 s -3 for the Volt </li></ul></ul></ul>
24. 24. SI Prefixes <ul><li>Instead of writing units in scientific notation physicists often use prefixes in front of the units instead.
25. 25. This makes it easier to read values in reports etc.
26. 26. In general use scientific notation in results tables and intermediate work (as this makes working with large numbers much easier) and use prefixes for the final answer. </li></ul>
27. 27. SI Prefixes Name Symbol Exponent Multiplier Peta P 10 15 1 000 000 000 000 000 Tera T 10 12 1 000 000 000 000 Giga G 10 9 1 000 000 000 Mega M 10 6 1 000 000 Kilo k 10 3 1 000 10 0 1 milli m 10 -3 0.001 micro μ 10 -6 0.000 001 nano n 10 -9 0.000 000 001 pico p 10 -12 0.000 000 000 001 femto f 10 -15 0.000 000 000 000 001
28. 28. SI Prefixes <ul><li>Express each of the following quantities in both Scientific notation and using prefixes </li><ul><li>27.647J
29. 29. 150 564 kL
30. 30. 0.003 672 nF
31. 31. 178.3671 μ C
32. 32. 258 245 mK </li></ul></ul>
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