Properties, quantities & Measurement

1. Properties, quantities & Measurement CHp 2

2. Topics • 1) Some common properties of matter • 2) Some important quantities & their measurement • 3) Speed & Distance-time graphs

3. physical properties

4. What is a property? • A physical property of a substance (or material) is something which describes the substance, AND is true no matter which sample of the substance • E.g. water freezes at 0 °C

5. Strength • How much a substance can support a heavy load without breaking • (note: avoid using the word “weak”) • E.g. Concrete, Steel

6. Hardness • Harder substances can scratch substances which are less hard • (note: avoid using the word “soft”) • The hardest substance on Earth is diamond

7. Flexibility • The ability to bend without breaking and to return to its original shape and size • Related properties: • A substance is ductile if we can stretch it without breaking • A substance is malleable if we can press it without it breaking (i.e. it flattens) • Uses?

8. Electrical Conductivity • We use the word “conductors”, “insulators” or “conductivity” in two ways. To avoid confusion, use the term “electrical conductivity” or “thermal conductivity” • Electrical conductivity is how easily electricity passes through it • A substance which is a very poor conductor is called a good insulator • Metals, water and graphite are good conductors of electricity • Most are materials (wood, rubber, etc) are insulators

9. thermal conductivity • Thermal conductivity is a measure of how easily heat passes through it • Objects which are not good conductors are called insulators • Metals are good conductors of heat • Most other materials (e.g. plastic, water) are good insulators of heat • The best insulator of heat is actually vacuum • In a frying pan, do you want to use a good conductor or good insulator of heat?

10. Melting point • “Freezing Point” means the same thing • The temperature which a pure substances melts (from solid to liquid) • Impure substances do not melt at a fix temperature, but instead over a range of temperatures • Melting point of water is 0° C • What is the lowest melting point of any substance? • Liquid helium freezes at -273 °C • Note: some substances do not melt into liquid, but turns directly to gas.

11. boiling point • The temperature at which a pure substance boils, and change its state from liquid to gas • Note: boiling is different from evaporation • Substances which are liquid at room temperature but have a low boiling point (e.g. alcohol) are also volatile (evaporate easily)

12. density • Video: density tower (http://www.youtube.com/watch?gl=SG&hl=en-GB&v=-CDkJuo_LYs) • How do you explain the Cartesian Diver? • Given your knowledge of the Cartesian Diver, can you explain how a submarine works? (i.e. it is able to sink or float whenever it wants to)

13. Density • Density is the property which determines whether an object floats or sinks in a liquid • Objects with greater density than the liquid sink in that liquid while objects with lesser density float • If a liquid has very high density, objects float more easily. This explains why it is easy to float in the dead sea. • Video: will a bowling ball float in the dead sea? http://www.youtube.com/watch?v=ZhL68D9BPiw • Density is not only a property, it is also a quantity (next part of the lesson)

14. physical quantities

15. What is a Physical quantity? • The term quantity means “number”. (e.g. quality vs quantity) • A physical quantity is a number representing an object • Sometimes physical quantities can be measured directly • Other times, physical quantities may only be derived through calculations

16. units • Most physical quantities have a unit. E.g. a unit for temperature is °C, a unit for distance is metres • However, some quantities have more than one unit (e.g. units of length include metre, kilometre, inches, feet, cubits, mile, etc.) • So one day, a group of scientists came together and said that there should be an official unit for everything (and other units are unofficial unit) • The official units are called S.I. Units • Original plan if for the whole world to use S.I. Units, but even today, this has not been accomplished yet

17. prefixes • What’s the difference between these units: • kilometre, metre, centimetre, millimetre • Those that are underlined are called prefixes • They adjust the main unit (metre) when the quantity is very large or very small • Refer to your notes for a table of different prefixes (from Giga to Nano)

18. important physical quantities • Length (distance) • Area • Volume • Mass • Density • Temperature • Time

19. Length • S.I. Unit for length is the metre (m) • You need to be familiar with the ruler, measuring tape and the verniercalipers to measure length • To what precision can the ruler measure length? • To what precision can the verniercalipers measure length? • What is one way where we can make an error in measuring length? • Parallax Error (refer to notes)

20. area • S.I. Unit for area is square metre (m2) • It is difficult to measure area directly • We usually calculate it (if it is a regular shaped object) or we estimate it (irregularly shaped object)

