Unit 3

Question 1

Proportional graph

Answer 1

• y · x should be constant
• y = cx
• y = c / x

Question 2

Why ohmeter more accurate

Answer 2

• With 2 metres (ammeter and voltmeter), more systematic error and larger % uncertainty
• % uncertainty would increase without it

Question 3

Validity

Answer 3

• A valid measurement measures what it is supposed to be measuring
• Depends on method and instruments

Question 4

Repeatable

Answer 4

• Similar results are obtained by a group/person repeating the same method

Question 5

Repdroducibility

Answer 5

• Similar results are obtained by different groups/persons using different methods/apparatus

Question 6

Accuracy

Answer 6

• How close to the true value
• Influenced by random and systematic error
• Can’t be expressed numerically

Question 7

Precision

Answer 7

• Precise if repeated measurements are consistently close together
• Influenced by random effects
• Can be expressed numerically

Question 8

Increase accuracy

Answer 8

• Object resolution should be small when compared to the size measured
• Measuring object sizes:
  · Take multiple measurements in different positions and take average
• Measuring lengths:
  · Avoid parallax error by looking at eye level (90º)
• Measuring mass/force:
  · Avoid systematic error by calibrating instrument before using it (make sure it reads 0 with no object present)

Question 9

Resolution

Answer 9

• Smallest measuring interval
• Source of uncertainty in a single reading
• Analogue: Gap between smalles gradations
• Digital: Smallest SF shown

Question 10

Significant figures

Answer 10

• Non-zero always, significant
• Leading zeros, not significant
• Trailing zeros after decimal, significant
• Zeros in between significant, significant

Question 11

Significant figures in calculations

Answer 11

• Answer always to the lowest significant figures of the values being used
• Include number of significant figures after result

Question 12

Systematic errors

Answer 12

• Fault in instrument or method
• Affects precision and accuracy
• Causes:
  · Not calibrated
  · Reading value at an angle
• Types:
  · Zero error: Values shifted by a constant (+ -)
  · Scale factor error: Values transformed by a constant multiplied (different gradient)

Question 13

Errors

Answer 13

• The difference between the measurement and the true value
• Due to systematic errors or random errors
• An error of unkown size is a source of uncertainty

Question 14

True value

Answer 14

• The value the would have been obtained in an ideal measurement
• With the exception of a fundamental
  constant the true value is considered unknowable

Question 15

Random error

Answer 15

• Unpredictable variations in measurement that cause them to be scattered around the true value
• Caused by fluctuations in the environment or unknown variatitons in the measuring instrument or technique
• Dealt with by averaging results from multiple measurements

Question 16

Anomaly

Answer 16

• Sample that deviates
• Not included in calculations

Question 17

Uncertainty

Answer 17

• Interval within which the true value can be considered to lie
• Stated after values as ± (uncertainty) in SAME units and generally just 1 SF

Question 18

Finding absolute uncertainty

Answer 18

• Methods:
  1. Find range of a group of reading and half it
  2. Find range between mean and furthest value from the mean
  3. Resolution of instrument:
  · Analogue: The resolution itself
  · Digital: The resolution itself

Question 19

Calculate percentage uncertainty

Answer 19

• (absolute uncertainty / obtained value) · 100