physical quantities, units & measurement Flashcards
Physical quantity
A physical quantity is a quantity that can be measured. It consists of a numerical magnitude and a unit.
Base quantity
Base quantities are a set of independent physical quantities. The SI units of the base quantities are called base units.
Length SI unit
Metre (m)
Mass SI unit
Kilogram (kg)
Time SI unit
Second (s)
Electric current SI unit
Ampere (A)
Temperature SI unit
Kelvin (K)
Amount of substance SI unit
Mole (mol)
Luminous intensity SI unit
Candela (cd)
Derived quantities
Derived quantities can be derived by the multiplication or division of two or more base quantities
10^12 prefix
Tera- (T)
10^9 prefix
Giga- (G)
10^6 prefix
Mega- (M)
10^3 prefix
Kilo- (k)
10^-1 prefix
Deci- (d)
10^-2 prefix
Centi- (c)
10^-3 prefix
Milli- (m)
10^-6 prefix
Micro- that squiggly line
10^-9 prefix
Nano- (n)
Example of object 10^-15m in size
Radius of a hydrogen nucleus
Example of object 10^-10m in size
Diameter of a typical atom
Example of object 10^-4m in size
Average width of human hair
Example of object 10^-3m in size
Diameter of a wire
Example of object 10^-1m in size
Length of chopstick
Example of object 10^3m in size
Height of Mount Everest
Example of object 10^5m in size
Diameter of earth
Example of object 10^15m in size
Distance travelled by light in one year
Example of object 10^22m in size
Distance from earth to the andromeda galaxy
Precision
The precision of an instrument is related to the smallest division of the instrument. An instrument is more precise when it has smaller divisions, and will usually make measurements to more decimal places for the same unit
Recording of measurements using measuring tape
0.1cm (1mm)
Recording of measurements using metre rule
0.1cm (1mm)
Recording of measurements using Digital calipers
0.01cm (0.1mm)
Recording of measurements using digital micrometer screw gauge
0.001 cm (0.01mm)
How to avoid parallex error
The eye should be positioned such that our line of sight is perpendicular to the mark on the scale.
Wear and tear on metre rule
Avoid using the zero mark of the metre rule as a start point to take measurements, this is because wear and tear may make it unsuitable for measuring purposes. The worn end may introduce errors to the readings.
Use of digital calipers
Digital calipers are used to measure the internal and external diameters of an object accurately using the inside and outside jaws. The tail of the calipers is used to measure the depth of an object.
Checking fer zero error when using digital calipers
When the jaws are closed, the display should show a reading of zero. If not, there is zero error. The digital calipers must be reset to zero before measurements are taken.
Recording of measurements using digital calipers
To account for other more significant sources of error when using the digital calipers, we record the measurements shown on the digital calipers to 0.01cm or 0.1mm, even though the display may show 1 more decimal place.
Digital micrometer screw gauge
A digital micrometer screw gauge is used to measure objects that are too small to be measured using the digital calipers.
Digital micrometer screw gauge range
0 to 2.5cm
Taking measurements on Digital micrometer screw gauge
Turn the thimble until the anvil and spindle gently touch the object.
Digital micrometer screw gauge ratchet
Turn the ratchet until it clicks once. This prevents over tightening of the digital micrometer screw gauge.
Checking for zero error when using the digital micrometer screw gauge
When the anvil and spindle touch each other, the display should show a reading of zero. If not, there is zero error. The digital micrometer screw gauge must be reset to zero before measurements are taken.
Recording of measurements on the digital micrometer screw gauge
To account for other more significant sources of error when using the digital micrometer screw gauge, we record the measurements shown on the digital micrometer screw gauge to 0.001 cm or 0.01mm, even though the display may show 1 more decimal place.
Oscillation
The regular, repeating, back-and-forth motion of a simple pendulum is an example of an oscillation
Period of the oscillation
The time taken for one complete oscillation is referred to as the period of the oscillation, T.
Factoring in human reaction time
A digital stopwatch can display readings to 0.01s. However, when it is used to time an event, time should not be recorded to the nearest 0.01s. This is because the human reaction time in starting and stopping the stopwatch will be more than a few hundredth of a second. To account for the uncertainty due to human reaction time, the recording of measured time should be to the nearest 0.1s instead.
Why is taking the average of multiple oscillations better?
The method gives a smaller proportion of the same human reaction time in the result.
