1.4 understanding measurement

Chapter 1
Introduction To Physics

1.4
Understanding Measurement

www.myfunphysicsworld.blogspot.com

Learning Outcomes
 Measure physical quantities
using appropriate instruments
 Explain :
1. accuracy, consistency, sensitivity
2. Types of experimental error
 Use appropriate technique to
reduce error

Measuring Instruments
 Metre Rule
 Measuring tape
 Micrometer Screw Gauge
 Vernier Calipers
 Stop watch
 Thermometer
 Ammeter
 Triple beam balance
 Spring balance

Accuracy

 How close the value of the
measurement to the actual
value


Consistency

The consistency of a measuring
instrument is its ability to
register the same reading
when a measurement is
repeated.


Sensitivity

 The instruments ability to detect a
small change in the quantity to be
measured
 Responds faster, by a larger extend
to small variations in quantity
measured


Accuracy


Consistency


Consistency

 Very Consistent :
 readings are all the same everytime
measured


How to measure
consistency?
 Calculate Relative Deviation
 High consistency:

Smaller value of relative deviation


Deviation & Relative
Deviation

A deviation is the difference between
a measured value and its mean
value or the average value.


READING CONSISTENCY
S AVERAGE
1 0.5 = 0.5 + 0.4 + 0.5+0.3+0.4
5
2 0.4 = 0.42
3 0.5
DEVIATION (1st Reading)
4 0.3 = 0.42 – 0.5
= 0.08
5 0.4
1. AVERAGE
2. DEVIATION
3. AVERAGE DEVIATION
4. RELATIVE DEVIATION : average deviation x
100%

READINGS Deviation
1 0.5 0.08
2 0.4 0.02
3 0.5 0.08
4 0.3 0.12
5 0.4 0.02
Average 0.42 0.064 = 0.06

RELATIVE DEVIATION :
= average deviation x 100%
average
= 0.06/0.42 x 100% = 14.29 %

4amanah until here
20/01


The consistency of a measuring
instrument can be improved by:

(a) eliminating parallax measurement.
(b) exercising greater care and effort
when taking readings.
(c) using an instrument which is not
defective


Sensitivity


Sensitivity

 The instruments ability to detect a
small change in the quantity to be
measured


Appropriate Instrument

 Musthave:
Appropriate sensitivity


Choosing Appropriate
Instrument
 Must consider Magnitude of
the quantity
 Should not exceed the
maximum and minimum
capasity of the instrument


Sensitivity

 The smaller the change which can
be measured by the instrument,
 The more sensitive the instrument
is


How to increase the
sensitivity?

 Narrow capillary
 Thin walled-bulb

Errors In Measurements

 Two types of experimental
errors:
1. Systematic errors
2. Random errors


Systematic errors


Systematics errors

 Due to
1. the error in calibration of instrument
which makes it defective
2. Zero error* of the instrument

Zero error – the pointer of the
instrument does not return to zero
when not in use

Systematics errors

 Will lead to a decrease in
accuracy
 Always occur with the same value
when we measure using the same
instrument in the same way
 Readings taken are always larger
or always smaller than the true
value www.myfunphysicsworld.blogspot.com

Can we eliminate the
systematic error by repeating
the measurement and
averaging out the result?

 No! Cannot!
 Only can by:

 calibrating or doing adjustment
to the instrument

Random errors


Random errors
 Due to
1. the carelessness of the observer
when making measurement
(parallax error)
2. Sudden change of ambient factor
(temperature, air circulation)
 Will lead to a decrease in
consistency

accurac
y


VERNIER CALLIPERS


 What is the accuracy of
 a) main scale

 b) vernier scale

 Reading = 3.2 + 0.04 = 3.24 cm

MICROMETER SCREW
GAUGE


What is the reading?

Reading = 2.0 + 0.22
= 2.22 mm


What is the reading?


Reading = 2.5 + 0.22
= 2.72 mm


ZERO ERROR


0

0

Zero Error = negative
= - (0.10 – 0.06)
= - 0.04 cm

0
main

0
vernier
Zero Error = positive
= +0.05 cm

4 amanah until here
27 jan
-reading msg already
- ze msg notyet


 Zero Error = +0.04 mm Correct reading
 meaning that 0.04 mm Micrometer reading -(+0.04)
must be = Micrometer reading - 0.04
 substracted from all

readings taken.

 Zero Error = -0.03 mm Correct reading
 meaning that 0.03 mm Micrometer reading - (-0.03)
must be = Micrometer reading + 0.03
 added to all readings

taken.

 Figure shows a micrometer screw
gauge used to measure the size of an
object. Determine the size of the
object if the micrometer has a zero
 (a) +0.01 mm (b) -0.03 mm

 Solution
 The reading of the gauge
 = 4.50 + 0.21 = 4.71 mm
 (a) Size of object =Reading- Zero Erro
= 4.71 – (+0.01)
= 4.70 mm
 (b) Size of object = Reading – Zero Error
= 4.71-(-0.03)
= 4.74 mm

1.4 understanding measurement

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