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OLevel

Physics (5054)

Physical Quantities, Units and Measurement

Paper 1 & 2

Physical Quantities, Units and Measurement

General physics

Physical quantities are measured in units. There are seven SI base quantities in Physics, as shown below table; 


Physical Quantities

Base Unit

Symbol

Length meter m
Mass kilogram Kg
Time seconds
Current ampere A
TemperaturekelvinK
Amount of Substancemolemol
Light Intensitycandela cd

Sometimes we need a prefix to clearly define a very large and a very small quantity. Prefixes are multiple and sub-multiple units of Physical quantities. Some commonly used SI prefixes are given below; 


Value 

Prefix 

Symbol 

10-1deci d
10-2centi c
10-3milim
10-4micro µ
10-5nenon
103kiloK
106megaM
109giga G


Measurements

Distance and volume can be measured by various means.

Distance 

Distance is one dimensional thing. There are several units to measure it, like centi-meters, meters, feet, etc. 

Rulers can be used to measure small distances of a few cm. They are able to measure to the nearest mm 


A ruler can measure small distances


When measuring larger distances (of a few meters) a tape measure is more appropriate or, when measuring even larger distances, a trundle wheel. Users walk to the distance to be measured with trundle wheel and at the end it gives us readings. 


Trundle wheels can be used to measure large distances

Volume 

Measuring cylinders can be used to measure the volume of liquids or, they can measure the volume of irregular shape solid object by measuring the change in volume,



Measuring cylinders can be used to determine the volume of a liquid or an irregular shaped solid


Time

Hour-glass and Stopwatches can be used to measure time intervals

An important factor when measuring time intervals is human reaction time. This can have a significant impact upon measurements when the measurements involved are very short (less than a second)


Multiple Readings

  • Suppose you have to measure the thickness of a sheet of paper. The thing that you are trying to measure is so small that it would be very difficult to get an accurate answer
  • If, however, you measure the thickness of 100 sheets of paper you can do so much more accurately. Dividing your answer by 100 will then give an accurate figure for the thickness of one sheet
  • This process of taking a reading of a large number of values and then dividing by the number, is a good way of getting accurate values for small figures, including (for example) the time period of a pendulum – measure the time taken for 10 swings and then divide that time by 10

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