La Mesure

Question 1

How are random and systematic errors different? Name some eg

Answer 1

• Random errors hv diff magnitudes & signs when measurements are repeated.
  eg parallax error
• Systematic errors hv same magnitude & sign when measurements are repeated.
  eg zero error

Question 2

What are the 7 base quantities and their symbols and units?

Answer 2

1. Mass (m) - kg
2. Length (l) - m
3. Time (t) - s
4. Temperature (T) - K
5. Current (I) - A
6. Amt of substance (n) - mol
7. Luminous intensity (L) - cd (candela, not in syllabus)

Question 3

Name sources of random errors

Answer 3

• Experimenter techniques: e.g. parallax errors where an observer reads a scale from an
  inconsistent angle, or human reaction time such as when timing an oscillation.
• Fluctuations in environmental conditions: e.g. room temperature changes over time.
• Irregularity in the object measured: e.g. measured diameter of a wire (or sphere) is
  slightly different at different points.
• Irregularity of the phenomenon being measured: e.g. time between bounces of a
  basketball.
• Limitation of the equipment: some equipment may be very sensitive and detect the
  slightest variation due to the way that we operate it (e.g. digital multimeter).
• Random nature of phenomenon: measured number of radioactive decays per unit time of
  a substance shows random fluctuations over time.

Question 4

How to identify and reduce random errors?

Answer 4

• scattered ard avg value
• reduced by taking repeated readings and averaging
• improving the experimental setup by:
  eg using more precise instruments, reference objects such as a plumb line, spirit level or fiducial marker,
  controlling environmental fluctuations by using clamps for improved stability,
  hot water baths to maintain a constant temperature.

Question 5

What are sources of systematic errors?

Answer 5

1. Zero error of instrument
   - There is already a reading on the instrument when it should
   be zero; all readings obtained will be incorrect by the same
   amount.
2. Incorrect calibration of an
   instrument
   - Readings are made against the calibration of an instrument.
   If the calibration is wrong, the readings will always be wrong
   in the same way (always above or always below accepted
   value).
3. Incorrect experimental
   technique
   e.g. viewing consistently from the same wrong angle for all
   readings.
   e.g. length of pendulum measured
   from a to b instead of a to c.

Question 6

How to identify and eliminate systematic error?

Answer 6

• affect all readings (not revealed by repeated measurement, cnt b reduced by averaging)
• may b detected by comparing values fr diff experimental technique, compare trends of pt in graph to equation relating quantities (eg zero error present if best fit line does not pass thru origin for eqn that should)
• if cause is known, readings can b corrected, which eliminates error

Question 7

What is difference btw accuracy & precision? What affects error each more?

Answer 7

• Accuracy: how close measured value is to true value (affected by systematic errors; less accurate if more systematic errors present)
• Precision: how close repeated measured values are to one another, w/o regard to true value (affected by random error; less precise if more random errors present)

Question 8

How to find relative velocity btw 2 moving bodies?

Answer 8

Let there by 2 bodies A & B

velocity A relative to B = velocity of A - velocity of B

(remember, direction is considered too, not just subtract values)