Home / CIE AS & A Level Physics 9702: Topic 1: Physical quantities and units- Unit : 1.1 Physical quantities Study Notes

CIE AS & A Level Physics 9702: Topic 1: Physical quantities and units- Unit : 1.1 Physical quantities Study Notes

AS Physics Physical quantities Study Notes

AS Physics Physical quantities Study Notes

AS Physics Physical quantities Study Notes at  IITian Academy  focus on  specific topic and type of questions asked in actual exam. Study Notes focus on AS Physics  Study Notes syllabus with guiding questions of

  1. understand that all physical quantities consist of a numerical magnitude and a unit
  2. make reasonable estimates of physical quantities included within the syllabus

Standard level and higher level: 3 hours
Additional higher level: 1 hour

AS Physics Study Notes- All Topics

1.1.1—Understand that all physical quantities consist of a numerical magnitude and a unit

Quantitative versus qualitative

  • Quantitative data is objective, represented in numbers and is measurable.
  • Qualitative data is subjective, descriptive and depends on interpretation.  Usually observed using five senses.
  • Most observation in physics are quantitative. Example, distance, velocity etc.
  • Descriptive observations (or qualitative) are usually imprecise.
  • For example, saying that water is hot or cold is a qualitative description and telling its accurate temperature is quantitative.
                                                     A thermometer is used to measure the temperature of warm water as shown below. The temperature recorded by the thermometer is. 
             Water is steaming hot. ( Qualitative Description)                                          Temperature of water is 121 °F. (Quantitative Description)

  • A physical quantity is any physical property that can be measured and consists of a magnitude and unit.
  • The unit describes which physical quantity is being described.
  • Some quantities cannot be less than zero like weight while some can have negative values like electronic charge.
  • Along with magnitude and unit, vector quantities are described using a direction. Example, velocity, displacement.
  • Scalar quantities are directionless. Example, speed, distance.
  • The quantities that do not have a unit are dimensionless and are defined only with a number. Example, refractive index.

Are classified into two types:
• Base or fundamental quantities
• Derived quantities

Base (Fundamental) quantity

Base quantities are the basic or fundamental terms that are independently defined.

In physics, there are 7 fundamental quantities namely, length (metre), mass (kilogram), thermodynamic temperature (Kelvin), time (second), amount of a substance (mole), luminous intensity (candela), electric current (Ampere).

For example:  brick – the basic building block of a house

Derived quantity

The quantities can be described in terms of these 7 fundamental units are derived quantities.
For example : It is like the house that was build up from a collection of bricks (basic quantity).

1.1.2—Make reasonable estimates of physical quantities included within the syllabus

  • Order of magnitude of a physical quantity gives the rough estimate of its value.
  • It is represented in the nearest power of 10.
  • For example, if value of weight is given as 300 kg, its estimate it described as 10kg.
  • Consider a quantity is given as B x 10n
  • If B<5, estimate would be 10and if B>5, estimate would be 10n+1 .
  • For example,  in case of 2.5 x 10estimate would be 10and in case of 6.2 x 1011, estimate would be 1012.
  • The same formula is applicable in case of decimal numbers or negative power of 10.
  • For example,  in case of 2.5 x 10-3 estimate would be 10-3   
 

Question 1. Estimate the ratio of
a) the mass of an electron to the mass of a human
b) the radius of earth to the size of the universe
Answers:
1. a) compare 10^-30 and 10^2, 10^32 is the difference hence 1:10^32  b) compare 10^7 and 10^26, 10^19 is the difference hence 1:10^19

AS Physics Physical quantities Worked Out Example

Question

The table shows some measurable quantities.  Which row gives the correct order of magnitude of the measurable quantity in the stated unit?

▶️Answer/Explanation

(A) The mass of a coin is approximately 5 grams hence it is much greater than 10^{-4} kg , hence the option is incorrect.

(B) The thickness of a paper is in millimeter hence it is less than 10^{-2} m therefore the option is incorrect.

(C) The mass of an apple is approximately 125 grams therefore the weight of the apple is

W = 0.100\times 9.8 = 0.98 N \approx 1 N

Hence the option (C) is correct 

Question

Which quantity is a physical quantity?
A atomic number
B efficiency
C number density of charge carriers
D strain

Answer/Explanation

Ans: C

(A) The atomic number is not a physical quantity because it does not have a unit. Atomic number is a chemical property. Hence the option is incorrect.

(B) The efficiency does not have a unit, hence it is not a physical quantity.

(C) The number density has a unit of N/m^3 therefore it is a physical quantity.

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