3NA Science (Physics)Chapter 1Physical Quantities, Units and Measurement

By the end of the lesson, you should be able to

Students should be able to:1.show understanding that all physical quantities consist of a numerical magnitude and a unit2.recall the following base quantities and their units: mass (kg), length (m), time (s), current (A), temperature (K)3.use the following prefixes and their symbols to indicate decimal sub-multiples and multiples of the SI units: nano (n), micro (μ), milli (m), centi (c), deci (d), kilo (k), mega (M)4.show an understanding of the orders of magnitude of the sizes of common objects ranging from a typical atom to the Earth5.describe how to measure a variety of lengths with appropriate accuracy by means of tapes, rules, micrometers and calipers, using a vernier scale as necessary6.describe how to measure a short interval of time including the period of a simple pendulum with appropriate accuracy using stopwatches or appropriate instruments

Physical Quantities

A physical property is a quantity that can be measured

Can you name some physical quantities? Length, speed, mass, weight, force,

pressure, time, temperature etc.

                                                                               

Imagine you ask about the age of a person, and the number is a numerical value.

The information conveyed during the conversation will not be very specific or useful.

How old is your little sister, Jon?

She is 12!

12? 12 months? 12 years old?

No NAKED numbers please!!!Do these numbers make sense to you? What do they lack?

45 897.00

67000

0.325 3 x 108

Quantities, Magnitude, Units The weight of a coin is 12 g.

Physical quantity magnitude

unit

All physical quantities are presented with a magnitude and a unit.

Base Quantities and its Unit

mmetrelength

molmoleAmount of substance

Aamperecurrent

ssecondtime

cdcandelaLuminous intensity

Kkelvintemperature

kgkilogrammass

SymbolBase unitBase quantity

Very small and very BIG quantities

Bacteria is very small…..it is about 0.000 001 m long…

The sun is 149 342 000 000 m away from us…

A more efficient way of expressing very big or very small number is to do so in scientific notation, using appropriate prefixes .

Prefixes

How to convert unit?

Now you try!

5500 mg = ……………… g  5500 x 10-3 = 5.5 g

0.45 kg = ……………....g 0.45 x 103 = 450 g

0.004 GB = …………… B 0.004 x 109 = 4 x 106 B

8900 nm = ……………….. m 8900 x 10-9 = 8.9 x 10-6 m

How to convert unit?

Your turn to try!

250 g = ………………… kg 250 103 = 0.25 kg

0.000345 m = …………….m 0.000345 10-6 = 345 m

45 600 Hz = ……………..MHz 45 600 106 = 0.0456 MHz

0. 765 A = ……………. mA  0.765 10-3 = 765 A

How big? How small?

Estimate the diameter of an atom. 10-10 m

Estimate the diameter of the earth. 1.2 x 107 m

For accurate length measurement using ruler, the eye should be positioned in line, perpendicular to the point to be read to avoid parallax error.

Measuring Length Accurately

Vernier Calipers

http://members.shaw.ca/ron.blond/Vern.APPLET/index.html

• The internal jaws can be used to measure the internal diameter of test tubes or beakers.

• The outside jaws can be used to measure the thickness of a book or the diameter of a sphere.

• The depth rod can be used to measure the depth of a test tube or cavity.

Main scale

Vernier scale

2.1? cm

0.04 cm

2.14 cm

Hence a vernier calipers can measure small length accurate to 2 decimal places in cm.

How to read

write down main scale reading

Add vernier scale reading

Using Vernier Calipers

Qn 1

2.15 cm

Qn 2

1.04 cm

Qn 3

3.19 cm

Qn 4 & 5

2.74 cm 4.04 cm

Micrometer Screw Gauge

http://www.upscale.utoronto.ca/PVB/Harrison/Micrometer/Flash/MicSimulation.html

How to read

write down main scale (sleeve) reading

Add thimble scale reading

Sleeve scale

Thimble scale

4.5? mm

0.12 mm

4.62 mm

Hence a micrometer screwgauge can measure small length accurate to 2 decimal places in mm.

Using Micrometer Screw Gauge

Precautions

Never tighten the thimble too much Clean the ends of the anvil and spindle

before measurement Check for zero error

Qn 1

7.32 mm

Qn 2

7.63 mm

Qn 3

7.00 mm

Qn 4 & 5

2.73 mm 8.57 mm

Time Measurement

Period of a Simple Pendulum

The time taken for a pendulum to complete one oscillation is called the pendulum's period.

For the pendulum on the right, one oscillation is from A to B then back to A again.

Factors affecting period of pendulum

When the length of the pendulum increases, the period increases.

The period is not affected by the size of the bob and the angle of displacement.

Period of Pendulum

Period = 0.35 x 4 = 1.40 s