21. volume • S.I. Unit for volume is cubic metre (m3) • Other commonly use units: litres, millilitres, pints, fluid ounces • We measure volume of liquids using a measuring cylinder • We can calculate volume for regularly shaped objects • How do we measure volume for irregularly shaped objects? • We can use a displacement can(Archimedes can, Eureka can)

22. measuring cylinder • How to read measuring cylinder: • Read the lowest point of the meniscus – be careful of parallax error! • Read to half the smallest division (if smallest division is 1 ml, then half smallest division is 0.5 ml) • We will try this in a later practical

23. mass • Mass in the measure of the amount of substance in an object • S.I. Unit is kg • We measure mass using a beam balance:

24. Mass • What’s the difference between mass and weight? • Weight is the amount of gravity pulling you downwards towards Earth • When an astronaut goes to the moon, his mass remains the same, but his weight decreases by 6 times! • In other space (zero gravity), your weight is zero!

25. Mass • In the lab you will be using an electronic balance to measure mass • In reality, what you’re doing is measuring the weight • But we approximate the weight to be the mass, since it is more convenient than using a beam balance

26. units for density • The S.I. units for density is kgm-3 • But usually we don’t measure things which are smaller, so another unit we commonly use for density is gcm-3 • 1 gcm-3 is 1 g divided by 1 cm3 • 1 kgm-3 = 1000 gcm-3

27. density • Density is the mass per unit volume of an object • It describes how much matter is squeezed into an amount of space • Formula: density = (mass)/(volume) • S.I. Unit for density is kg/m3 or kgm-3 • It is difficult to measure density directly, usually we measure the mass and the volume, and divide the two • Recall that density is also a property (the same substance will also have the same density)

28. Liquid-in-glass thermometer • Consider the red liquid (alcohol) in the thermometer below • When the temperature increases, what is happening to the alcohol? • What is happening to the volume of alcohol? • What is happening to the density of alcohol?

29. temperature • The S.I. Units for Temperature is Kelvin (K) • However, °C is more commonly used (in real life as well as the lab as well) • In the lab, we measure temperature using an alcohol thermometer • Note: in the past we use mercury thermometers. Be careful if you ever handle a mercury thermometer; do not break it, mercury is toxic • How does an in-ear thermometer work?

30. time • The S.I. Units for time is seconds (s) • In the lab we use a stopwatch to measure time. Record time from a stopwatch to 1 decimal place • In the lab you have already done the pendulum experiment • Did you notice: the time taken for one complete oscillation (period) is approximately constant, if the length of the pendulum is constant? This is the principle behind pendulum clocks (i.e. old-grandfather clocks) • What is the principle behind quartz clocks and watches?

31. summary

32. distance-time graphs

33. Distance-time graphs • In the previous unit, you have been introduced to graphs and graph-plotting • now we will pay particular attention to a specific graph: distance-time graph • This graph represents the distance travelled by an object with respect to time • Vertical axis: distance • Horizontal axis: time • Note: usually, a distance-time graph is NOT the graph of a scientific experiment.

34. stationary • Stationary objects appear as a flat line distance /m 5 time/s

35. uniform speed • Uniform speed means the speed of the object is constant (not changing) • Objects travelling with uniform speed appear as a straight line going upwards distance /m 20 time/s 5

36. non-uniform speed • Non-uniform speed means the speed of the object is not constant • It may either be going faster and faster, or slower and slower distance /m distance /m time/s time/s

37. Ticker Tape • The ticker-tape is a machine which draws dots at a regular frequency • The faster the machine is travelling, the further apart the dots are • When the ticker-tape machine is moving at a uniform speed, it will draw dots which are equally spaced apart • If the ticker-tape machine is going faster and faster, it will draw dots further and further apart • If the ticker-tape machine is going slower and slower, it will draw dots closer and closer together

38. ticker tape

39. Average speed • There are two ways to understand speed: • 1) instantaneous speed • 2) average speed • When we say an object is getting faster and faster, we are referring to its instantaneous speed • However, even if an object does not have uniform speed, we can still calculate its average speed given this formula: • Average Speed = Total Distance / Total Time

40. Summary • Physical Properties • Strength • Hardness • Flexibility • Electrical Conductivity • Thermal Conductivity • Boiling Point • Melting Point • Density • Physical Quantities • Units & Prefixes

41. SUmmary • Physical Quantities (cont’d) • Length • Area • Volume • Mass • Density • Temperature • Time • Distance-Time Graphs • Uniform Speed • Non-Uniform Speed • Ticker-Tape • Average Speed