Friday, August 14, 2015

Measurement Techniques | 9702 Physics Summary Notes

  • 2. Measurement Techniques | 9702 Physics Summary Notes





Quantity
Accuracy
Instrument
Length
1 cm
Tape
0.1 cm
Ruler
0.01 cm
Vernier caliper
0.001 cm
Micrometer screw gauge
Volume
1 cm3
Measuring cylinder
0.05 cm3
Pipette/burette
Angle
0.5o
Protractor
Time
1 min
Clocks
0.01 sec
Stopwatch
-axis scale
Time base of c.r.o
Temperature
1oC
Thermometer
0.5oC
Thermocouple
P.d.
0.01 V
Voltmeter
Current
0.01 A
Ammeter
0.0001 A
Galvanometer






2.1 Using a Cathode Ray Oscilloscope


Example: A supply of peak value 5.0 V and of frequency 50 Hz is connected to a c.r.o with time-base at 10 ms per division and Y-gain at 5.0V per division. Which trace is obtained?








2.2 Systematic and Random Errors

·  Systematic error:
o Constant error in one direction; too big or too small
o Cannot be eliminated by repeating or averaging
o If systematic error small, measurement accurate
o Accuracy: refers to degree of agreement between result of a measurement and true value of quantity.

·  Random error:
o Random fluctuations or scatter about a true value
o Can be reduced by repeating and averaging
o When random error small, measurement precise
o Precision: refers to degree of agreement of repeated measurements of the same quantity (regardless of whether it is correct or not)







2.3 Calculations Involving Errors

For a quantity x = (2.0±0.1)mm
· Absolute uncertainty = ∆x = ±0.1mm
· Fractional uncertainty = ∆x/x =0.05  
· Percentage uncertainty =  (∆x/x) × 100% = 5%
· Combining errors:
o When values added or subtracted, add absolute error
If p = (2x+y) / 3 or p = (2x-y) / 3, then ∆p = (2∆x+∆y) / 3
o When values multiplied or divided, add % errors
o When values are powered (e.g. squared), multiply percentage error with power
If r = 2xy3 or r = 2x / y3, then ∆r/r = ∆x/x+ 3 (∆y/y)




2.4 Treatment of Significant Figures

· Actual error: recorded to only 1 significant figure
· Number of decimal places for a calculated quantity is equal to number of decimal places in actual error.
· During a practical, when calculating using a measured quantity, give answers to the same significant figure as the measurement or one less








2.5 Micrometer Screw Gauge



·  Measures objects up to 0.01mm
·  Place object between anvil & spindle
·  Rotate thimble until object firmly held by jaws
·  Add together value from main scale and rotating scale




2.6 Vernier Scale


Measures objects up to 0.1mm
·  Place object on rule
·  Push slide scale to edge of object.
·  The sliding scale is 0.9mm long & is divided into 10 equal divisions.
·  Check which line division on sliding scale matches with a line division on rule
·  Subtract the value from the sliding scale (0.09×Divisions) by the value from the rule.



4 comments:

  1. Do you have solutions for 04/O/N/06 Question 2?

    ReplyDelete
    Replies
    1. See solution 984 at
      http://physics-ref.blogspot.com/2015/09/physics-9702-doubts-help-page-204.html

      Delete
  2. Hi can you please explain why it is impossible to measure time any closer than the nearest one tenth of a second using a stopwatch that has a precision of 0.01 s. Please reply asap I have a practical exam

    ReplyDelete
    Replies
    1. It's because of the human reaction time which is about 0.1s. We need to start and stop the clock manually.

      Delete

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